添加水印在android开发中比较常见，对于那些固定水印直接可以用图片平铺就可以实现了。如果需要根据文字生成动态的水印，可以使用自定义Drawable重写onDraw()方法，来绘制相应的文字。

但如果需要在RecyclerView 上绘制动态水印。你使用上面的方法就会发现，当RecyclerView滑动的时候，水印并不会随着滑动，而是一直固定在哪里。如果需求是需要水印也跟着RecyclerView滑动，那我们应该怎么实现呢？

实现步骤：

1、自定义一个Drawabl用于动态显示水印。

2、用ItemDecoration来绘制水印，并且跟踪滚动位置。

3、由于是重现ItemDecoration的onDraw（），所以该水印的位置在ItemView之下，如果ItemView有背景色将会遮挡住水印

效果图：

单个水印

多个水印

核心代码WatermarkDecoration.java

package eebochina.com.testtechniques.watermark;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.ColorFilter;import android.graphics.Paint;import android.graphics.PixelFormat;import android.graphics.Rect;import android.graphics.drawable.Drawable;import android.support.annotation.ColorInt;import android.support.annotation.NonNull;import android.support.annotation.Nullable;import android.support.v7.widget.RecyclerView;import java.util.List;/** * recyclerView 添加水印 */public class WatermarkDecoration extends RecyclerView.ItemDecoration {        WatermarkParams mWatermarkParams;    WaterMarkDrawable mDrawable;    int mScrollY = 0;    int mListDrawTextSize = -1;        private WatermarkDecoration(WatermarkParams mWatermarkParams) {        this.mWatermarkParams = mWatermarkParams;        mDrawable = new WaterMarkDrawable();        mListDrawTextSize = mWatermarkParams.mDrawTexts != null                ? mWatermarkParams.mDrawTexts.size() : -1;    }        @Override    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {        super.onDraw(c, parent, state);        int top = -mScrollY;        int right = parent.getWidth() / mWatermarkParams.mColumnNum;        int start = Math.abs(mScrollY / mWatermarkParams.mRowHeight);        int max = (parent.getHeight() + mScrollY) / mWatermarkParams.mRowHeight + 1;        for (int i = start; i < max; i++) {            int tempTop = top + (mWatermarkParams.mRowHeight * start);            int tempLeft = right / 3;            for (int j = 0; j < mWatermarkParams.mColumnNum; j++) {                mDrawable.setBounds(tempLeft, top, tempLeft + right, tempTop + mWatermarkParams.mRowHeight);                if (mListDrawTextSize == -1) {                    mDrawable.setDrawTex(mWatermarkParams.mDrawText);                } else {                    mDrawable.setDrawTex(mWatermarkParams.mDrawTexts.get((i * mWatermarkParams.mColumnNum + j) % mListDrawTextSize));                }                mDrawable.draw(c);                tempLeft += right;            }            top += mWatermarkParams.mRowHeight;        }            }        //跟踪recyclerView 滚动值    public void setScrollY(int dy) {        this.mScrollY += dy;    }        class WaterMarkDrawable extends Drawable {                Paint mPaint;        String mTempText;                public WaterMarkDrawable() {            mPaint = new Paint();            mPaint.setColor(mWatermarkParams.mTextColor);            mPaint.setTextSize(mWatermarkParams.mTextSize);            mPaint.setAlpha((int) (255 * mWatermarkParams.mAlpha));            mPaint.setAntiAlias(true);            mPaint.setTextAlign(Paint.Align.LEFT);        }                @Override        public void draw(@NonNull Canvas canvas) {            Rect rect = getBounds();            canvas.save();            canvas.rotate(mWatermarkParams.mDegrees, rect.left, rect.bottom);            canvas.drawText(mTempText, rect.left, rect.bottom, mPaint);            canvas.restore();        }                @Override        public void setAlpha(int alpha) {                }                @Override        public void setColorFilter(@Nullable ColorFilter colorFilter) {                }                @Override        public int getOpacity() {            return PixelFormat.UNKNOWN;        }                public void setDrawTex(String mDrawTex) {            this.mTempText = mDrawTex;        }    }        public static class Builder {        private WatermarkParams mWatermarkParams;                //水印背景字符        public Builder(String drawText) {            mWatermarkParams = new WatermarkParams(drawText);        }                //多个水印背景字符        public Builder(List
    
      drawTexts) {            mWatermarkParams = new WatermarkParams(drawTexts);        }                //文字大小        public Builder setTextSize(int textSize) {            mWatermarkParams.mTextSize = textSize;            return this;        }                //文字颜色        public Builder setTextColor(@ColorInt int textColor) {            mWatermarkParams.mTextColor = textColor;            return this;        }                //展示多少列        public Builder setColumnNum(int columnNum) {            mWatermarkParams.mColumnNum = columnNum;            return this;        }                //行高        public Builder setRowHeight(int rowHeight) {            mWatermarkParams.mRowHeight = rowHeight;            return this;        }                //倾斜角度        public Builder setDegrees(int degrees) {            mWatermarkParams.mDegrees = degrees;            return this;        }                //透明度 0-1        public Builder setAlpha(float alpha) {            mWatermarkParams.mAlpha = alpha;            return this;        }                public WatermarkDecoration builder() {            return new WatermarkDecoration(mWatermarkParams);        }    }        static class WatermarkParams {        String mDrawText;        List
     
       mDrawTexts;        int mTextColor = Color.parseColor("#ebebeb");        int mTextSize = 40;        int mColumnNum = 3;        int mRowHeight = 240;        int mDegrees = -30;        float mAlpha = 0.5f;                        public WatermarkParams(String mDrawText) {            this.mDrawText = mDrawText;        }                public WatermarkParams(List
      
        mDrawTexts) {            this.mDrawTexts = mDrawTexts;        }    }}复制代码
      
     
    

如何使用：

private void setWatermark() {//        WatermarkDecoration.Builder builder = new WatermarkDecoration.Builder("单个水印")//                .setColumnNum(3)//                .setTextColor(Color.GRAY)//                .setTextSize(35);        //多个水印        WatermarkDecoration.Builder builder = new WatermarkDecoration.Builder(getMultiple())                .setColumnNum(3)                .setTextColor(Color.GRAY)                .setTextSize(35);                mWatermarkDecoration = builder.builder();                mRecyclerView.addItemDecoration(mWatermarkDecoration);        mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {            @Override            public void onScrolled(RecyclerView recyclerView, int dx, int dy) {                super.onScrolled(recyclerView, dx, dy);                //设置水印滚动位置                mWatermarkDecoration.setScrollY(dy);            }        });    }复制代码

完整代码： https://github.com/hu5080126/SimpleExample/tree/master/watermark