RecyclerView的position==0位置上的View

RecyclerView的position==0位置上的View高度为0时，canScrollVertically方法无法判断是否已经滑动到顶部？

看View中 canScrollVertically 方法的调用：

    /**
     * Check if this view can be scrolled vertically in a certain direction.
     *
     * @param direction Negative to check scrolling up, positive to check scrolling down.
     * @return true if this view can be scrolled in the specified direction, false otherwise.
     */
    public boolean canScrollVertically(int direction) {
        final int offset = computeVerticalScrollOffset();
        final int range = computeVerticalScrollRange() - computeVerticalScrollExtent();
        if (range == 0) return false;
        if (direction < 0) {
            return offset > 0;
        } else {
            return offset < range - 1;
        }
    }

这里边主要调用了三个方法，分别是computeVerticalScrollOffset()表示纵向滑动的距离，如果距离为0，表示已经滑动到了顶部,computeVerticalScrollRange()指的是整体高度，包括所显示区域和屏幕区域外的高度,computeVerticalScrollExtent()表示的是显示区域的高度。

问题出现的原因

当滑动到顶部的时候，computeVerticalScrollOffset()返回的高度一直是大于0的数，那么canScrollVertically就一致返回true,表示还没有滑动到顶部。接下来我们RecyclerView中方法computeVerticalScrollOffset()的实现。�RecyclerView这个方法的实现是直接调用LayoutManager的方法computeVerticalScrollOffset，我使用的LinearLayoutManager,所以直接看这个里边方法的实现。代码如下：

@Override
    public int computeVerticalScrollOffset(RecyclerView.State state) {
        return computeScrollOffset(state);
    }

这里又直接调用的computeScrollOffset方法

    private int computeScrollOffset(RecyclerView.State state) {
        if (getChildCount() == 0) {
            return 0;
        }
        ensureLayoutState();
        return ScrollbarHelper.computeScrollOffset(state, mOrientationHelper,
                findFirstVisibleChildClosestToStart(!mSmoothScrollbarEnabled, true),
                findFirstVisibleChildClosestToEnd(!mSmoothScrollbarEnabled, true),
                this, mSmoothScrollbarEnabled, mShouldReverseLayout);
    }

这里在主要看一下findFirstVisibleChildClosestToStart方法，这个方法主要是用来获取屏幕中第一个可见的childView，找到并返回这个childView。

    /**
     * Convenience method to find the visible child closes to start. Caller should check if it has
     * enough children.
     *
     * @param completelyVisible Whether child should be completely visible or not
     * @return The first visible child closest to start of the layout from user's perspective.
     */
    private View findFirstVisibleChildClosestToStart(boolean completelyVisible,
                                                     boolean acceptPartiallyVisible) {
        if (mShouldReverseLayout) {
            return findOneVisibleChild(getChildCount() - 1, -1, completelyVisible,
                    acceptPartiallyVisible);
        } else {
            return findOneVisibleChild(0, getChildCount(), completelyVisible,
                    acceptPartiallyVisible);
        }
    }

下边是查找第一个可见childView的详细代码，看完下边的代码就发现，如果第一个childView的高度为0的时候，会继续往下查找，找到第一个不为0的childView并返回。

View findOneVisibleChild(int fromIndex, int toIndex, boolean completelyVisible,
                             boolean acceptPartiallyVisible) {
        ensureLayoutState();
        final int start = mOrientationHelper.getStartAfterPadding();
        final int end = mOrientationHelper.getEndAfterPadding();
        final int next = toIndex > fromIndex ? 1 : -1;
        View partiallyVisible = null;
        for (int i = fromIndex; i != toIndex; i += next) {
            final View child = getChildAt(i);
            final int childStart = mOrientationHelper.getDecoratedStart(child);
            final int childEnd = mOrientationHelper.getDecoratedEnd(child);
            //如果第一个childView高度为0，childStart和childEnd都等于0；下边的条件不成立，会继续往下边寻找。
            if (childStart < end && childEnd > start) {
                if (completelyVisible) {
                    if (childStart >= start && childEnd <= end) {
                        return child;
                    } else if (acceptPartiallyVisible && partiallyVisible == null) {
                        partiallyVisible = child;
                    }
                } else {
                    return child;
                }
            }
        }
        return partiallyVisible;
    }

computeVerticalScrollOffset()的最终返回值来自这里

/**
     * @param startChild View closest to start of the list. (top or left)
     * @param endChild   View closest to end of the list (bottom or right)
     */
    static int computeScrollOffset(RecyclerView.State state, OrientationHelper orientation,
                                   View startChild, View endChild, RecyclerView.LayoutManager lm,
                                   boolean smoothScrollbarEnabled, boolean reverseLayout) {
        if (lm.getChildCount() == 0 || state.getItemCount() == 0 || startChild == null ||
                endChild == null) {
            return 0;
        }
        //获取第一个高度不为0的childView的position，如果RecylerView的第一个item高度为0，那么minPosition的值肯定大于0
        final int minPosition = Math.min(lm.getPosition(startChild),
                lm.getPosition(endChild));
        final int maxPosition = Math.max(lm.getPosition(startChild),
                lm.getPosition(endChild));
        //reverseLayout默认是false，所以itemBefore基本上就是等于minPosition的值
        final int itemsBefore = reverseLayout
                ? Math.max(0, state.getItemCount() - maxPosition - 1)
                : Math.max(0, minPosition);
        if (!smoothScrollbarEnabled) {
            return itemsBefore;
        }
        final int laidOutArea = Math.abs(orientation.getDecoratedEnd(endChild) -
                orientation.getDecoratedStart(startChild));
        final int itemRange = Math.abs(lm.getPosition(startChild) -
                lm.getPosition(endChild)) + 1;
        final float avgSizePerRow = (float) laidOutArea / itemRange;
        //通过返回的值，如果minPosition != 0,返回值都大于0。
        return Math.round(itemsBefore * avgSizePerRow + (orientation.getStartAfterPadding()
                - orientation.getDecoratedStart(startChild)));
    }

解决方案

要解决这个这个问题，需要将第一个item的高度设置为1像素就可以啦。