尊重他人劳动成果，转载请说明出处：http://blog.csdn.net/bingospunky/article/details/44278533
在该系列文章第四篇，我准备介绍一下viewpager的touch事件处理。
如果想了解touch和click的那些事，请浏览touch事件传递系列的第一篇http://blog.csdn.net/bingospunky/article/details/43603397
如果想了解touch事件一步一步传递的路线，请浏览touch事件传递系列的第二篇http://blog.csdn.net/bingospunky/article/details/43735497
如果想从源码角度什么理解viewgroup的dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent如何实现，请浏览touch事件传递系列的第二篇http://blog.csdn.net/bingospunky/article/details/44156771
源码
代码A：boolean android.support.v4.view.ViewPager.onInterceptTouchEvent(MotionEvent ev)
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; if (mVelocityTracker != null) { mVelocityTracker.recycle(); mVelocityTracker = null; } 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 - mInitialMotionY); if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff); if (dx != 0 && !isGutterDrag(mLastMotionX, dx) && canScroll(this, false, (int) dx, (int) x, (int) y)) { // Nested view has scrollable area under this point. Let it be handled there. mLastMotionX = x; mLastMotionY = y; mIsUnableToDrag = true; return false; } if (xDiff > mTouchSlop && xDiff * 0.5f > yDiff) { if (DEBUG) Log.v(TAG, "Starting drag!"); mIsBeingDragged = true; requestParentDisallowInterceptTouchEvent(true); setScrollState(SCROLL_STATE_DRAGGING); mLastMotionX = dx > 0 ? mInitialMotionX + mTouchSlop : mInitialMotionX - mTouchSlop; mLastMotionY = y; 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; } if (mIsBeingDragged) { // Scroll to follow the motion event if (performDrag(x)) { ViewCompat.postInvalidateOnAnimation(this); } } break; } case MotionEvent.ACTION_DOWN: { /* * Remember location of down touch. * ACTION_DOWN always refers to pointer index 0. */ mLastMotionX = mInitialMotionX = ev.getX(); mLastMotionY = mInitialMotionY = ev.getY(); mActivePointerId = MotionEventCompat.getPointerId(ev, 0); mIsUnableToDrag = false; mScroller.computeScrollOffset(); if (mScrollState == SCROLL_STATE_SETTLING && Math.abs(mScroller.getFinalX() - mScroller.getCurrX()) > mCloseEnough) { // Let the user 'catch' the pager as it animates. mScroller.abortAnimation(); mPopulatePending = false; populate(); mIsBeingDragged = true; requestParentDisallowInterceptTouchEvent(true); setScrollState(SCROLL_STATE_DRAGGING); } else { completeScroll(false); mIsBeingDragged = false; } if (DEBUG) Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY + " mIsBeingDragged=" + mIsBeingDragged + "mIsUnableToDrag=" + mIsUnableToDrag); break; } case MotionEventCompat.ACTION_POINTER_UP: onSecondaryPointerUp(ev); break; } if (mVelocityTracker == null) { mVelocityTracker = VelocityTracker.obtain(); } mVelocityTracker.addMovement(ev); /* * The only time we want to intercept motion events is if we are in the * drag mode. */ return mIsBeingDragged; }代码B：boolean android.support.v4.view.ViewPager.onTouchEvent(MotionEvent ev)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: { mScroller.abortAnimation(); mPopulatePending = false; populate(); // Remember where the motion event started mLastMotionX = mInitialMotionX = ev.getX(); mLastMotionY = mInitialMotionY = ev.getY(); 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; requestParentDisallowInterceptTouchEvent(true); mLastMotionX = x - mInitialMotionX > 0 ? mInitialMotionX + mTouchSlop : mInitialMotionX - mTouchSlop; mLastMotionY = y; setScrollState(SCROLL_STATE_DRAGGING); setScrollingCacheEnabled(true); // Disallow Parent Intercept, just in case ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } } } // Not else! Note that mIsBeingDragged can be set above. if (mIsBeingDragged) { // Scroll to follow the motion event final int activePointerIndex = MotionEventCompat.findPointerIndex( ev, mActivePointerId); final float x = MotionEventCompat.getX(ev, activePointerIndex); needsInvalidate |= performDrag(x); } 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 width = getClientWidth(); final int scrollX = getScrollX(); final ItemInfo ii = infoForCurrentScrollPosition(); final int currentPage = ii.position; final float pageOffset = (((float) scrollX / width) - ii.offset) / ii.widthFactor; final int activePointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId); final float x = MotionEventCompat.getX(ev, activePointerIndex); final int totalDelta = (int) (x - mInitialMotionX); int nextPage = determineTargetPage(currentPage, pageOffset, initialVelocity, totalDelta); setCurrentItemInternal(nextPage, true, true, initialVelocity); mActivePointerId = INVALID_POINTER; endDrag(); needsInvalidate = mLeftEdge.onRelease() | mRightEdge.onRelease(); } break; case MotionEvent.ACTION_CANCEL: if (mIsBeingDragged) { scrollToItem(mCurItem, true, 0, false); 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) { ViewCompat.postInvalidateOnAnimation(this); } return true; }代码C：void android.support.v4.view.ViewPager.onSecondaryPointerUp(MotionEvent ev)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(); } } }总结
在这里就不想上篇文章那样分各种情况去讨论了，那样有些无聊，这里就重要点的说明一下我的理解，最后附一个我写的demo供大家参考学习。
1、viewpager处理touch的思路：不截断touch，如果touch移动满足了一定的条件，再截断touch由该viewgroup处理。
2、我们可以看到onInterceptTouchEvent和onTouchEvent方法里的方法很相似，对于一个touch事件，这两个方法基本不会被都执行，只有很少的情况下这两个方法都会被执行。
3、viewpager有趣的现象，操作：单点操作viewpager，使viewpager响应事件，再加一个点触碰viewpager，如此不断加点，可以看到viewpager响应后加上去点的事件，这是为什么呢？代码B第108--111行已经告诉我们了，不再解释。
4、viewpager有趣的现象，操作：对于前面不不断加上去的触点，假设现在一共有4个触点，现在响应第4个触点的事件，如果第4个触点抬起，那么viewpager响应哪个触点的事件呢？答案是第一个。规律是什么呢？抬起的触点不是当前响应的，那么没影响；如果是当前响应的：如果抬起的触点的pointerIndex不是0，那么由pointerIndex最小的触点来响应，所以由第一个来响应。代码C已经很明显告诉我们了。如果这里你看不懂，那么你要学习一下android是怎么处理多触点事件的就会明白了。
Demo
下载地址：http://download.csdn.net/detail/u011647962/8507523