Android——RecyclerView.scrollBy源码分析

最近有个需求是RecyclerView左右滚动时，如果焦点View超过屏幕中间就把焦点View滚到屏幕中间。实现思路为获取焦点View在屏幕上的坐标，并根据屏幕宽度/2来计算滚动距离。代码如下：

FocusLinearLayoutManager layoutManager = (FocusLinearLayoutManager) provincesList.getLayoutManager();
int firstVisiblePos = layoutManager.findFirstVisibleItemPosition();
View view = provincesList.getChildAt(position - firstVisiblePos);
int[] location = new int[2];
view.getLocationOnScreen(location);
int half_width = DensityUtil.getScreenWidth(this) / 2;
provincesList.scrollBy(location[0] - half_width, 0);

今天要分析的问题，并不是上面的需求如何实现，而是分析下scrollBy如何实现的。
scrollBy会执行到scrollByInternal方法中，下面是scrollByInternal的一段代码。

 int unconsumedX = 0, unconsumedY = 0;
        int consumedX = 0, consumedY = 0;

        consumePendingUpdateOperations();
        if (mAdapter != null) {
            eatRequestLayout();
            onEnterLayoutOrScroll();
            TraceCompat.beginSection(TRACE_SCROLL_TAG);
            fillRemainingScrollValues(mState);
            if (x != 0) {
                consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState);
                unconsumedX = x - consumedX;
            }
            if (y != 0) {
                consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
                unconsumedY = y - consumedY;
            }
            TraceCompat.endSection();
            repositionShadowingViews();
            onExitLayoutOrScroll();
            resumeRequestLayout(false);
        }

由mLayout.scrollHorizontallyBy(x, mRecycler, mState);可见，RecyclerView把滚动交给了Layoutmanager来实现，我们继续跟进。(这里以横向滚动为例)

LinearlayoutManager.scrollHorizontalBy:

    public int scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler,
            RecyclerView.State state) {
        if (mOrientation == VERTICAL) {
            return 0;
        }
        return scrollBy(dx, recycler, state);
    }

int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (getChildCount() == 0 || dy == 0) {
            return 0;
        }
        mLayoutState.mRecycle = true;
        ensureLayoutState();
        final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;
        final int absDy = Math.abs(dy);
        updateLayoutState(layoutDirection, absDy, true, state);
        final int consumed = mLayoutState.mScrollingOffset
                + fill(recycler, mLayoutState, state, false);
        if (consumed < 0) {
            if (DEBUG) {
                Log.d(TAG, "Don't have any more elements to scroll");
            }
            return 0;
        }
        final int scrolled = absDy > consumed ? layoutDirection * consumed : dy;
        mOrientationHelper.offsetChildren(-scrolled);
        if (DEBUG) {
            Log.d(TAG, "scroll req: " + dy + " scrolled: " + scrolled);
        }
        mLayoutState.mLastScrollDelta = scrolled;
        return scrolled;
    }

scrollBy方法中mOrientationHelper.offsetChildren(-scrolled);表明了，接下要进行View的移动。

public void offsetChildren(int amount) {
	mLayoutManager.offsetChildrenHorizontal(amount);
}

由上述代码可见，最终代码的执行又回到了Layoutmanager。

public void offsetChildrenHorizontal(int dx) {
       if (mRecyclerView != null) {
             mRecyclerView.offsetChildrenHorizontal(dx);
        }
}

public void offsetChildrenHorizontal(int dx) {
        final int childCount = mChildHelper.getChildCount();
        for (int i = 0; i < childCount; i++) {
            mChildHelper.getChildAt(i).offsetLeftAndRight(dx);
        }
}

这里敲黑板了！！！真想即将大白，最后的最后还是回到了View中处理

   public void offsetLeftAndRight(int offset) {
        if (offset != 0) {
            final boolean matrixIsIdentity = hasIdentityMatrix();
            if (matrixIsIdentity) {
                if (isHardwareAccelerated()) {
                    invalidateViewProperty(false, false);
                } else {
                    final ViewParent p = mParent;
                    if (p != null && mAttachInfo != null) {
                        final Rect r = mAttachInfo.mTmpInvalRect;
                        int minLeft;
                        int maxRight;
                        if (offset < 0) {
                            minLeft = mLeft + offset;
                            maxRight = mRight;
                        } else {
                            minLeft = mLeft;
                            maxRight = mRight + offset;
                        }
                        r.set(0, 0, maxRight - minLeft, mBottom - mTop);
                        p.invalidateChild(this, r);
                    }
                }
            } else {
                invalidateViewProperty(false, false);
            }
        }
    }

其中最重要的是这几行，重新计算View的坐标。

 if (offset < 0) {
	minLeft = mLeft + offset;
    maxRight = mRight;
} else {
    minLeft = mLeft;
    maxRight = mRight + offset;
}

看到这里，想必文章开头中RecyclerView如何计算滚动距离也不用解释。向左滚动x坐标左移，向右滚动x坐标右移。

分析的不是很细，主要是为了理清流程，了解原理。