RecyclerView系列 - 分割线源码分析

1. 说明

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前边两节课我们已经写了ListView和GridView样式的分割线的具体实现，那么这节课我们就从RecyclerView的源码角度来分析下 - 分割线。

2. 源码分析

2.1 猜想1：
RecyclerView既然考虑了分割线，那么子View在测量的时候肯定要去考虑分割线留出的位置，直接看RecyclerView源码中的 measureChild()方法，源码方法如下：

/**
         * Measure a child view using standard measurement policy, taking the padding
         * of the parent RecyclerView and any added item decorations into account.
         *
         * 
If the RecyclerView can be scrolled in either dimension the caller may
         * pass 0 as the widthUsed or heightUsed parameters as they will be irrelevant.
         *
         * @param child Child view to measure
         * @param widthUsed Width in pixels currently consumed by other views, if relevant
         * @param heightUsed Height in pixels currently consumed by other views, if relevant
         */
        public void measureChild(View child, int widthUsed, int heightUsed) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            // 这个是最主要的方法  返回的是分割线所要考虑的Rect，这个Rect就相当于是个矩形，其实就是回调的getItemOffsets()，可以返回top、bottom、left、right ，所以在自定义的时候可以覆盖getItemOffsets()这个方法
            final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
            // 测量的时候要考虑的分割线距离
            widthUsed += insets.left + insets.right;
            heightUsed += insets.top + insets.bottom;
            final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
                    getPaddingLeft() + getPaddingRight() + widthUsed, lp.width,
                    canScrollHorizontally());
            final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
                    getPaddingTop() + getPaddingBottom() + heightUsed, lp.height,
                    canScrollVertically());
            if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
                child.measure(widthSpec, heightSpec);
            }
        }

2.2 猜想2：分割线会调用绘制的方法onDraw()

@Override
    public void onDraw(Canvas c) {
        super.onDraw(c);

        final int count = mItemDecorations.size();
        for (int i = 0; i < count; i++) {

            //  这个地方会回调分割线的onDraw()方法，所以你在自定义的那两个分割线的类中的onDraw()方法中想怎样绘制就怎样绘制，它都会回调回来的
            mItemDecorations.get(i).onDraw(c, this, mState);
        }
    }