CoordinatorLayout + AppBarLayout + NestScrollView 向上滑动卡顿问题解决方案
问题描述
在商品详情页使用CoordinatorLayout + AppBarLayout + NestScrollView布局组合展示。当向上滚动时，不会产生Fling效果，每当手指离开屏幕，布局就会停止滚动。当向下滚动时没有该问题。
问题分析
嵌套滑动的原理为由NestScrollView接收触摸事件并反馈到父视图，产生联动效果。猜测没有产生Fling效果的原因有可能是在NestScrollView中计算手指滑动速度有错误。
代码分析
关键代码在NestScrollView的OnTouchEvent方法中：
@Override
public boolean onTouchEvent(MotionEvent ev) {
//部分代码省略
//手指滑动事件
case MotionEvent.ACTION_MOVE:
final int activePointerIndex = MotionEventCompat.findPointerIndex(ev,
mActivePointerId);
if (activePointerIndex == -1) {
Log.e(TAG, "Invalid pointerId=" + mActivePointerId + " in onTouchEvent");
break;
}
final int y = (int) MotionEventCompat.getY(ev, activePointerIndex);
int deltaY = mLastMotionY - y;
if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset)) {
deltaY -= mScrollConsumed[1];
vtev.offsetLocation(0, mScrollOffset[1]);
mNestedYOffset += mScrollOffset[1];
}
if (!mIsBeingDragged && Math.abs(deltaY) > mTouchSlop) {
final ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
//此处将mIsBeingGragged赋值为true
mIsBeingDragged = true;
if (deltaY > 0) {
deltaY -= mTouchSlop;
} else {
deltaY += mTouchSlop;
}
}
//部分代码省略
case MotionEvent.ACTION_UP:
//计算手指滑动速率的标准为mIsBeingGragged为True
if (mIsBeingDragged) {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) VelocityTrackerCompat.getYVelocity(velocityTracker,
mActivePointerId);
if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
flingWithNestedDispatch(-initialVelocity);
} else if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0,
getScrollRange())) {
ViewCompat.postInvalidateOnAnimation(this);
}
}
mActivePointerId = INVALID_POINTER;
endDrag();
break;
由以上代码可以看出，计算手势滑动速率的前提是滑动增量大于最小滑动距离，即Math.abs(deltaY) > mTouchSlop。而在15行-20行代码中有一些对deltaY的计算dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset)。
方法调用跟踪
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
return mChildHelper.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
} public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
if (isNestedScrollingEnabled() && mNestedScrollingParent != null) {
if (dx != 0 || dy != 0) {
int startX = 0;
int startY = 0;
if (offsetInWindow != null) {
mView.getLocationInWindow(offsetInWindow);
startX = offsetInWindow[0];
startY = offsetInWindow[1];
}
if (consumed == null) {
if (mTempNestedScrollConsumed == null) {
mTempNestedScrollConsumed = new int[2];
}
consumed = mTempNestedScrollConsumed;
}
consumed[0] = 0;
consumed[1] = 0;
//再次调用 ViewParentCompat.onNestedPreScroll(mNestedScrollingParent, mView, dx, dy, consumed);
if (offsetInWindow != null) {
mView.getLocationInWindow(offsetInWindow);
offsetInWindow[0] -= startX;
offsetInWindow[1] -= startY;
}
return consumed[0] != 0 || consumed[1] != 0;
} else if (offsetInWindow != null) {
offsetInWindow[0] = 0;
offsetInWindow[1] = 0;
}
}
return false;
}CoordinatorLayout的onNestPreScroll方法
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
int xConsumed = 0;
int yConsumed = 0;
boolean accepted = false;
final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View view = getChildAt(i);
final LayoutParams lp = (LayoutParams) view.getLayoutParams();
if (!lp.isNestedScrollAccepted()) {
continue;
}
final Behavior viewBehavior = lp.getBehavior();
if (viewBehavior != null) {
mTempIntPair[0] = mTempIntPair[1] = 0;
viewBehavior.onNestedPreScroll(this, view, target, dx, dy, mTempIntPair);
xConsumed = dx > 0 ? Math.max(xConsumed, mTempIntPair[0])
: Math.min(xConsumed, mTempIntPair[0]);
yConsumed = dy > 0 ? Math.max(yConsumed, mTempIntPair[1])
: Math.min(yConsumed, mTempIntPair[1]);
accepted = true;
}
}
consumed[0] = xConsumed;
//将Y轴滚动赋值给consumed[1]
consumed[1] = yConsumed;
if (accepted) {
dispatchOnDependentViewChanged(true);
}
} @Override
public void onNestedPreScroll(CoordinatorLayout coordinatorLayout, AppBarLayout child,
View target, int dx, int dy, int[] consumed) {
if (dy != 0 && !mSkipNestedPreScroll) {
int min, max;
if (dy < 0) {
// We're scrolling down
min = -child.getTotalScrollRange();
max = min + child.getDownNestedPreScrollRange();
} else {
// We're scrolling up
min = -child.getUpNestedPreScrollRange();
max = 0;
}
consumed[1] = scroll(coordinatorLayout, child, dy, min, max);
}
}NestScrollView :: OnTouchEvent() —>ACTION_MOVE —> dispatchNestedPreScroll()方法将NestedScrollView的滚动距离分发给parentView(AppBarLayout)，
会调用AppBarLayout:: onNestedPreScroll()方法，
该方法会根据滚动距离设置Header的顶部和底部偏移(setHeaderTopBottomOffset())，并返回偏移的距离即滚动距离。
返回到NestScrollView的OnTouchEvent方法后，会重新计算Y轴滚动距离deltaY -= mScrollConsumed[1];计算完成之后deltaY结果为0。
当手指抬起的时候ACTION_UP，此时mIsBeingGragged为false，所以不会Fling。
解决方案
为AppBarLayout自定义Behavior，在嵌套滚动之前(onNestedPreScroll)记录手指滑动速率，在滚动完成之后(onStopNestedScroll)，再次手动调用onNestedFling方法。
代码实现：
public final class FlingBehavior extends AppBarLayout.Behavior {
private static final String TAG = FlingBehavior.class.getName();
private static final int TOP_CHILD_FLING_THRESHOLD = 1;
private static final float OPTIMAL_FLING_VELOCITY = 3500;
private static final float MIN_FLING_VELOCITY = 20;
boolean shouldFling = false;
float flingVelocityY = 0;
public FlingBehavior() {
}
public FlingBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public void onNestedPreScroll(CoordinatorLayout coordinatorLayout, AppBarLayout child, View target,
int velocityX, int velocityY, int[] consumed) {
super.onNestedPreScroll(coordinatorLayout, child, target, velocityX, velocityY, consumed);
if (velocityY > MIN_FLING_VELOCITY) {
shouldFling = true;
flingVelocityY = velocityY;
} else {
shouldFling = false;
}
}
@Override
public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, AppBarLayout abl, View target) {
super.onStopNestedScroll(coordinatorLayout, abl, target);
if (shouldFling) {
Log.d(TAG, "onNestedPreScroll: running nested fling, velocityY is " + flingVelocityY);
onNestedFling(coordinatorLayout, abl, target, 0, flingVelocityY, true);
}
}
@Override
public boolean onNestedFling(CoordinatorLayout coordinatorLayout, AppBarLayout child, View target,
float velocityX, float velocityY, boolean consumed) {
if (target instanceof RecyclerView && velocityY < 0) {
Log.d(TAG, "onNestedFling: target is recyclerView");
final RecyclerView recyclerView = (RecyclerView) target;
final View firstChild = recyclerView.getChildAt(0);
final int childAdapterPosition = recyclerView.getChildAdapterPosition(firstChild);
consumed = childAdapterPosition > TOP_CHILD_FLING_THRESHOLD;
}
// prevent fling flickering when going up
if (target instanceof NestedScrollView && velocityY > 0) {
consumed = true;
}
if (Math.abs(velocityY) < OPTIMAL_FLING_VELOCITY) {
velocityY = OPTIMAL_FLING_VELOCITY * (velocityY < 0 ? -1 : 1);
}
Log.d(TAG, "onNestedFling: velocityY - " + velocityY + ", consumed - " + consumed);
return super.onNestedFling(coordinatorLayout, child, target, velocityX, velocityY, consumed);
}
}在布局中添加这个Behavior：
.support.design.widget.AppBarLayout
android:id="@+id/appbar_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_behavior="packagename.FlingBehavior"
android:fitsSystemWindows="true">