/**
         *聊天区域顶部的渐变效果
         *
         */
        Paint mPaint = new Paint();
        // 融合器
        final Xfermode xfermode = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
        mPaint.setXfermode(xfermode);
        // 创造一个颜色渐变，作为聊天区顶部效果
        LinearGradient linearGradient = new LinearGradient(0.0f, 0.0f, 0.0f, 100.0f, new int[]{0, Color.BLACK}, null, Shader.TileMode.CLAMP);

        recyc_pull.addItemDecoration(new RecyclerView.ItemDecoration() {

            private int layerId;

            // 滑动RecyclerView，渲染之后每次都会回调这个方法，就在这里进行融合
            @Override
            public void onDrawOver(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
                super.onDrawOver(canvas, parent, state);

                mPaint.setXfermode(xfermode);
                mPaint.setShader(linearGradient);
                canvas.drawRect(0.0f, 0.0f, parent.getRight(), 200.0f, mPaint);
                mPaint.setXfermode(null);
                canvas.restoreToCount(layerId);
            }

            @Override
            public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
                super.onDraw(c, parent, state);
                layerId = c.saveLayer(0.0f, 0.0f, (float) parent.getWidth(), (float) parent.getHeight(), mPaint, Canvas.ALL_SAVE_FLAG);
            }

            @Override
            public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
                super.getItemOffsets(outRect, view, parent, state);
            }
        });