android使用ItemDecoration给RecyclerView 添加水印

public class MyDrawable extends Drawable {
 Paint mPaint;

 public MyDrawable() {
  mPaint = new Paint();
  mPaint.setColor(Color.parseColor("#1A000000"));
  mPaint.setAntiAlias(true);
  mPaint.setTextAlign(Paint.Align.LEFT);//从字的最左边开始画
  mPaint.setTextSize(54);
 }

 @Override public void draw(@NonNull Canvas canvas) {
  Rect r = getBounds();

  //画斜着的字
  canvas.save();
  canvas.rotate(-30, r.left, r.bottom);
  canvas.drawText("哈哈哈哈哈哈哈", r.left, r.bottom, mPaint);

  canvas.restore();
 }

 /*
  复写这两个方法是为了当在控件wrap_content时能自己测量出高，同时也方便布局。
 */ 

 //倾斜30度，可以算出高来
 @Override public int getIntrinsicHeight() {
  return (int) (Math.sqrt(3) / 3 * getIntrinsicWidth() + 0.5F);

 }

 @Override public int getIntrinsicWidth() {
  return (int) (mPaint.measureText("DecorationDraw") + 0.5F);
 }

 //...模板方法省略

}

public class MyDecoration extends RecyclerView.ItemDecoration {
 private Drawable mDivider;
 private int mScrollY;

 public MyDecoration() {
  mDivider = new MyDrawable();
 }

 @Override public void onDraw(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
  //清除之前画的
  // canvas.drawColor(Color.WHITE);

  /*
   * 跟着滑动是因为bounds在不停的变化，就是top
   */
  int top = UIUtil.dp(20) - mScrollY;

  // 对于画多少个水印，根据业务需求来，这里直接画count个
  int itemCount = parent.getAdapter().getItemCount();

  // 进行布局
  for (int i = 0; i < itemCount; ++i) {
   int left = i % 2 == 0 ? UIUtil.dp(20) : parent.getMeasuredWidth() -mDivider.getIntrinsicWidth() - UIUtil.dp(20);

   //通过setBounds来控制水印的左右
   mDivider.setBounds(left, top, parent.getMeasuredWidth(), top + mDivider.getIntrinsicHeight());
   mDivider.draw(canvas);

   if (i % 2 == 0) {
    top += UIUtil.dp(20) + mDivider.getIntrinsicHeight();
   } else {
    top += UIUtil.dp(140) + mDivider.getIntrinsicHeight();
   }
  }

 }

 /*
  mScrollY用于监测recyclerView的滑动距离，此处使用的是onScrollListener中dy的累加值，当item不发生删除添加操作时是准确的
 */ 
 public void setScrollY(int scrollY) {
  this.mScrollY = scrollY;
 }
}

private int totallyY = 0;

recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
 @Override public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
  totallyY += dy;
  myDecoration.setScrollY(totallyY);
  }
});