1.说明

2 实现

布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <LinearLayout
        android:id="@+id/main_linlayout"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:orientation="vertical">
    </LinearLayout>

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <Button
            android:id="@+id/bt"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="确定"/>
        <Button
            android:id="@+id/bt_pre"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="上一步"/>
        <Button
            android:id="@+id/bt_clear"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="清空"/>
    </LinearLayout>
    <ImageView
        android:id="@+id/img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="5dp"
        android:layout_gravity="center"/>

</LinearLayout>

MainActivity

package com.example.myview;

import androidx.appcompat.app.AppCompatActivity;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BlurMaskFilter;
import android.graphics.EmbossMaskFilter;
import android.os.Build;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;

public class MainActivity extends Activity implements View.OnClickListener {
    EmbossMaskFilter emboss;
    BlurMaskFilter blur;
    DrawView drawView;
    private LinearLayout main_linlayout;
    private Button bt,bt_clear,bt_pre;
    private ImageView img;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
        getWH();
    }
    //获取我们xml布局中view的宽高
    private void getWH() {
        // 获取创建的宽度和高度
        DisplayMetrics displayMetrics = new DisplayMetrics();
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
            getWindowManager().getDefaultDisplay().getRealMetrics(displayMetrics);
        }
        // 创建一个DrawView，该DrawView的宽度、高度与该Activity保持相同
        main_linlayout = (LinearLayout)findViewById(R.id.main_linlayout);
        drawView = new DrawView(this, displayMetrics.widthPixels, displayMetrics.heightPixels);

        main_linlayout.addView(drawView);
        drawView.requestFocus();
    }
    private void init() {
        bt = (Button)findViewById(R.id.bt);
        bt.setOnClickListener(this);
        bt_clear = (Button)findViewById(R.id.bt_clear);
        bt_clear.setOnClickListener(this);
        bt_pre=findViewById(R.id.bt_pre);
        bt_pre.setOnClickListener(this);
        img = (ImageView) findViewById(R.id.img);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.bt:
                Bitmap bit = drawView.getPaintBitmap();
                img .setImageBitmap(bit);
                break;
            case R.id.bt_pre:
                drawView.returnPre();
                break;
            case R.id.bt_clear:
                drawView.clear();
                break;
        }
    }
}

DrawView

package com.example.myview;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import androidx.annotation.Nullable;
import java.util.ArrayList;

public class DrawView extends View {
    // 定义记录前一个拖动事件发生点的坐标
    float preX;
    float preY;
    private Path path;
    private Path prePath;//之前路径存储
    public Paint paint = null;
    // 定义一个内存中的图片，该图片将作为缓冲区
    Bitmap cacheBitmap = null;
    // 定义cacheBitmap上的Canvas对象
    Canvas cacheCanvas = null;
    ArrayList<Path> cachePath=new ArrayList<>();

    public DrawView(Context context) {
        super(context);
    }

    public DrawView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public DrawView(Context context, int width , int height) {
        super(context);
        cacheBitmap=Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        cacheCanvas=new Canvas();
        path=new Path();

        // 设置cacheCanvas将会绘制到内存中的cacheBitmap上
        cacheCanvas.setBitmap(cacheBitmap);
        // 设置画笔的颜色
        paint = new Paint(Paint.DITHER_FLAG);
        paint.setColor(Color.RED);
        // 设置画笔风格
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(3);
        // 反锯齿
        paint.setAntiAlias(true);
        paint.setDither(true);

    }

    public boolean onTouchEvent(MotionEvent event){
        float x=event.getX();
        float y=event.getY();
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                // 从前一个点绘制到当前点之后，把当前点定义成下次绘制的前一个点
                path.moveTo(x, y);
                preX = x;
                preY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                // 从前一个点绘制到当前点之后，把当前点定义成下次绘制的前一个点
                path.quadTo(preX, preY, x, y);
                preX = x;
                preY = y;
                break;
            case MotionEvent.ACTION_UP:
                prePath=new Path();
                prePath.addPath(path);
                cachePath.add(prePath);
                Log.d("绘制次数",cachePath.size()+"");
                cacheCanvas.drawPath(path, paint); // ①
                path.reset();
                break;
        }
        invalidate();
        // 返回true表明处理方法已经处理该事件
        Log.d("onTouchEvent","onTouchEvent");
        return true;
    }
    @Override
    public void onDraw(Canvas canvas)
    {
        Log.d("onDraw","onDraw");
        Paint bmpPaint = new Paint();
        // 将cacheBitmap绘制到该View组件上
        canvas.drawBitmap(cacheBitmap, 0, 0, bmpPaint); // ②
        // 沿着path绘制
        canvas.drawPath(path, paint);
    }
    public Bitmap getPaintBitmap(){
        return resizeImage(cacheBitmap, 620, 780);
    }
    // 缩放
    public static Bitmap resizeImage(Bitmap bitmap, int width, int height) {
        //获取图片的宽高
        int originWidth = bitmap.getWidth();
        int originHeight = bitmap.getHeight();
        //这里缩放我们的尺寸，缩放多少自己去定义
        float scaleWidth = ((float) width) / originWidth;
        float scaleHeight = ((float) height) / originHeight;
        //进行缩放
        Matrix matrix = new Matrix();
        matrix.postScale(scaleWidth, scaleHeight);
        Bitmap resizedBitmap = Bitmap.createBitmap(bitmap, 0, 0, originWidth,
                originHeight, matrix, true);
        return resizedBitmap;
    }
    //清除画板
    public void clear() {
        if (cacheBitmap != null) {
            path.reset();
            cachePath.clear();
            cacheCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
            invalidate();
        }
    }
    //返回上一步绘制的情况
    public void returnPre(){
        path.reset();
        cacheCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
        invalidate();
        if(cachePath.size()!=0){
            for(int i=0;i<cachePath.size()-1;i++){
                cacheCanvas.drawPath(cachePath.get(i),paint);
            }
            cachePath.remove(cachePath.size()-1);
        }
        Log.d("绘制次数",cachePath.size()+"");
        invalidate();
    }
}