recycleVIew 上下 渐变

要在RecyclerView中实现上下渐变，你可以使用一个叫做`ItemDecoration`的类来实现。在`ItemDecoration`中，你可以重写`onDrawOver()`方法，然后在该方法中绘制你想要的渐变效果。

下面是一个简单的实现示例，可以实现RecyclerView顶部和底部的渐变效果：

public class GradientItemDecoration extends RecyclerView.ItemDecoration {
    private int mGradientHeight = 100; //渐变高度，单位为px
    private int mGradientColor = Color.TRANSPARENT; //渐变颜色，默认为透明

    public GradientItemDecoration(int gradientHeight, int gradientColor) {
        mGradientHeight = gradientHeight;
        mGradientColor = gradientColor;
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

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

        //绘制顶部渐变
        LinearGradient topGradient = new LinearGradient(0, 0, 0, mGradientHeight,
                mGradientColor, Color.TRANSPARENT, Shader.TileMode.CLAMP);
        Paint topPaint = new Paint();
        topPaint.setShader(topGradient);
        c.drawRect(0, 0, width, mGradientHeight, topPaint);

        //绘制底部渐变
        LinearGradient bottomGradient = new LinearGradient(0, height - mGradientHeight, 0,
                height, mGradientColor, Color.TRANSPARENT, Shader.TileMode.CLAMP);
        Paint bottomPaint = new Paint();
        bottomPaint.setShader(bottomGradient);
        c.drawRect(0, height - mGradientHeight, width, height, bottomPaint);
    }
}

你可以在适当的地方创建一个`GradientItemDecoration`对象，并将其添加到你的RecyclerView中：

GradientItemDecoration decoration = new GradientItemDecoration(100, Color.BLACK);
recyclerView.addItemDecoration(decoration);

这样就可以实现RecyclerView的上下渐变效果了。