OverScroller、Scroll到edge时的阻尼效果,overScrollBy和scrollTo，AbsListView和ScrollView的滚动实现区别

刺骨的言语ヽ痛彻心扉 2022-03-10 10:58 250阅读 0赞

OverScroller

和Scroller类似，都只是根据duration、已过去的时间，start position,final position，根据某种interpolator计算某个时刻的scrollX和scrollY（这里说的scrollX/Y和View的成员mScrollX/Y无关）。AbsListView和ScrollView添加阻尼效果使用的Scroller都是OverScroller，当然，还需其他类配合。

如果需要使用判断边界和绘制某种边界效果，使用View.overScrollBy()方法代替直接使用View.scrollTo()。而是否允许overScrollmode，则可由android:overScrollMode="true|false"来设定。

如果想要制造阻尼效果，可以参考ScrollView的实现，简单明了，AbsListView太复杂了。ScrollView使用computeScroll()驱动scroll过程的(AbsListView使用的其内部的FlingRunnable，通过不断postOnAnimation(Runnable)驱动scroll过程)。因为滚动计算和滚动变化AbsListView更复杂，所以夹杂在其中的阻尼效果代码的流程就不那么明朗，但是ListView也是使用了OverScroller，overScrollBy(),onOverScroll,EdgeEffect来实现阻尼效果的。

下面看看ScrollView\#computeScroll():

@Override
        public void computeScroll() {
            if (mScroller.computeScrollOffset()) {
                // This is called at drawing time by ViewGroup.  We don't want to
                // re-show the scrollbars at this point, which scrollTo will do,
                // so we replicate most of scrollTo here.
                //
                //         It's a little odd to call onScrollChanged from inside the drawing.
                //
                //         It is, except when you remember that computeScroll() is used to
                //         animate scrolling. So unless we want to defer the onScrollChanged()
                //         until the end of the animated scrolling, we don't really have a
                //         choice here.
                //
                //         I agree.  The alternative, which I think would be worse, is to post
                //         something and tell the subclasses later.  This is bad because there
                //         will be a window where mScrollX/Y is different from what the app
                //         thinks it is.
                //
                int oldX = mScrollX;
                int oldY = mScrollY;
                int x = mScroller.getCurrX();
                int y = mScroller.getCurrY();
    
                if (oldX != x || oldY != y) {
                    final int range = getScrollRange();
                    final int overscrollMode = getOverScrollMode();
                    final boolean canOverscroll = overscrollMode == OVER_SCROLL_ALWAYS ||
                            (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0);
    //主要是计算过程滚动过程，然后回调onOverScroll()方法，overScrollBy不用复写，复写
    //onOverScroll()方法。这里是使用overScrollBy代替直接使用scrollTo。在onOverScroll()
    //中还是使用scrollTo
                    overScrollBy(x - oldX, y - oldY, oldX, oldY, 0, range,
                            0, mOverflingDistance, false);
                    onScrollChanged(mScrollX, mScrollY, oldX, oldY);
    
                    if (canOverscroll) {//使用EdgeEffect绘制阻尼效果
                        if (y < 0 && oldY >= 0) {
                            mEdgeGlowTop.onAbsorb((int) mScroller.getCurrVelocity());
                        } else if (y > range && oldY <= range) {
                            mEdgeGlowBottom.onAbsorb((int) mScroller.getCurrVelocity());
                        }
                    }
                }
    
                if (!awakenScrollBars()) {
                    // Keep on drawing until the animation has finished.
                    postInvalidateOnAnimation();
                }
            } else {
                if (mFlingStrictSpan != null) {
                    mFlingStrictSpan.finish();
                    mFlingStrictSpan = null;
                }
            }
        }

看ScrollView复写的onOverScroll():

@Override
        protected void onOverScrolled(int scrollX, int scrollY,
                boolean clampedX, boolean clampedY) {//如果clamped为true，则代表某个方向到边界了
    //到边界后，scrollX/Y会和之前的值相同
            // Treat animating scrolls differently; see #computeScroll() for why.
            if (!mScroller.isFinished()) {
                final int oldX = mScrollX;
                final int oldY = mScrollY;
                mScrollX = scrollX;
                mScrollY = scrollY;
                invalidateParentIfNeeded();
                onScrollChanged(mScrollX, mScrollY, oldX, oldY);
                if (clampedY) {//说明到了需要overScroll并需要产生阻尼效果或其他效果，看Android版本的实现。该方法中会计算速度SplineOverScroller.mVelocity，EdgeEffect会用到这个数据
                    mScroller.springBack(mScrollX, mScrollY, 0, 0, 0, getScrollRange());
                }
            } else {
    //如果还没有到边界，就使用scrollTo就好。所以View.overScrollBy还将scrollX/Y计算好了
                super.scrollTo(scrollX, scrollY);
            }
    
            awakenScrollBars();
        }

AbsListView滚动使用的变换是offsetChildrenTopAndBottom(),ScrollView使用的是scrollTo().

public void offsetChildrenTopAndBottom(int offset) {
            final int count = mChildrenCount;
            final View[] children = mChildren;
            boolean invalidate = false;
     
            for (int i = 0; i < count; i++) {
                final View v = children[i];
                v.mTop += offset;
                v.mBottom += offset;
                if (v.mRenderNode != null) {
                    invalidate = true;
                    v.mRenderNode.offsetTopAndBottom(offset);
                }
            }
     
            if (invalidate) {
                invalidateViewProperty(false, false);
            }
            notifySubtreeAccessibilityStateChangedIfNeeded();}

ListView的滚动实现：

FlingMode：FlingRunnbale作为一次滚动单一任务，一次Fling需要很多个FlingRunnbale来完成滚动，每次执行FlingRunnbale都会通过View\#postOnAnimation(),把FlingRunnbale重新放入Choreographer,dattachInfo.mViewRootImpl.mChoreographer.postCallback( Choreographer.CALLBACK\_ANIMATION, action, null);通过OverScroller\#computeScrollOffset()和OverScroller\#getCurrY()实现获得deltaY，然后trackMotionScroll，其中调用offsetChildrenTopAndBottom(deltaY)实现滚动。

View\#postOnAnimation()

/**
         * <p>Causes the Runnable to execute on the next animation time step.
         * The runnable will be run on the user interface thread.</p>
         *
         * @param action The Runnable that will be executed.
         *
         * @see #postOnAnimationDelayed
         * @see #removeCallbacks
         */
        public void postOnAnimation(Runnable action) {
            final AttachInfo attachInfo = mAttachInfo;
            if (attachInfo != null) {
                attachInfo.mViewRootImpl.mChoreographer.postCallback(
                        Choreographer.CALLBACK_ANIMATION, action, null);
            } else {
                // Postpone the runnable until we know
                // on which thread it needs to run.
                getRunQueue().post(action);
            }
        }

ScrollMode：直接使用trackMotionScroll，然后调用offsetChildrenTopAndBottom(deltaY)实现滚动，而不用使用动画。