上周客户反馈Contacts快速滑动界面切换tab有明显卡顿，让优化。
自己验证又没发现卡顿现象，但总得给客户一个技术性的回复，于是看了一下ViewPager源码中处理滑动切换tab的过程。
ViewPager 源码位置： androidframeworkssupportv4javaandroidsupportv4viewViewPager.java
ViewPager其实就是一个重写的ViewGroup，使用ViewPager可以参考SDK中的demo:sdkextrasandroidsupportsamples
ViewPager.java开头的注释中有推荐一个demo,使用了supportv13
* <p>Here is a more complicated example of ViewPager, using it in conjuction * with {@link android.app.ActionBar} tabs. You can find other examples of using * ViewPager in the API 4+ Support Demos and API 13+ Support Demos sample code. * * {@sample development/samples/Support13Demos/src/com/example/android/supportv13/app/ActionBarTabsPager.java * complete}
ViewPager滑动是处理Touch事件，所以有必要了解Touch事件的分发过程。可以参考这篇 http://blog.csdn.net/guolin_blog/article/details/9153747
public class ViewPager extends ViewGroup{ ...... @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; 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); boolean isGutterDrag = isGutterDrag(mLastMotionX, dx); boolean canScroll = canScroll(this, false, (int) dx, (int) x, (int) y); if (dx != 0 && !isGutterDrag && canScroll) { // 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; } @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: { 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; } ...... }
按照ACTION_DOWN, ACTION_MOVE, ACTION_UP顺序分析滑动页面
1，ACTION_DOWN
第一次按下时onInterceptTouchEvent先处理
第一次按下时mIsBeingDragged = false;所以ACTION_DOWN传给ViewPager当前页面子View处理——如:联系人列表，直至ACTION_DOWN处理完
2，ACTION_MOVE
如果处理ACTION_DOWN时没执行requestParentDisallowInterceptTouchEvent(true);则 onInterceptTouchEvent处理ACTION_MOVE
//判断水平移动，一般canScroll都为false,所以不会进入这里。如果进入，则onInterceptTouchEvent返回false,ACTION_MOVE传递给当前页面中的View处理。 if (dx != 0 && !isGutterDrag && canScroll) { //isGutterDrag——是否从屏幕边缘滑动， canScroll——ViewPager当前页面中的子View是否支持水平滑动 //Contacts中canScroll始终未false, 所以不会进入这里。 // Nested view has scrollable area under this point. Let it be handled there. mLastMotionX = x; mLastMotionY = y; mIsUnableToDrag = true; return false; } //判断水平，竖直位移 if (xDiff > mTouchSlop && xDiff * 0.5f > yDiff) { //mTouchSlop在ViewPager初始化时获得为16dp, 也就是（水平位移>16dp && 水平位移/2>竖直位移） if (DEBUG) Log.v(TAG, "Starting drag!"); mIsBeingDragged = true; //mIsBeingDragged是onInterceptTouchEvent返回的返回值，true表示onTouchEvent要处理ACTION_MOVE，不再往下传递 requestParentDisallowInterceptTouchEvent(true);//disallowIntercept设为true，即将发生的ACTION_UP不会进入onInterceptTouchEvent setScrollState(SCROLL_STATE_DRAGGING); mLastMotionX = dx > 0 ? mInitialMotionX + mTouchSlop : mInitialMotionX - mTouchSlop; mLastMotionY = y; setScrollingCacheEnabled(true); } else if (yDiff > mTouchSlop) { //如果不满足（水平位移>16dp && 水平位移/2>竖直位移），但——竖直位移>16dp， onInterceptTouchEvent返回false,ACTION_MOVE传递给当前页面中的View处理 // 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);//页面滑动 } } //满足（水平位移>16dp && 水平位移/2>竖直位移）onTouchEvent处理ACTION_MOVE 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); } ACTION_MOVE事件是滑动页面时执行最多的，
3，ACTION_UP
如果第2步执行requestParentDisallowInterceptTouchEvent(true)并且return rue, 则由onTouchEvent直接处理。
case MotionEvent.ACTION_UP: if (mIsBeingDragged) { ......//获得要切换的页面 int nextPage = determineTargetPage(currentPage, pageOffset, initialVelocity, totalDelta); //这里实现页面切换 setCurrentItemInternal(nextPage, true, true, initialVelocity); ...... }
setCurrentItemInternal(....)方法实现页面切换，切换到哪个页面时由determineTargetPage(....)返回的值决定。
private int determineTargetPage(int currentPage, float pageOffset, int velocity, int deltaX) { int targetPage; Log.i("antoon", TAG+", determineTargetPage, Math.abs(deltaX) = "+Math.abs(deltaX)+", mFlingDistance = "+mFlingDistance); Log.i("antoon", TAG+", determineTargetPage, Math.abs(velocity) = "+Math.abs(velocity)+", mMinimumVelocity = "+mMinimumVelocity); if (Math.abs(deltaX) > mFlingDistance && Math.abs(velocity) > mMinimumVelocity) { targetPage = velocity > 0 ? currentPage : currentPage + 1; } else { final float truncator = currentPage >= mCurItem ? 0.4f : 0.6f; targetPage = (int) (currentPage + pageOffset + truncator); } if (mItems.size() > 0) { final ItemInfo firstItem = mItems.get(0); final ItemInfo lastItem = mItems.get(mItems.size() - 1); // Only let the user target pages we have items for targetPage = Math.max(firstItem.position, Math.min(targetPage, lastItem.position)); } return targetPage; }
经Log输出,mFlingDistance=75, mMinimumVelocity=1200, 所以对于快速滑动要满足 (水平滑动距离>75px && 滑动速率>1200px/s)才会切换页面
else {//这是对缓慢滑动的处理。 pageOffset决定切换哪个页面 。 final float truncator = currentPage >= mCurItem ? 0.4f : 0.6f; targetPage = (int) (currentPage + pageOffset + truncator); }
综上ViewPager快速滑动切换页面需要满足条件： 1，（水平位移>16dp && 水平位移/2>竖直位移）触发页面滑动，
2，(水平滑动距离>75px && 滑动速率>1200px/s)触发页面切换。