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完美解决关于禁止ViewPager预加载的相关问题

转载  2017-09-04   作者:Plumcot_hz   我要评论

本篇文章主要介绍了完美解决关于禁止ViewPager预加载的相关问题,具有一定的参考价值,感兴趣的小伙伴们可以参考一下

我最近上班又遇到一个小难题了,就是如题所述:ViewPager预加载的问题。相信用过ViewPager的人大抵都有遇到过这种情况,网上的解决办法也就那么几个,终于在我自己不断试验之下,完美解决了(禁止了)ViewPager的预加载。

好了,首先来说明一下,什么是ViewPager的预加载:ViewPager有一个 “预加载”的机制,默认会把ViewPager当前位置的左右相邻页面预先初始化(俗称的预加载),它的默认值是 1,这样做的好处就是ViewPager左右滑动会更加流畅。

可是我的情况很特殊,因为我 5 个Fragment里有一个Fragment是有SurfaceView的,这样造成的问题就是,我ViewPager滑动到其相邻页面时,含有SurfaceView的页面就会被预先初始化,然后SurfaceView就开始预览了,只是我们看不到而已。同样的,当我们从含有SurfaceView的页面滑倒其相邻的页面时,SurfaceView并不会回调其surfaceDestory方法。于是这给我造成了极大的困扰。

ok,下面言归正传,到底该怎么禁止ViewPager的这个预加载问题呢?

方案1:网上大多数说法是 懒加载,即让ViewPager预加载初始化UI,而具体一些数据,网络访问请求等延迟加载。这是靠Fragment里有一个setUserVisibleHint(boolean isVisibleToUser)的方法,我们可以在这个方法里做判断,当其True可见时(即切换到某一个具体Fragment)时才去加载数据,这样可以省流量。但这里并不满足我的需求,因为某一个Fragment并不会在ViewPager滑动到其相邻的Fragment时销毁。这个只可以解决部分人问题。

首先我们来深入了解下ViewPager的预加载机制:

上文提到过,ViewPager默认预加载的数量是1,这一点我们可以在ViewPager源码里看到。

DEFAULT_OFFSCREEN_PAGES 这里就定义了默认值是1, 所以网上 有种解决方案 说调用ViewPager的setOffscreenPageLimit(int limit),来设置ViewPager预加载的数量,但是这里很明确的告诉你,这种方案是不可行的,如下图ViewPager源码:

我们可以看到,如果你调用该方法传进来的值小于1是无效的,会被强行的拽回1。而且DEFAULT_OFFSCREEN_PAGES 这个值是private的,子类继承ViewPager也是不可见的。

方案2:然后网上有第二种说法,自定义一个ViewPager,把原生ViewPager全拷过来,修改这个DEFAULT_OFFSCREEN_PAGES 值为0。对,就是这种解决方案!!但是!!

但是!!!但是什么呢?但是我试过,没用。为什么呢?接下来就是本文的重点了。

因为现在Android都6.0了,版本都老高了,其实android虽然每个版本都有v4包,但是这些v4包是有差异的。这就造成高版本v4包里的ViewPager,即使你Copy它,将其DEFAULT_OFFSCREEN_PAGES的值改为0,还是不起作用的,其中的逻辑变了。具体哪里变了导致无效我也没有继续研究了,毕竟公司不会花时间让我研究这些啊。偷笑

完美解决方案:ok,所以关于禁止ViewPager预加载的完美解决方案就是,使用低版本v4包里的ViewPager,完全copy一份,将其中的DEFAULT_OFFSCREEN_PAGES值改为0即可。博主亲测 API 14 即 Android 4.0的v4包里ViewPager 有效。

当然,谷歌既然有这么一种ViewPager的机制肯定有它的道理,所以一般还是预加载的好。

最后,因为低版本的源码越来越少的人会去下载,这里直接把这个禁止了预加载的ViewPager贴上来,需要的人就拿去吧。copy就能用了。

package com.plumcot.usb.view; 
 
/* 
 * Copyright (C) 2011 The Android Open Source Project 
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); 
 * you may not use this file except in compliance with the License. 
 * You may obtain a copy of the License at 
 * 
 *   http://www.apache.org/licenses/LICENSE-2.0 
 * 
 * Unless required by applicable law or agreed to in writing, software 
 * distributed under the License is distributed on an "AS IS" BASIS, 
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 * See the License for the specific language governing permissions and 
 * limitations under the License. 
 */ 
 
 
import android.content.Context; 
import android.database.DataSetObserver; 
import android.graphics.Canvas; 
import android.graphics.Rect; 
import android.graphics.drawable.Drawable; 
import android.os.Parcel; 
import android.os.Parcelable; 
import android.os.SystemClock; 
import android.support.v4.os.ParcelableCompat; 
import android.support.v4.os.ParcelableCompatCreatorCallbacks; 
import android.support.v4.view.KeyEventCompat; 
import android.support.v4.view.MotionEventCompat; 
import android.support.v4.view.PagerAdapter; 
import android.support.v4.view.VelocityTrackerCompat; 
import android.support.v4.view.ViewCompat; 
import android.support.v4.view.ViewConfigurationCompat; 
import android.support.v4.widget.EdgeEffectCompat; 
import android.util.AttributeSet; 
import android.util.Log; 
import android.view.FocusFinder; 
import android.view.KeyEvent; 
import android.view.MotionEvent; 
import android.view.SoundEffectConstants; 
import android.view.VelocityTracker; 
import android.view.View; 
import android.view.ViewConfiguration; 
import android.view.ViewGroup; 
import android.view.ViewParent; 
import android.view.accessibility.AccessibilityEvent; 
import android.view.animation.Interpolator; 
import android.widget.Scroller; 
 
import java.util.ArrayList; 
import java.util.Collections; 
import java.util.Comparator; 
 
/** 
 * Layout manager that allows the user to flip left and right 
 * through pages of data. You supply an implementation of a 
 * {@link android.support.v4.view.PagerAdapter} to generate the pages that the view shows. 
 * 
 * <p>Note this class is currently under early design and 
 * development. The API will likely change in later updates of 
 * the compatibility library, requiring changes to the source code 
 * of apps when they are compiled against the newer version.</p> 
 */ 
public class NoPreloadViewPager extends ViewGroup { 
  private static final String TAG = "<span style="font-family:Arial, Helvetica, sans-serif;">NoPreLoadViewPager</span>"; 
  private static final boolean DEBUG = false; 
 
  private static final boolean USE_CACHE = false; 
 
  private static final int DEFAULT_OFFSCREEN_PAGES = 0;//默认是1 
  private static final int MAX_SETTLE_DURATION = 600; // ms 
 
  static class ItemInfo { 
    Object object; 
    int position; 
    boolean scrolling; 
  } 
 
  private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>(){ 
    @Override 
    public int compare(ItemInfo lhs, ItemInfo rhs) { 
      return lhs.position - rhs.position; 
    }}; 
 
  private static final Interpolator sInterpolator = new Interpolator() { 
    public float getInterpolation(float t) { 
      // _o(t) = t * t * ((tension + 1) * t + tension) 
      // o(t) = _o(t - 1) + 1 
      t -= 1.0f; 
      return t * t * t + 1.0f; 
    } 
  }; 
 
  private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>(); 
 
  private PagerAdapter mAdapter; 
  private int mCurItem;  // Index of currently displayed page. 
  private int mRestoredCurItem = -1; 
  private Parcelable mRestoredAdapterState = null; 
  private ClassLoader mRestoredClassLoader = null; 
  private Scroller mScroller; 
  private PagerObserver mObserver; 
 
  private int mPageMargin; 
  private Drawable mMarginDrawable; 
 
  private int mChildWidthMeasureSpec; 
  private int mChildHeightMeasureSpec; 
  private boolean mInLayout; 
 
  private boolean mScrollingCacheEnabled; 
 
  private boolean mPopulatePending; 
  private boolean mScrolling; 
  private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES; 
 
  private boolean mIsBeingDragged; 
  private boolean mIsUnableToDrag; 
  private int mTouchSlop; 
  private float mInitialMotionX; 
  /** 
   * Position of the last motion event. 
   */ 
  private float mLastMotionX; 
  private float mLastMotionY; 
  /** 
   * ID of the active pointer. This is used to retain consistency during 
   * drags/flings if multiple pointers are used. 
   */ 
  private int mActivePointerId = INVALID_POINTER; 
  /** 
   * Sentinel value for no current active pointer. 
   * Used by {@link #mActivePointerId}. 
   */ 
  private static final int INVALID_POINTER = -1; 
 
  /** 
   * Determines speed during touch scrolling 
   */ 
  private VelocityTracker mVelocityTracker; 
  private int mMinimumVelocity; 
  private int mMaximumVelocity; 
  private float mBaseLineFlingVelocity; 
  private float mFlingVelocityInfluence; 
 
  private boolean mFakeDragging; 
  private long mFakeDragBeginTime; 
 
  private EdgeEffectCompat mLeftEdge; 
  private EdgeEffectCompat mRightEdge; 
 
  private boolean mFirstLayout = true; 
 
  private OnPageChangeListener mOnPageChangeListener; 
 
  /** 
   * Indicates that the pager is in an idle, settled state. The current page 
   * is fully in view and no animation is in progress. 
   */ 
  public static final int SCROLL_STATE_IDLE = 0; 
 
  /** 
   * Indicates that the pager is currently being dragged by the user. 
   */ 
  public static final int SCROLL_STATE_DRAGGING = 1; 
 
  /** 
   * Indicates that the pager is in the process of settling to a final position. 
   */ 
  public static final int SCROLL_STATE_SETTLING = 2; 
 
  private int mScrollState = SCROLL_STATE_IDLE; 
 
  /** 
   * Callback interface for responding to changing state of the selected page. 
   */ 
  public interface OnPageChangeListener { 
 
    /** 
     * This method will be invoked when the current page is scrolled, either as part 
     * of a programmatically initiated smooth scroll or a user initiated touch scroll. 
     * 
     * @param position Position index of the first page currently being displayed. 
     *         Page position+1 will be visible if positionOffset is nonzero. 
     * @param positionOffset Value from [0, 1) indicating the offset from the page at position. 
     * @param positionOffsetPixels Value in pixels indicating the offset from position. 
     */ 
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels); 
 
    /** 
     * This method will be invoked when a new page becomes selected. Animation is not 
     * necessarily complete. 
     * 
     * @param position Position index of the new selected page. 
     */ 
    public void onPageSelected(int position); 
 
    /** 
     * Called when the scroll state changes. Useful for discovering when the user 
     * begins dragging, when the pager is automatically settling to the current page, 
     * or when it is fully stopped/idle. 
     * 
     * @param state The new scroll state. 
     * @see android.support.v4.view.ViewPager#SCROLL_STATE_IDLE 
     * @see android.support.v4.view.ViewPager#SCROLL_STATE_DRAGGING 
     * @see android.support.v4.view.ViewPager#SCROLL_STATE_SETTLING 
     */ 
    public void onPageScrollStateChanged(int state); 
  } 
 
  /** 
   * Simple implementation of the {@link android.support.v4.view.LazyViewPager.OnPageChangeListener} interface with stub 
   * implementations of each method. Extend this if you do not intend to override 
   * every method of {@link android.support.v4.view.LazyViewPager.OnPageChangeListener}. 
   */ 
  public static class SimpleOnPageChangeListener implements OnPageChangeListener { 
    @Override 
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { 
      // This space for rent 
    } 
 
    @Override 
    public void onPageSelected(int position) { 
      // This space for rent 
    } 
 
    @Override 
    public void onPageScrollStateChanged(int state) { 
      // This space for rent 
    } 
  } 
 
  public NoPreloadViewPager(Context context) { 
    super(context); 
    initViewPager(); 
  } 
 
  public NoPreloadViewPager(Context context, AttributeSet attrs) { 
    super(context, attrs); 
    initViewPager(); 
  } 
 
  void initViewPager() { 
    setWillNotDraw(false); 
    setDescendantFocusability(FOCUS_AFTER_DESCENDANTS); 
    setFocusable(true); 
    final Context context = getContext(); 
    mScroller = new Scroller(context, sInterpolator); 
    final ViewConfiguration configuration = ViewConfiguration.get(context); 
    mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration); 
    mMinimumVelocity = configuration.getScaledMinimumFlingVelocity(); 
    mMaximumVelocity = configuration.getScaledMaximumFlingVelocity(); 
    mLeftEdge = new EdgeEffectCompat(context); 
    mRightEdge = new EdgeEffectCompat(context); 
 
    float density = context.getResources().getDisplayMetrics().density; 
    mBaseLineFlingVelocity = 2500.0f * density; 
    mFlingVelocityInfluence = 0.4f; 
  } 
 
  private void setScrollState(int newState) { 
    if (mScrollState == newState) { 
      return; 
    } 
 
    mScrollState = newState; 
    if (mOnPageChangeListener != null) { 
      mOnPageChangeListener.onPageScrollStateChanged(newState); 
    } 
  } 
 
  public void setAdapter(PagerAdapter adapter) { 
    if (mAdapter != null) { 
//      mAdapter.unregisterDataSetObserver(mObserver); 
      mAdapter.startUpdate(this); 
      for (int i = 0; i < mItems.size(); i++) { 
        final ItemInfo ii = mItems.get(i); 
        mAdapter.destroyItem(this, ii.position, ii.object); 
      } 
      mAdapter.finishUpdate(this); 
      mItems.clear(); 
      removeAllViews(); 
      mCurItem = 0; 
      scrollTo(0, 0); 
    } 
 
    mAdapter = adapter; 
 
    if (mAdapter != null) { 
      if (mObserver == null) { 
        mObserver = new PagerObserver(); 
      } 
//      mAdapter.registerDataSetObserver(mObserver); 
      mPopulatePending = false; 
      if (mRestoredCurItem >= 0) { 
        mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader); 
        setCurrentItemInternal(mRestoredCurItem, false, true); 
        mRestoredCurItem = -1; 
        mRestoredAdapterState = null; 
        mRestoredClassLoader = null; 
      } else { 
        populate(); 
      } 
    } 
  } 
 
  public PagerAdapter getAdapter() { 
    return mAdapter; 
  } 
 
  /** 
   * Set the currently selected page. If the ViewPager has already been through its first 
   * layout there will be a smooth animated transition between the current item and the 
   * specified item. 
   * 
   * @param item Item index to select 
   */ 
  public void setCurrentItem(int item) { 
    mPopulatePending = false; 
    setCurrentItemInternal(item, !mFirstLayout, false); 
  } 
 
  /** 
   * Set the currently selected page. 
   * 
   * @param item Item index to select 
   * @param smoothScroll True to smoothly scroll to the new item, false to transition immediately 
   */ 
  public void setCurrentItem(int item, boolean smoothScroll) { 
    mPopulatePending = false; 
    setCurrentItemInternal(item, smoothScroll, false); 
  } 
 
  public int getCurrentItem() { 
    return mCurItem; 
  } 
 
  void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) { 
    setCurrentItemInternal(item, smoothScroll, always, 0); 
  } 
 
  void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) { 
    if (mAdapter == null || mAdapter.getCount() <= 0) { 
      setScrollingCacheEnabled(false); 
      return; 
    } 
    if (!always && mCurItem == item && mItems.size() != 0) { 
      setScrollingCacheEnabled(false); 
      return; 
    } 
    if (item < 0) { 
      item = 0; 
    } else if (item >= mAdapter.getCount()) { 
      item = mAdapter.getCount() - 1; 
    } 
    final int pageLimit = mOffscreenPageLimit; 
    if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) { 
      // We are doing a jump by more than one page. To avoid 
      // glitches, we want to keep all current pages in the view 
      // until the scroll ends. 
      for (int i=0; i<mItems.size(); i++) { 
        mItems.get(i).scrolling = true; 
      } 
    } 
 
    final boolean dispatchSelected = mCurItem != item; 
    mCurItem = item; 
    populate(); 
    final int destX = (getWidth() + mPageMargin) * item; 
    if (smoothScroll) { 
      smoothScrollTo(destX, 0, velocity); 
      if (dispatchSelected && mOnPageChangeListener != null) { 
        mOnPageChangeListener.onPageSelected(item); 
      } 
    } else { 
      if (dispatchSelected && mOnPageChangeListener != null) { 
        mOnPageChangeListener.onPageSelected(item); 
      } 
      completeScroll(); 
      scrollTo(destX, 0); 
    } 
  } 
 
  public void setOnPageChangeListener(OnPageChangeListener listener) { 
    mOnPageChangeListener = listener; 
  } 
 
  /** 
   * Returns the number of pages that will be retained to either side of the 
   * current page in the view hierarchy in an idle state. Defaults to 1. 
   * 
   * @return How many pages will be kept offscreen on either side 
   * @see #setOffscreenPageLimit(int) 
   */ 
  public int getOffscreenPageLimit() { 
    return mOffscreenPageLimit; 
  } 
 
  /** 
   * Set the number of pages that should be retained to either side of the 
   * current page in the view hierarchy in an idle state. Pages beyond this 
   * limit will be recreated from the adapter when needed. 
   * 
   * <p>This is offered as an optimization. If you know in advance the number 
   * of pages you will need to support or have lazy-loading mechanisms in place 
   * on your pages, tweaking this setting can have benefits in perceived smoothness 
   * of paging animations and interaction. If you have a small number of pages (3-4) 
   * that you can keep active all at once, less time will be spent in layout for 
   * newly created view subtrees as the user pages back and forth.</p> 
   * 
   * <p>You should keep this limit low, especially if your pages have complex layouts. 
   * This setting defaults to 1.</p> 
   * 
   * @param limit How many pages will be kept offscreen in an idle state. 
   */ 
  public void setOffscreenPageLimit(int limit) { 
    if (limit < DEFAULT_OFFSCREEN_PAGES) { 
      Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " + 
          DEFAULT_OFFSCREEN_PAGES); 
      limit = DEFAULT_OFFSCREEN_PAGES; 
    } 
    if (limit != mOffscreenPageLimit) { 
      mOffscreenPageLimit = limit; 
      populate(); 
    } 
  } 
 
  /** 
   * Set the margin between pages. 
   * 
   * @param marginPixels Distance between adjacent pages in pixels 
   * @see #getPageMargin() 
   * @see #setPageMarginDrawable(android.graphics.drawable.Drawable) 
   * @see #setPageMarginDrawable(int) 
   */ 
  public void setPageMargin(int marginPixels) { 
    final int oldMargin = mPageMargin; 
    mPageMargin = marginPixels; 
 
    final int width = getWidth(); 
    recomputeScrollPosition(width, width, marginPixels, oldMargin); 
 
    requestLayout(); 
  } 
 
  /** 
   * Return the margin between pages. 
   * 
   * @return The size of the margin in pixels 
   */ 
  public int getPageMargin() { 
    return mPageMargin; 
  } 
 
  /** 
   * Set a drawable that will be used to fill the margin between pages. 
   * 
   * @param d Drawable to display between pages 
   */ 
  public void setPageMarginDrawable(Drawable d) { 
    mMarginDrawable = d; 
    if (d != null) refreshDrawableState(); 
    setWillNotDraw(d == null); 
    invalidate(); 
  } 
 
  /** 
   * Set a drawable that will be used to fill the margin between pages. 
   * 
   * @param resId Resource ID of a drawable to display between pages 
   */ 
  public void setPageMarginDrawable(int resId) { 
    setPageMarginDrawable(getContext().getResources().getDrawable(resId)); 
  } 
 
  @Override 
  protected boolean verifyDrawable(Drawable who) { 
    return super.verifyDrawable(who) || who == mMarginDrawable; 
  } 
 
  @Override 
  protected void drawableStateChanged() { 
    super.drawableStateChanged(); 
    final Drawable d = mMarginDrawable; 
    if (d != null && d.isStateful()) { 
      d.setState(getDrawableState()); 
    } 
  } 
 
  // We want the duration of the page snap animation to be influenced by the distance that 
  // the screen has to travel, however, we don't want this duration to be effected in a 
  // purely linear fashion. Instead, we use this method to moderate the effect that the distance 
  // of travel has on the overall snap duration. 
  float distanceInfluenceForSnapDuration(float f) { 
    f -= 0.5f; // center the values about 0. 
    f *= 0.3f * Math.PI / 2.0f; 
    return (float) Math.sin(f); 
  } 
 
  /** 
   * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of immediately. 
   * 
   * @param x the number of pixels to scroll by on the X axis 
   * @param y the number of pixels to scroll by on the Y axis 
   */ 
  void smoothScrollTo(int x, int y) { 
    smoothScrollTo(x, y, 0); 
  } 
 
  /** 
   * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of immediately. 
   * 
   * @param x the number of pixels to scroll by on the X axis 
   * @param y the number of pixels to scroll by on the Y axis 
   * @param velocity the velocity associated with a fling, if applicable. (0 otherwise) 
   */ 
  void smoothScrollTo(int x, int y, int velocity) { 
    if (getChildCount() == 0) { 
      // Nothing to do. 
      setScrollingCacheEnabled(false); 
      return; 
    } 
    int sx = getScrollX(); 
    int sy = getScrollY(); 
    int dx = x - sx; 
    int dy = y - sy; 
    if (dx == 0 && dy == 0) { 
      completeScroll(); 
      setScrollState(SCROLL_STATE_IDLE); 
      return; 
    } 
 
    setScrollingCacheEnabled(true); 
    mScrolling = true; 
    setScrollState(SCROLL_STATE_SETTLING); 
 
    final float pageDelta = (float) Math.abs(dx) / (getWidth() + mPageMargin); 
    int duration = (int) (pageDelta * 100); 
 
    velocity = Math.abs(velocity); 
    if (velocity > 0) { 
      duration += (duration / (velocity / mBaseLineFlingVelocity)) * mFlingVelocityInfluence; 
    } else { 
      duration += 100; 
    } 
    duration = Math.min(duration, MAX_SETTLE_DURATION); 
 
    mScroller.startScroll(sx, sy, dx, dy, duration); 
    invalidate(); 
  } 
 
  void addNewItem(int position, int index) { 
    ItemInfo ii = new ItemInfo(); 
    ii.position = position; 
    ii.object = mAdapter.instantiateItem(this, position); 
    if (index < 0) { 
      mItems.add(ii); 
    } else { 
      mItems.add(index, ii); 
    } 
  } 
 
  void dataSetChanged() { 
    // This method only gets called if our observer is attached, so mAdapter is non-null. 
 
    boolean needPopulate = mItems.size() < 3 && mItems.size() < mAdapter.getCount(); 
    int newCurrItem = -1; 
 
    for (int i = 0; i < mItems.size(); i++) { 
      final ItemInfo ii = mItems.get(i); 
      final int newPos = mAdapter.getItemPosition(ii.object); 
 
      if (newPos == PagerAdapter.POSITION_UNCHANGED) { 
        continue; 
      } 
 
      if (newPos == PagerAdapter.POSITION_NONE) { 
        mItems.remove(i); 
        i--; 
        mAdapter.destroyItem(this, ii.position, ii.object); 
        needPopulate = true; 
 
        if (mCurItem == ii.position) { 
          // Keep the current item in the valid range 
          newCurrItem = Math.max(0, Math.min(mCurItem, mAdapter.getCount() - 1)); 
        } 
        continue; 
      } 
 
      if (ii.position != newPos) { 
        if (ii.position == mCurItem) { 
          // Our current item changed position. Follow it. 
          newCurrItem = newPos; 
        } 
 
        ii.position = newPos; 
        needPopulate = true; 
      } 
    } 
 
    Collections.sort(mItems, COMPARATOR); 
 
    if (newCurrItem >= 0) { 
      // TODO This currently causes a jump. 
      setCurrentItemInternal(newCurrItem, false, true); 
      needPopulate = true; 
    } 
    if (needPopulate) { 
      populate(); 
      requestLayout(); 
    } 
  } 
 
  void populate() { 
    if (mAdapter == null) { 
      return; 
    } 
 
    // Bail now if we are waiting to populate. This is to hold off 
    // on creating views from the time the user releases their finger to 
    // fling to a new position until we have finished the scroll to 
    // that position, avoiding glitches from happening at that point. 
    if (mPopulatePending) { 
      if (DEBUG) Log.i(TAG, "populate is pending, skipping for now..."); 
      return; 
    } 
 
    // Also, don't populate until we are attached to a window. This is to 
    // avoid trying to populate before we have restored our view hierarchy 
    // state and conflicting with what is restored. 
    if (getWindowToken() == null) { 
      return; 
    } 
 
    mAdapter.startUpdate(this); 
 
    final int pageLimit = mOffscreenPageLimit; 
    final int startPos = Math.max(0, mCurItem - pageLimit); 
    final int N = mAdapter.getCount(); 
    final int endPos = Math.min(N-1, mCurItem + pageLimit); 
 
    if (DEBUG) Log.v(TAG, "populating: startPos=" + startPos + " endPos=" + endPos); 
 
    // Add and remove pages in the existing list. 
    int lastPos = -1; 
    for (int i=0; i<mItems.size(); i++) { 
      ItemInfo ii = mItems.get(i); 
      if ((ii.position < startPos || ii.position > endPos) && !ii.scrolling) { 
        if (DEBUG) Log.i(TAG, "removing: " + ii.position + " @ " + i); 
        mItems.remove(i); 
        i--; 
        mAdapter.destroyItem(this, ii.position, ii.object); 
      } else if (lastPos < endPos && ii.position > startPos) { 
        // The next item is outside of our range, but we have a gap 
        // between it and the last item where we want to have a page 
        // shown. Fill in the gap. 
        lastPos++; 
        if (lastPos < startPos) { 
          lastPos = startPos; 
        } 
        while (lastPos <= endPos && lastPos < ii.position) { 
          if (DEBUG) Log.i(TAG, "inserting: " + lastPos + " @ " + i); 
          addNewItem(lastPos, i); 
          lastPos++; 
          i++; 
        } 
      } 
      lastPos = ii.position; 
    } 
 
    // Add any new pages we need at the end. 
    lastPos = mItems.size() > 0 ? mItems.get(mItems.size()-1).position : -1; 
    if (lastPos < endPos) { 
      lastPos++; 
      lastPos = lastPos > startPos ? lastPos : startPos; 
      while (lastPos <= endPos) { 
        if (DEBUG) Log.i(TAG, "appending: " + lastPos); 
        addNewItem(lastPos, -1); 
        lastPos++; 
      } 
    } 
 
    if (DEBUG) { 
      Log.i(TAG, "Current page list:"); 
      for (int i=0; i<mItems.size(); i++) { 
        Log.i(TAG, "#" + i + ": page " + mItems.get(i).position); 
      } 
    } 
 
    ItemInfo curItem = null; 
    for (int i=0; i<mItems.size(); i++) { 
      if (mItems.get(i).position == mCurItem) { 
        curItem = mItems.get(i); 
        break; 
      } 
    } 
    mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null); 
 
    mAdapter.finishUpdate(this); 
 
    if (hasFocus()) { 
      View currentFocused = findFocus(); 
      ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null; 
      if (ii == null || ii.position != mCurItem) { 
        for (int i=0; i<getChildCount(); i++) { 
          View child = getChildAt(i); 
          ii = infoForChild(child); 
          if (ii != null && ii.position == mCurItem) { 
            if (child.requestFocus(FOCUS_FORWARD)) { 
              break; 
            } 
          } 
        } 
      } 
    } 
  } 
 
  public static class SavedState extends BaseSavedState { 
    int position; 
    Parcelable adapterState; 
    ClassLoader loader; 
 
    public SavedState(Parcelable superState) { 
      super(superState); 
    } 
 
    @Override 
    public void writeToParcel(Parcel out, int flags) { 
      super.writeToParcel(out, flags); 
      out.writeInt(position); 
      out.writeParcelable(adapterState, flags); 
    } 
 
    @Override 
    public String toString() { 
      return "FragmentPager.SavedState{" 
          + Integer.toHexString(System.identityHashCode(this)) 
          + " position=" + position + "}"; 
    } 
 
    public static final Creator<SavedState> CREATOR 
        = ParcelableCompat.newCreator(new ParcelableCompatCreatorCallbacks<SavedState>() { 
          @Override 
          public SavedState createFromParcel(Parcel in, ClassLoader loader) { 
            return new SavedState(in, loader); 
          } 
          @Override 
          public SavedState[] newArray(int size) { 
            return new SavedState[size]; 
          } 
        }); 
 
    SavedState(Parcel in, ClassLoader loader) { 
      super(in); 
      if (loader == null) { 
        loader = getClass().getClassLoader(); 
      } 
      position = in.readInt(); 
      adapterState = in.readParcelable(loader); 
      this.loader = loader; 
    } 
  } 
 
  @Override 
  public Parcelable onSaveInstanceState() { 
    Parcelable superState = super.onSaveInstanceState(); 
    SavedState ss = new SavedState(superState); 
    ss.position = mCurItem; 
    if (mAdapter != null) { 
      ss.adapterState = mAdapter.saveState(); 
    } 
    return ss; 
  } 
 
  @Override 
  public void onRestoreInstanceState(Parcelable state) { 
    if (!(state instanceof SavedState)) { 
      super.onRestoreInstanceState(state); 
      return; 
    } 
 
    SavedState ss = (SavedState)state; 
    super.onRestoreInstanceState(ss.getSuperState()); 
 
    if (mAdapter != null) { 
      mAdapter.restoreState(ss.adapterState, ss.loader); 
      setCurrentItemInternal(ss.position, false, true); 
    } else { 
      mRestoredCurItem = ss.position; 
      mRestoredAdapterState = ss.adapterState; 
      mRestoredClassLoader = ss.loader; 
    } 
  } 
 
  @Override 
  public void addView(View child, int index, LayoutParams params) { 
    if (mInLayout) { 
      addViewInLayout(child, index, params); 
      child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec); 
    } else { 
      super.addView(child, index, params); 
    } 
 
    if (USE_CACHE) { 
      if (child.getVisibility() != GONE) { 
        child.setDrawingCacheEnabled(mScrollingCacheEnabled); 
      } else { 
        child.setDrawingCacheEnabled(false); 
      } 
    } 
  } 
 
  ItemInfo infoForChild(View child) { 
    for (int i=0; i<mItems.size(); i++) { 
      ItemInfo ii = mItems.get(i); 
      if (mAdapter.isViewFromObject(child, ii.object)) { 
        return ii; 
      } 
    } 
    return null; 
  } 
 
  ItemInfo infoForAnyChild(View child) { 
    ViewParent parent; 
    while ((parent=child.getParent()) != this) { 
      if (parent == null || !(parent instanceof View)) { 
        return null; 
      } 
      child = (View)parent; 
    } 
    return infoForChild(child); 
  } 
 
  @Override 
  protected void onAttachedToWindow() { 
    super.onAttachedToWindow(); 
    mFirstLayout = true; 
  } 
 
  @Override 
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
    // For simple implementation, or internal size is always 0. 
    // We depend on the container to specify the layout size of 
    // our view. We can't really know what it is since we will be 
    // adding and removing different arbitrary views and do not 
    // want the layout to change as this happens. 
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec), 
        getDefaultSize(0, heightMeasureSpec)); 
 
    // Children are just made to fill our space. 
    mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() - 
        getPaddingLeft() - getPaddingRight(), MeasureSpec.EXACTLY); 
    mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() - 
        getPaddingTop() - getPaddingBottom(), MeasureSpec.EXACTLY); 
 
    // Make sure we have created all fragments that we need to have shown. 
    mInLayout = true; 
    populate(); 
    mInLayout = false; 
 
    // Make sure all children have been properly measured. 
    final int size = getChildCount(); 
    for (int i = 0; i < size; ++i) { 
      final View child = getChildAt(i); 
      if (child.getVisibility() != GONE) { 
        if (DEBUG) Log.v(TAG, "Measuring #" + i + " " + child 
        + ": " + mChildWidthMeasureSpec); 
        child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec); 
      } 
    } 
  } 
 
  @Override 
  protected void onSizeChanged(int w, int h, int oldw, int oldh) { 
    super.onSizeChanged(w, h, oldw, oldh); 
 
    // Make sure scroll position is set correctly. 
    if (w != oldw) { 
      recomputeScrollPosition(w, oldw, mPageMargin, mPageMargin); 
    } 
  } 
 
  private void recomputeScrollPosition(int width, int oldWidth, int margin, int oldMargin) { 
    final int widthWithMargin = width + margin; 
    if (oldWidth > 0) { 
      final int oldScrollPos = getScrollX(); 
      final int oldwwm = oldWidth + oldMargin; 
      final int oldScrollItem = oldScrollPos / oldwwm; 
      final float scrollOffset = (float) (oldScrollPos % oldwwm) / oldwwm; 
      final int scrollPos = (int) ((oldScrollItem + scrollOffset) * widthWithMargin); 
      scrollTo(scrollPos, getScrollY()); 
      if (!mScroller.isFinished()) { 
        // We now return to your regularly scheduled scroll, already in progress. 
        final int newDuration = mScroller.getDuration() - mScroller.timePassed(); 
        mScroller.startScroll(scrollPos, 0, mCurItem * widthWithMargin, 0, newDuration); 
      } 
    } else { 
      int scrollPos = mCurItem * widthWithMargin; 
      if (scrollPos != getScrollX()) { 
        completeScroll(); 
        scrollTo(scrollPos, getScrollY()); 
      } 
    } 
  } 
 
  @Override 
  protected void onLayout(boolean changed, int l, int t, int r, int b) { 
    mInLayout = true; 
    populate(); 
    mInLayout = false; 
 
    final int count = getChildCount(); 
    final int width = r-l; 
 
    for (int i = 0; i < count; i++) { 
      View child = getChildAt(i); 
      ItemInfo ii; 
      if (child.getVisibility() != GONE && (ii=infoForChild(child)) != null) { 
        int loff = (width + mPageMargin) * ii.position; 
        int childLeft = getPaddingLeft() + loff; 
        int childTop = getPaddingTop(); 
        if (DEBUG) Log.v(TAG, "Positioning #" + i + " " + child + " f=" + ii.object 
        + ":" + childLeft + "," + childTop + " " + child.getMeasuredWidth() 
        + "x" + child.getMeasuredHeight()); 
        child.layout(childLeft, childTop, 
            childLeft + child.getMeasuredWidth(), 
            childTop + child.getMeasuredHeight()); 
      } 
    } 
    mFirstLayout = false; 
  } 
 
  @Override 
  public void computeScroll() { 
    if (DEBUG) Log.i(TAG, "computeScroll: finished=" + mScroller.isFinished()); 
    if (!mScroller.isFinished()) { 
      if (mScroller.computeScrollOffset()) { 
        if (DEBUG) Log.i(TAG, "computeScroll: still scrolling"); 
        int oldX = getScrollX(); 
        int oldY = getScrollY(); 
        int x = mScroller.getCurrX(); 
        int y = mScroller.getCurrY(); 
 
        if (oldX != x || oldY != y) { 
          scrollTo(x, y); 
        } 
 
        if (mOnPageChangeListener != null) { 
          final int widthWithMargin = getWidth() + mPageMargin; 
          final int position = x / widthWithMargin; 
          final int offsetPixels = x % widthWithMargin; 
          final float offset = (float) offsetPixels / widthWithMargin; 
          mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels); 
        } 
 
        // Keep on drawing until the animation has finished. 
        invalidate(); 
        return; 
      } 
    } 
 
    // Done with scroll, clean up state. 
    completeScroll(); 
  } 
 
  private void completeScroll() { 
    boolean needPopulate = mScrolling; 
    if (needPopulate) { 
      // Done with scroll, no longer want to cache view drawing. 
      setScrollingCacheEnabled(false); 
      mScroller.abortAnimation(); 
      int oldX = getScrollX(); 
      int oldY = getScrollY(); 
      int x = mScroller.getCurrX(); 
      int y = mScroller.getCurrY(); 
      if (oldX != x || oldY != y) { 
        scrollTo(x, y); 
      } 
      setScrollState(SCROLL_STATE_IDLE); 
    } 
    mPopulatePending = false; 
    mScrolling = false; 
    for (int i=0; i<mItems.size(); i++) { 
      ItemInfo ii = mItems.get(i); 
      if (ii.scrolling) { 
        needPopulate = true; 
        ii.scrolling = false; 
      } 
    } 
    if (needPopulate) { 
      populate(); 
    } 
  } 
 
  @Override 
  public boolean onInterceptTouchEvent(MotionEvent ev) { 
    /* 
     * This method JUST determines whether we want to intercept the motion. 
     * If we return true, onMotionEvent will be called and we do the actual 
     * scrolling there. 
     */ 
 
    final int action = ev.getAction() & MotionEventCompat.ACTION_MASK; 
 
    // Always take care of the touch gesture being complete. 
    if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) { 
      // Release the drag. 
      if (DEBUG) Log.v(TAG, "Intercept done!"); 
      mIsBeingDragged = false; 
      mIsUnableToDrag = false; 
      mActivePointerId = INVALID_POINTER; 
      return false; 
    } 
 
    // Nothing more to do here if we have decided whether or not we 
    // are dragging. 
    if (action != MotionEvent.ACTION_DOWN) { 
      if (mIsBeingDragged) { 
        if (DEBUG) Log.v(TAG, "Intercept returning true!"); 
        return true; 
      } 
      if (mIsUnableToDrag) { 
        if (DEBUG) Log.v(TAG, "Intercept returning false!"); 
        return false; 
      } 
    } 
 
    switch (action) { 
      case MotionEvent.ACTION_MOVE: { 
        /* 
         * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check 
         * whether the user has moved far enough from his original down touch. 
         */ 
 
        /* 
        * Locally do absolute value. mLastMotionY is set to the y value 
        * of the down event. 
        */ 
        final int activePointerId = mActivePointerId; 
        if (activePointerId == INVALID_POINTER) { 
          // If we don't have a valid id, the touch down wasn't on content. 
          break; 
        } 
 
        final int pointerIndex = MotionEventCompat.findPointerIndex(ev, activePointerId); 
        final float x = MotionEventCompat.getX(ev, pointerIndex); 
        final float dx = x - mLastMotionX; 
        final float xDiff = Math.abs(dx); 
        final float y = MotionEventCompat.getY(ev, pointerIndex); 
        final float yDiff = Math.abs(y - mLastMotionY); 
        final int scrollX = getScrollX(); 
        final boolean atEdge = (dx > 0 && scrollX == 0) || (dx < 0 && mAdapter != null && 
            scrollX >= (mAdapter.getCount() - 1) * getWidth() - 1); 
        if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff); 
 
        if (canScroll(this, false, (int) dx, (int) x, (int) y)) { 
          // Nested view has scrollable area under this point. Let it be handled there. 
          mInitialMotionX = mLastMotionX = x; 
          mLastMotionY = y; 
          return false; 
        } 
        if (xDiff > mTouchSlop && xDiff > yDiff) { 
          if (DEBUG) Log.v(TAG, "Starting drag!"); 
          mIsBeingDragged = true; 
          setScrollState(SCROLL_STATE_DRAGGING); 
          mLastMotionX = x; 
          setScrollingCacheEnabled(true); 
        } else { 
          if (yDiff > mTouchSlop) { 
            // The finger has moved enough in the vertical 
            // direction to be counted as a drag... abort 
            // any attempt to drag horizontally, to work correctly 
            // with children that have scrolling containers. 
            if (DEBUG) Log.v(TAG, "Starting unable to drag!"); 
            mIsUnableToDrag = true; 
          } 
        } 
        break; 
      } 
 
      case MotionEvent.ACTION_DOWN: { 
        /* 
         * Remember location of down touch. 
         * ACTION_DOWN always refers to pointer index 0. 
         */ 
        mLastMotionX = mInitialMotionX = ev.getX(); 
        mLastMotionY = ev.getY(); 
        mActivePointerId = MotionEventCompat.getPointerId(ev, 0); 
 
        if (mScrollState == SCROLL_STATE_SETTLING) { 
          // Let the user 'catch' the pager as it animates. 
          mIsBeingDragged = true; 
          mIsUnableToDrag = false; 
          setScrollState(SCROLL_STATE_DRAGGING); 
        } else { 
          completeScroll(); 
          mIsBeingDragged = false; 
          mIsUnableToDrag = false; 
        } 
 
        if (DEBUG) Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY 
            + " mIsBeingDragged=" + mIsBeingDragged 
            + "mIsUnableToDrag=" + mIsUnableToDrag); 
        break; 
      } 
 
      case MotionEventCompat.ACTION_POINTER_UP: 
        onSecondaryPointerUp(ev); 
        break; 
    } 
 
    /* 
    * The only time we want to intercept motion events is if we are in the 
    * drag mode. 
    */ 
    return mIsBeingDragged; 
  } 
 
  @Override 
  public boolean onTouchEvent(MotionEvent ev) { 
    if (mFakeDragging) { 
      // A fake drag is in progress already, ignore this real one 
      // but still eat the touch events. 
      // (It is likely that the user is multi-touching the screen.) 
      return true; 
    } 
 
    if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) { 
      // Don't handle edge touches immediately -- they may actually belong to one of our 
      // descendants. 
      return false; 
    } 
 
    if (mAdapter == null || mAdapter.getCount() == 0) { 
      // Nothing to present or scroll; nothing to touch. 
      return false; 
    } 
 
    if (mVelocityTracker == null) { 
      mVelocityTracker = VelocityTracker.obtain(); 
    } 
    mVelocityTracker.addMovement(ev); 
 
    final int action = ev.getAction(); 
    boolean needsInvalidate = false; 
 
    switch (action & MotionEventCompat.ACTION_MASK) { 
      case MotionEvent.ACTION_DOWN: { 
        /* 
         * If being flinged and user touches, stop the fling. isFinished 
         * will be false if being flinged. 
         */ 
        completeScroll(); 
 
        // Remember where the motion event started 
        mLastMotionX = mInitialMotionX = ev.getX(); 
        mActivePointerId = MotionEventCompat.getPointerId(ev, 0); 
        break; 
      } 
      case MotionEvent.ACTION_MOVE: 
        if (!mIsBeingDragged) { 
          final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId); 
          final float x = MotionEventCompat.getX(ev, pointerIndex); 
          final float xDiff = Math.abs(x - mLastMotionX); 
          final float y = MotionEventCompat.getY(ev, pointerIndex); 
          final float yDiff = Math.abs(y - mLastMotionY); 
          if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff); 
          if (xDiff > mTouchSlop && xDiff > yDiff) { 
            if (DEBUG) Log.v(TAG, "Starting drag!"); 
            mIsBeingDragged = true; 
            mLastMotionX = x; 
            setScrollState(SCROLL_STATE_DRAGGING); 
            setScrollingCacheEnabled(true); 
          } 
        } 
        if (mIsBeingDragged) { 
          // Scroll to follow the motion event 
          final int activePointerIndex = MotionEventCompat.findPointerIndex( 
              ev, mActivePointerId); 
          final float x = MotionEventCompat.getX(ev, activePointerIndex); 
          final float deltaX = mLastMotionX - x; 
          mLastMotionX = x; 
          float oldScrollX = getScrollX(); 
          float scrollX = oldScrollX + deltaX; 
          final int width = getWidth(); 
          final int widthWithMargin = width + mPageMargin; 
 
          final int lastItemIndex = mAdapter.getCount() - 1; 
          final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin); 
          final float rightBound = 
              Math.min(mCurItem + 1, lastItemIndex) * widthWithMargin; 
          if (scrollX < leftBound) { 
            if (leftBound == 0) { 
              float over = -scrollX; 
              needsInvalidate = mLeftEdge.onPull(over / width); 
            } 
            scrollX = leftBound; 
          } else if (scrollX > rightBound) { 
            if (rightBound == lastItemIndex * widthWithMargin) { 
              float over = scrollX - rightBound; 
              needsInvalidate = mRightEdge.onPull(over / width); 
            } 
            scrollX = rightBound; 
          } 
          // Don't lose the rounded component 
          mLastMotionX += scrollX - (int) scrollX; 
          scrollTo((int) scrollX, getScrollY()); 
          if (mOnPageChangeListener != null) { 
            final int position = (int) scrollX / widthWithMargin; 
            final int positionOffsetPixels = (int) scrollX % widthWithMargin; 
            final float positionOffset = (float) positionOffsetPixels / widthWithMargin; 
            mOnPageChangeListener.onPageScrolled(position, positionOffset, 
                positionOffsetPixels); 
          } 
        } 
        break; 
      case MotionEvent.ACTION_UP: 
        if (mIsBeingDragged) { 
          final VelocityTracker velocityTracker = mVelocityTracker; 
          velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity); 
          int initialVelocity = (int) VelocityTrackerCompat.getXVelocity( 
              velocityTracker, mActivePointerId); 
          mPopulatePending = true; 
          final int widthWithMargin = getWidth() + mPageMargin; 
          final int scrollX = getScrollX(); 
          final int currentPage = scrollX / widthWithMargin; 
          int nextPage = initialVelocity > 0 ? currentPage : currentPage + 1; 
          setCurrentItemInternal(nextPage, true, true, initialVelocity); 
 
          mActivePointerId = INVALID_POINTER; 
          endDrag(); 
          needsInvalidate = mLeftEdge.onRelease() | mRightEdge.onRelease(); 
        } 
        break; 
      case MotionEvent.ACTION_CANCEL: 
        if (mIsBeingDragged) { 
          setCurrentItemInternal(mCurItem, true, true); 
          mActivePointerId = INVALID_POINTER; 
          endDrag(); 
          needsInvalidate = mLeftEdge.onRelease() | mRightEdge.onRelease(); 
        } 
        break; 
      case MotionEventCompat.ACTION_POINTER_DOWN: { 
        final int index = MotionEventCompat.getActionIndex(ev); 
        final float x = MotionEventCompat.getX(ev, index); 
        mLastMotionX = x; 
        mActivePointerId = MotionEventCompat.getPointerId(ev, index); 
        break; 
      } 
      case MotionEventCompat.ACTION_POINTER_UP: 
        onSecondaryPointerUp(ev); 
        mLastMotionX = MotionEventCompat.getX(ev, 
            MotionEventCompat.findPointerIndex(ev, mActivePointerId)); 
        break; 
    } 
    if (needsInvalidate) { 
      invalidate(); 
    } 
    return true; 
  } 
 
  @Override 
  public void draw(Canvas canvas) { 
    super.draw(canvas); 
    boolean needsInvalidate = false; 
 
    final int overScrollMode = ViewCompat.getOverScrollMode(this); 
    if (overScrollMode == ViewCompat.OVER_SCROLL_ALWAYS || 
        (overScrollMode == ViewCompat.OVER_SCROLL_IF_CONTENT_SCROLLS && 
            mAdapter != null && mAdapter.getCount() > 1)) { 
      if (!mLeftEdge.isFinished()) { 
        final int restoreCount = canvas.save(); 
        final int height = getHeight() - getPaddingTop() - getPaddingBottom(); 
 
        canvas.rotate(270); 
        canvas.translate(-height + getPaddingTop(), 0); 
        mLeftEdge.setSize(height, getWidth()); 
        needsInvalidate |= mLeftEdge.draw(canvas); 
        canvas.restoreToCount(restoreCount); 
      } 
      if (!mRightEdge.isFinished()) { 
        final int restoreCount = canvas.save(); 
        final int width = getWidth(); 
        final int height = getHeight() - getPaddingTop() - getPaddingBottom(); 
        final int itemCount = mAdapter != null ? mAdapter.getCount() : 1; 
 
        canvas.rotate(90); 
        canvas.translate(-getPaddingTop(), 
            -itemCount * (width + mPageMargin) + mPageMargin); 
        mRightEdge.setSize(height, width); 
        needsInvalidate |= mRightEdge.draw(canvas); 
        canvas.restoreToCount(restoreCount); 
      } 
    } else { 
      mLeftEdge.finish(); 
      mRightEdge.finish(); 
    } 
 
    if (needsInvalidate) { 
      // Keep animating 
      invalidate(); 
    } 
  } 
 
  @Override 
  protected void onDraw(Canvas canvas) { 
    super.onDraw(canvas); 
 
    // Draw the margin drawable if needed. 
    if (mPageMargin > 0 && mMarginDrawable != null) { 
      final int scrollX = getScrollX(); 
      final int width = getWidth(); 
      final int offset = scrollX % (width + mPageMargin); 
      if (offset != 0) { 
        // Pages fit completely when settled; we only need to draw when in between 
        final int left = scrollX - offset + width; 
        mMarginDrawable.setBounds(left, 0, left + mPageMargin, getHeight()); 
        mMarginDrawable.draw(canvas); 
      } 
    } 
  } 
 
  /** 
   * Start a fake drag of the pager. 
   * 
   * <p>A fake drag can be useful if you want to synchronize the motion of the ViewPager 
   * with the touch scrolling of another view, while still letting the ViewPager 
   * control the snapping motion and fling behavior. (e.g. parallax-scrolling tabs.) 
   * Call {@link #fakeDragBy(float)} to simulate the actual drag motion. Call 
   * {@link #endFakeDrag()} to complete the fake drag and fling as necessary. 
   * 
   * <p>During a fake drag the ViewPager will ignore all touch events. If a real drag 
   * is already in progress, this method will return false. 
   * 
   * @return true if the fake drag began successfully, false if it could not be started. 
   * 
   * @see #fakeDragBy(float) 
   * @see #endFakeDrag() 
   */ 
  public boolean beginFakeDrag() { 
    if (mIsBeingDragged) { 
      return false; 
    } 
    mFakeDragging = true; 
    setScrollState(SCROLL_STATE_DRAGGING); 
    mInitialMotionX = mLastMotionX = 0; 
    if (mVelocityTracker == null) { 
      mVelocityTracker = VelocityTracker.obtain(); 
    } else { 
      mVelocityTracker.clear(); 
    } 
    final long time = SystemClock.uptimeMillis(); 
    final MotionEvent ev = MotionEvent.obtain(time, time, MotionEvent.ACTION_DOWN, 0, 0, 0); 
    mVelocityTracker.addMovement(ev); 
    ev.recycle(); 
    mFakeDragBeginTime = time; 
    return true; 
  } 
 
  /** 
   * End a fake drag of the pager. 
   * 
   * @see #beginFakeDrag() 
   * @see #fakeDragBy(float) 
   */ 
  public void endFakeDrag() { 
    if (!mFakeDragging) { 
      throw new IllegalStateException("No fake drag in progress. Call beginFakeDrag first."); 
    } 
 
    final VelocityTracker velocityTracker = mVelocityTracker; 
    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity); 
    int initialVelocity = (int)VelocityTrackerCompat.getYVelocity( 
        velocityTracker, mActivePointerId); 
    mPopulatePending = true; 
    if ((Math.abs(initialVelocity) > mMinimumVelocity) 
        || Math.abs(mInitialMotionX-mLastMotionX) >= (getWidth()/3)) { 
      if (mLastMotionX > mInitialMotionX) { 
        setCurrentItemInternal(mCurItem-1, true, true); 
      } else { 
        setCurrentItemInternal(mCurItem+1, true, true); 
      } 
    } else { 
      setCurrentItemInternal(mCurItem, true, true); 
    } 
    endDrag(); 
 
    mFakeDragging = false; 
  } 
 
  /** 
   * Fake drag by an offset in pixels. You must have called {@link #beginFakeDrag()} first. 
   * 
   * @param xOffset Offset in pixels to drag by. 
   * @see #beginFakeDrag() 
   * @see #endFakeDrag() 
   */ 
  public void fakeDragBy(float xOffset) { 
    if (!mFakeDragging) { 
      throw new IllegalStateException("No fake drag in progress. Call beginFakeDrag first."); 
    } 
 
    mLastMotionX += xOffset; 
    float scrollX = getScrollX() - xOffset; 
    final int width = getWidth(); 
    final int widthWithMargin = width + mPageMargin; 
 
    final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin); 
    final float rightBound = 
        Math.min(mCurItem + 1, mAdapter.getCount() - 1) * widthWithMargin; 
    if (scrollX < leftBound) { 
      scrollX = leftBound; 
    } else if (scrollX > rightBound) { 
      scrollX = rightBound; 
    } 
    // Don't lose the rounded component 
    mLastMotionX += scrollX - (int) scrollX; 
    scrollTo((int) scrollX, getScrollY()); 
    if (mOnPageChangeListener != null) { 
      final int position = (int) scrollX / widthWithMargin; 
      final int positionOffsetPixels = (int) scrollX % widthWithMargin; 
      final float positionOffset = (float) positionOffsetPixels / widthWithMargin; 
      mOnPageChangeListener.onPageScrolled(position, positionOffset, 
          positionOffsetPixels); 
    } 
 
    // Synthesize an event for the VelocityTracker. 
    final long time = SystemClock.uptimeMillis(); 
    final MotionEvent ev = MotionEvent.obtain(mFakeDragBeginTime, time, MotionEvent.ACTION_MOVE, 
        mLastMotionX, 0, 0); 
    mVelocityTracker.addMovement(ev); 
    ev.recycle(); 
  } 
 
  /** 
   * Returns true if a fake drag is in progress. 
   * 
   * @return true if currently in a fake drag, false otherwise. 
   * 
   * @see #beginFakeDrag() 
   * @see #fakeDragBy(float) 
   * @see #endFakeDrag() 
   */ 
  public boolean isFakeDragging() { 
    return mFakeDragging; 
  } 
 
  private void onSecondaryPointerUp(MotionEvent ev) { 
    final int pointerIndex = MotionEventCompat.getActionIndex(ev); 
    final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex); 
    if (pointerId == mActivePointerId) { 
      // This was our active pointer going up. Choose a new 
      // active pointer and adjust accordingly. 
      final int newPointerIndex = pointerIndex == 0 ? 1 : 0; 
      mLastMotionX = MotionEventCompat.getX(ev, newPointerIndex); 
      mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex); 
      if (mVelocityTracker != null) { 
        mVelocityTracker.clear(); 
      } 
    } 
  } 
 
  private void endDrag() { 
    mIsBeingDragged = false; 
    mIsUnableToDrag = false; 
 
    if (mVelocityTracker != null) { 
      mVelocityTracker.recycle(); 
      mVelocityTracker = null; 
    } 
  } 
 
  private void setScrollingCacheEnabled(boolean enabled) { 
    if (mScrollingCacheEnabled != enabled) { 
      mScrollingCacheEnabled = enabled; 
      if (USE_CACHE) { 
        final int size = getChildCount(); 
        for (int i = 0; i < size; ++i) { 
          final View child = getChildAt(i); 
          if (child.getVisibility() != GONE) { 
            child.setDrawingCacheEnabled(enabled); 
          } 
        } 
      } 
    } 
  } 
 
  /** 
   * Tests scrollability within child views of v given a delta of dx. 
   * 
   * @param v View to test for horizontal scrollability 
   * @param checkV Whether the view v passed should itself be checked for scrollability (true), 
   *        or just its children (false). 
   * @param dx Delta scrolled in pixels 
   * @param x X coordinate of the active touch point 
   * @param y Y coordinate of the active touch point 
   * @return true if child views of v can be scrolled by delta of dx. 
   */ 
  protected boolean canScroll(View v, boolean checkV, int dx, int x, int y) { 
    if (v instanceof ViewGroup) { 
      final ViewGroup group = (ViewGroup) v; 
      final int scrollX = v.getScrollX(); 
      final int scrollY = v.getScrollY(); 
      final int count = group.getChildCount(); 
      // Count backwards - let topmost views consume scroll distance first. 
      for (int i = count - 1; i >= 0; i--) { 
        // TODO: Add versioned support here for transformed views. 
        // This will not work for transformed views in Honeycomb+ 
        final View child = group.getChildAt(i); 
        if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight() && 
            y + scrollY >= child.getTop() && y + scrollY < child.getBottom() && 
            canScroll(child, true, dx, x + scrollX - child.getLeft(), 
                y + scrollY - child.getTop())) { 
          return true; 
        } 
      } 
    } 
 
    return checkV && ViewCompat.canScrollHorizontally(v, -dx); 
  } 
 
  @Override 
  public boolean dispatchKeyEvent(KeyEvent event) { 
    // Let the focused view and/or our descendants get the key first 
    return super.dispatchKeyEvent(event) || executeKeyEvent(event); 
  } 
 
  /** 
   * You can call this function yourself to have the scroll view perform 
   * scrolling from a key event, just as if the event had been dispatched to 
   * it by the view hierarchy. 
   * 
   * @param event The key event to execute. 
   * @return Return true if the event was handled, else false. 
   */ 
  public boolean executeKeyEvent(KeyEvent event) { 
    boolean handled = false; 
    if (event.getAction() == KeyEvent.ACTION_DOWN) { 
      switch (event.getKeyCode()) { 
        case KeyEvent.KEYCODE_DPAD_LEFT: 
          handled = arrowScroll(FOCUS_LEFT); 
          break; 
        case KeyEvent.KEYCODE_DPAD_RIGHT: 
          handled = arrowScroll(FOCUS_RIGHT); 
          break; 
        case KeyEvent.KEYCODE_TAB: 
          if (KeyEventCompat.hasNoModifiers(event)) { 
            handled = arrowScroll(FOCUS_FORWARD); 
          } else if (KeyEventCompat.hasModifiers(event, KeyEvent.META_SHIFT_ON)) { 
            handled = arrowScroll(FOCUS_BACKWARD); 
          } 
          break; 
      } 
    } 
    return handled; 
  } 
 
  public boolean arrowScroll(int direction) { 
    View currentFocused = findFocus(); 
    if (currentFocused == this) currentFocused = null; 
 
    boolean handled = false; 
 
    View nextFocused = FocusFinder.getInstance().findNextFocus(this, currentFocused, 
        direction); 
    if (nextFocused != null && nextFocused != currentFocused) { 
      if (direction == View.FOCUS_LEFT) { 
        // If there is nothing to the left, or this is causing us to 
        // jump to the right, then what we really want to do is page left. 
        if (currentFocused != null && nextFocused.getLeft() >= currentFocused.getLeft()) { 
          handled = pageLeft(); 
        } else { 
          handled = nextFocused.requestFocus(); 
        } 
      } else if (direction == View.FOCUS_RIGHT) { 
        // If there is nothing to the right, or this is causing us to 
        // jump to the left, then what we really want to do is page right. 
        if (currentFocused != null && nextFocused.getLeft() <= currentFocused.getLeft()) { 
          handled = pageRight(); 
        } else { 
          handled = nextFocused.requestFocus(); 
        } 
      } 
    } else if (direction == FOCUS_LEFT || direction == FOCUS_BACKWARD) { 
      // Trying to move left and nothing there; try to page. 
      handled = pageLeft(); 
    } else if (direction == FOCUS_RIGHT || direction == FOCUS_FORWARD) { 
      // Trying to move right and nothing there; try to page. 
      handled = pageRight(); 
    } 
    if (handled) { 
      playSoundEffect(SoundEffectConstants.getContantForFocusDirection(direction)); 
    } 
    return handled; 
  } 
 
  boolean pageLeft() { 
    if (mCurItem > 0) { 
      setCurrentItem(mCurItem-1, true); 
      return true; 
    } 
    return false; 
  } 
 
  boolean pageRight() { 
    if (mAdapter != null && mCurItem < (mAdapter.getCount()-1)) { 
      setCurrentItem(mCurItem+1, true); 
      return true; 
    } 
    return false; 
  } 
 
  /** 
   * We only want the current page that is being shown to be focusable. 
   */ 
  @Override 
  public void addFocusables(ArrayList<View> views, int direction, int focusableMode) { 
    final int focusableCount = views.size(); 
 
    final int descendantFocusability = getDescendantFocusability(); 
 
    if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) { 
      for (int i = 0; i < getChildCount(); i++) { 
        final View child = getChildAt(i); 
        if (child.getVisibility() == VISIBLE) { 
          ItemInfo ii = infoForChild(child); 
          if (ii != null && ii.position == mCurItem) { 
            child.addFocusables(views, direction, focusableMode); 
          } 
        } 
      } 
    } 
 
    // we add ourselves (if focusable) in all cases except for when we are 
    // FOCUS_AFTER_DESCENDANTS and there are some descendants focusable. this is 
    // to avoid the focus search finding layouts when a more precise search 
    // among the focusable children would be more interesting. 
    if ( 
      descendantFocusability != FOCUS_AFTER_DESCENDANTS || 
        // No focusable descendants 
        (focusableCount == views.size())) { 
      // Note that we can't call the superclass here, because it will 
      // add all views in. So we need to do the same thing View does. 
      if (!isFocusable()) { 
        return; 
      } 
      if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE && 
          isInTouchMode() && !isFocusableInTouchMode()) { 
        return; 
      } 
      if (views != null) { 
        views.add(this); 
      } 
    } 
  } 
 
  /** 
   * We only want the current page that is being shown to be touchable. 
   */ 
  @Override 
  public void addTouchables(ArrayList<View> views) { 
    // Note that we don't call super.addTouchables(), which means that 
    // we don't call View.addTouchables(). This is okay because a ViewPager 
    // is itself not touchable. 
    for (int i = 0; i < getChildCount(); i++) { 
      final View child = getChildAt(i); 
      if (child.getVisibility() == VISIBLE) { 
        ItemInfo ii = infoForChild(child); 
        if (ii != null && ii.position == mCurItem) { 
          child.addTouchables(views); 
        } 
      } 
    } 
  } 
 
  /** 
   * We only want the current page that is being shown to be focusable. 
   */ 
  @Override 
  protected boolean onRequestFocusInDescendants(int direction, 
      Rect previouslyFocusedRect) { 
    int index; 
    int increment; 
    int end; 
    int count = getChildCount(); 
    if ((direction & FOCUS_FORWARD) != 0) { 
      index = 0; 
      increment = 1; 
      end = count; 
    } else { 
      index = count - 1; 
      increment = -1; 
      end = -1; 
    } 
    for (int i = index; i != end; i += increment) { 
      View child = getChildAt(i); 
      if (child.getVisibility() == VISIBLE) { 
        ItemInfo ii = infoForChild(child); 
        if (ii != null && ii.position == mCurItem) { 
          if (child.requestFocus(direction, previouslyFocusedRect)) { 
            return true; 
          } 
        } 
      } 
    } 
    return false; 
  } 
 
  @Override 
  public boolean dispatchPopulateAccessibilityEvent(AccessibilityEvent event) { 
    // ViewPagers should only report accessibility info for the current page, 
    // otherwise things get very confusing. 
 
    // TODO: Should this note something about the paging container? 
 
    final int childCount = getChildCount(); 
    for (int i = 0; i < childCount; i++) { 
      final View child = getChildAt(i); 
      if (child.getVisibility() == VISIBLE) { 
        final ItemInfo ii = infoForChild(child); 
        if (ii != null && ii.position == mCurItem && 
            child.dispatchPopulateAccessibilityEvent(event)) { 
          return true; 
        } 
      } 
    } 
 
    return false; 
  } 
 
  private class PagerObserver extends DataSetObserver { 
 
    @Override 
    public void onChanged() { 
      dataSetChanged(); 
    } 
 
    @Override 
    public void onInvalidated() { 
      dataSetChanged(); 
    } 
  } 
} 

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持脚本之家。

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