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： android\frameworks\support\v4\java\android\support\v4\view\ViewPager.java
ViewPager其实就是一个重写的ViewGroup，使用ViewPager可以参考SDK中的demo:sdk\extras\android\support\samples
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。