/**
 * An ItemDecoration allows the application to add a special drawing and layout offset
 * to specific item views from the adapter's data set. This can be useful for drawing dividers
 * between items, highlights, visual grouping boundaries and more.
 *
 * <p>All ItemDecorations are drawn in the order they were added, before the item
 * views (in {@link ItemDecoration#onDraw(Canvas, RecyclerView, RecyclerView.State) onDraw()}
 * and after the items (in {@link ItemDecoration#onDrawOver(Canvas, RecyclerView,
 * RecyclerView.State)}.</p>
 */
public abstract static class ItemDecoration {
    /**
     * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
     * Any content drawn by this method will be drawn before the item views are drawn,
     * and will thus appear underneath the views.
     *
     * @param c Canvas to draw into
     * @param parent RecyclerView this ItemDecoration is drawing into
     * @param state The current state of RecyclerView
     */
    public void onDraw(Canvas c, RecyclerView parent, State state) {
        onDraw(c, parent);
    }

    /**
     * @deprecated
     * Override {@link #onDraw(Canvas, RecyclerView, RecyclerView.State)}
     */
    @Deprecated
    public void onDraw(Canvas c, RecyclerView parent) {
    }

    /**
     * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
     * Any content drawn by this method will be drawn after the item views are drawn
     * and will thus appear over the views.
     *
     * @param c Canvas to draw into
     * @param parent RecyclerView this ItemDecoration is drawing into
     * @param state The current state of RecyclerView.
     */
    public void onDrawOver(Canvas c, RecyclerView parent, State state) {
        onDrawOver(c, parent);
    }

    /**
     * @deprecated
     * Override {@link #onDrawOver(Canvas, RecyclerView, RecyclerView.State)}
     */
    @Deprecated
    public void onDrawOver(Canvas c, RecyclerView parent) {
    }


    /**
     * @deprecated
     * Use {@link #getItemOffsets(Rect, View, RecyclerView, State)}
     */
    @Deprecated
    public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
        outRect.set(0, 0, 0, 0);
    }

    /**
     * Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies
     * the number of pixels that the item view should be inset by, similar to padding or margin.
     * The default implementation sets the bounds of outRect to 0 and returns.
     *
     * <p>
     * If this ItemDecoration does not affect the positioning of item views, it should set
     * all four fields of <code>outRect</code> (left, top, right, bottom) to zero
     * before returning.
     *
     * <p>
     * If you need to access Adapter for additional data, you can call
     * {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the
     * View.
     *
     * @param outRect Rect to receive the output.
     * @param view    The child view to decorate
     * @param parent  RecyclerView this ItemDecoration is decorating
     * @param state   The current state of RecyclerView.
     */
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
        getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
                parent);
    }
}

outRect.top =20;
outRect.bottom =20;

outRect.left=60;
outRect.right =60;

outRect.top =20;
outRect.bottom =20;

outRect.left=60;
outRect.right =0;

outRect.top =60;
outRect.bottom =60;

outRect.left=20;
outRect.right =20;

public YdDividerItemDecoration(){
    mPaint = new Paint();
    mPaint.setColor(Color.GREEN);
    mPaint.setAntiAlias(true);
}


@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    super.onDraw(c, parent, state);
    Logger.d("yidong -- onDraw");

    int width = parent.getWidth();
    int height = parent.getHeight();

    int radius = Math.min(width, height)/2;

    c.drawCircle(width/2, height/2, radius, mPaint);
}

@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    if (parent.getLayoutManager() == null || mDivider == null) {
        return;
    }
    if (mOrientation == VERTICAL) {
        drawVertical(c, parent);
    } else {
        drawHorizontal(c, parent);
    }
}

private void drawVertical(Canvas canvas, RecyclerView parent) {
     canvas.save();
     final int left;
     final int right;
     //noinspection AndroidLintNewApi - NewApi lint fails to handle overrides.
     if (parent.getClipToPadding()) {
         left = parent.getPaddingLeft();
         right = parent.getWidth() - parent.getPaddingRight();
         canvas.clipRect(left, parent.getPaddingTop(), right,
                 parent.getHeight() - parent.getPaddingBottom());
     } else {
         left = 0;
         right = parent.getWidth();
     }

     final int childCount = parent.getChildCount();
     for (int i = 0; i < childCount; i++) {
         final View child = parent.getChildAt(i);
         parent.getDecoratedBoundsWithMargins(child, mBounds);
         final int bottom = mBounds.bottom + Math.round(child.getTranslationY());
         final int top = bottom - mDivider.getIntrinsicHeight();
         mDivider.setBounds(left, top, right, bottom);
         mDivider.draw(canvas);
     }
     canvas.restore();
 }

@Override
public void draw(Canvas c) {
    //这里是RecyclerView的绘制过程，和View的绘制过程类似，先是背景，然后是调用
    //recyclerview的onDraw方法绘制自身，然后通过dispatchDraw来绘制子View
    // 而onDraw(下面)执行的就是ItemDecoration的onDraw方法，所以顺序很明显了。
    //ItemDecoration－> ItemView -> ItemDecoration.onDrawOver
    super.draw(c);
    // 执行ItemDecoration onDrawOver方法，最顶层
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDrawOver(c, this, mState);
    }
    ……

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

    // 执行ItemDecoration的onDraw方法，最底层
    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDraw(c, this, mState);
    }
}