【Android 事件分发】ItemTouchHelper 事件分发源码分析 ( 绑定 RecyclerView )（二）

mRecyclerView.addItemDecoration(this);

public class RecyclerView extends ViewGroup implements ScrollingView,
        NestedScrollingChild2, NestedScrollingChild3 {
    /**
     * Add an {@link ItemDecoration} to this RecyclerView. Item decorations can
     * affect both measurement and drawing of individual item views.
     *
     * <p>Item decorations are ordered. Decorations placed earlier in the list will
     * be run/queried/drawn first for their effects on item views. Padding added to views
     * will be nested; a padding added by an earlier decoration will mean further
     * item decorations in the list will be asked to draw/pad within the previous decoration's
     * given area.</p>
     *
     * @param decor Decoration to add
     * @param index Position in the decoration chain to insert this decoration at. If this value
     *              is negative the decoration will be added at the end.
     */
    public void addItemDecoration(@NonNull ItemDecoration decor, int index) {
        if (mLayout != null) {
            mLayout.assertNotInLayoutOrScroll("Cannot add item decoration during a scroll  or"
                    + " layout");
        }
        if (mItemDecorations.isEmpty()) {
            setWillNotDraw(false);
        }
        // 将多个 ItemDecoration 添加到 mItemDecorations 集合中
        if (index < 0) {
            mItemDecorations.add(decor);
        } else {
            mItemDecorations.add(index, decor);
        }
        markItemDecorInsetsDirty();
        // 开始进行绘制 
        requestLayout();
    }
}

void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull State state) 方法是绘制方法 , 在该方法中调用了 void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent) 方法 ,
    public abstract static class ItemDecoration {
        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull State state) {
            onDraw(c, parent);
        }
        @Deprecated
        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent) {
        }
    }

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        if (parent.getLayoutManager() == null || mDivider == null) {
            return;
        }
        if (mOrientation == VERTICAL) {
          // 绘制垂直分割线
            drawVertical(c, parent);
        } else {
          // 绘制水平分割线
            drawHorizontal(c, parent);
        }
    }
}