RecyclerView.ItemDecoration

RecyclerView.ItemDecoration

这个类包含三个方法 1：
•onDraw(Canvas c, RecyclerView parent, State state)
•onDrawOver(Canvas c, RecyclerView parent, State state)
•getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)

getItemOffsets 中为 outRect 设置的4个方向的值，将对child的大小有影响。
outRect.set(left, top, right, bottom);

Rect getItemDecorInsetsForChild(View child) {

    final LayoutParams lp = (LayoutParams) child.getLayoutParams();

    if (!lp.mInsetsDirty) {

        return lp.mDecorInsets;

    }



    final Rect insets = lp.mDecorInsets;

    insets.set(0, 0, 0, 0);

    final int decorCount = mItemDecorations.size();

    for (int i = 0; i < decorCount; i++) {

        mTempRect.set(0, 0, 0, 0);

        mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);

        insets.left += mTempRect.left;

        insets.top += mTempRect.top;

        insets.right += mTempRect.right;

        insets.bottom += mTempRect.bottom;

    }

    lp.mInsetsDirty = false;

    return insets;

}

 public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
            widthUsed += insets.left + insets.right;
            heightUsed += insets.top + insets.bottom;

            final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
                    getPaddingLeft() + getPaddingRight() +
                            lp.leftMargin + lp.rightMargin + widthUsed, lp.width,
                    canScrollHorizontally());
            final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
                    getPaddingTop() + getPaddingBottom() +
                            lp.topMargin + lp.bottomMargin + heightUsed, lp.height,
                    canScrollVertically());
            if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
                child.measure(widthSpec, heightSpec);
            }
        }

在计算child的大小的时候，将outRect设置的四边减去了。

1、当RecyclerView可以水平滑动的时候
outRect设置的top，bottom将影响item的高度大小
2、当RecyclerView可以竖直滑动的时候
outRect设置的top，bottom将影响item的宽度大小

 public static int getChildMeasureSpec(int parentSize, int parentMode, int padding,
                int childDimension, boolean canScroll) {
            int size = Math.max(0, parentSize - padding);
            int resultSize = 0;
            int resultMode = 0;
            if (canScroll) {
                if (childDimension >= 0) {
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == LayoutParams.MATCH_PARENT) {
                    switch (parentMode) {
                        case MeasureSpec.AT_MOST:
                        case MeasureSpec.EXACTLY:
                            resultSize = size;
                            resultMode = parentMode;
                            break;
                        case MeasureSpec.UNSPECIFIED:
                            resultSize = 0;
                            resultMode = MeasureSpec.UNSPECIFIED;
                            break;
                    }
                } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                    resultSize = 0;
                    resultMode = MeasureSpec.UNSPECIFIED;
                }
            } else {
                if (childDimension >= 0) {
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == LayoutParams.MATCH_PARENT) {
                    resultSize = size;
                    resultMode = parentMode;
                } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                    resultSize = size;
                    if (parentMode == MeasureSpec.AT_MOST || parentMode == MeasureSpec.EXACTLY) {
                        resultMode = MeasureSpec.AT_MOST;
                    } else {
                        resultMode = MeasureSpec.UNSPECIFIED;
                    }

                }
            }
            //noinspection WrongConstant
            return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
        }

onDraw和onDrawOver画的区域不受outRect影响。绘制时，先执行ItemDecoration的onDraw，然后执行绘制child，最后执行ItemDecoration的onDrawOver.