面试官: 为了信息安全，来给聊天界面加上水印

Android扫地僧

发布于 2020-03-19 16:04:13

1.3K0

发布于 2020-03-19 16:04:13

文章被收录于专栏：Android进阶

“为了信息安全，来给聊天界面加上水印，怎么实现？”

这个是我曾经面试遇到过的一个问题，回答完就让我回去等消息了

先来看看效果：情景纯属虚构，如有雷同，概不负责

你乍得一听，简单！上手就写，直接给ReyclerView加个背景，你会发现聊天界面滚动，水印不动，甚至水印不显示；或者我直接给Item加个背景，也不行，被item背景色覆盖了，并且受item布局控制

首先拆解一下这个需求：

1、聊天界面一般都是基于RecyclerView / ListView实现，所以我们的水印是要加在RecyclerView / ListView上

2、水印不会覆盖聊天消息也就是遮挡Item

3、水印会随着聊天界面滚动

懵逼了吗？其实这个场景在很多企业软件中都有用到，企业微信，钉钉，一般都是将员工姓名和工号，添加在一些保密级别比较高的界面上，防止截图泄露内容。

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Drawable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、由于是重写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

我知道你会说：

Talk is cheap, show me the code.

核心代码WatermarkDecoration.java

package com.duoshou7.app.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<String> 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<String> 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<String> mDrawTexts) {
            this.mDrawTexts = mDrawTexts;
        }
    }
}

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);
            }
        });
    }

mark一下，你一定会用得上。

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原始发表：2020-02-20，如有侵权请联系 cloudcommunity@tencent.com 删除

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