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“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
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先来看看效果：情景纯属虚构，如有雷同，概不负责
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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。
<article style="margin: 0px; padding: 0px; max-width: 100%; overflow-wrap: break-word !important; box-sizing: border-box !important;">
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
image
多个水印
image
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。
对于自定义View这方面还不太熟悉的同学，建议这一块重点提升，回复【电子书】即可获取以下热门Android开发书籍，包括启舰的《Android自定义控件开发入门与实战》，直接一本书的内容来讲述自定义View。
记得按时学习
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