自定义View实现随机数

  MainActivity

  package com.example.view1;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn=new Button(this);
        findViewById(R.id.cv).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
              //  ((CustemView)view).setPaint();
                Toast.makeText(MainActivity.this,"aaaa",Toast.LENGTH_SHORT).show();
            }
        });
    }
}

CustemView
package com.example.view1;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

import java.util.Random;

/**
 * Created by tangxiaoying on 2017/9/28.
 * onMeasure 测量方法
 * onLayout摆放位置   在自定义viewGroup的时候需要使用
 * onDarw  画布
 */
public class CustemView extends View {
    Paint paint;
    char[] array=new char[4];
    Random r=new Random();
    float downX=0;
    //在类中直接使用new关键实例化控件时使用的构造方法
    public CustemView(Context context) {
        super(context);
        init();
    }
        //在布局文件中创建控件，使用的是第二个构造方法
    public CustemView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
    //canvas是画布对象
    public CustemView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    public void init(){
        paint=new Paint();
        //抗锯齿
        paint.setAntiAlias(true);
        paint.setColor(Color.RED);
        paint.setStrokeWidth(10);
    }
    //canvas是画布对象
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
     /*   canvas.drawLine(0,0,100,100,paint);
        paint.setColor(Color.BLUE);
       paint.setStyle(Paint.Style.STROKE);
       canvas.drawCircle(300,300,200,paint);
       Bitmap bitmap= BitmapFactory.decodeResource(getResources(),R.mipmap.ic_launcher);
       canvas.drawBitmap(bitmap,100,100,paint);*/

        for(int i=0;i<4;i++){
        String a=r.nextInt(10)+"";
            char[] temp=a.toCharArray();
            array[i]=temp[0];
        }
        canvas.drawRect(100,100,300,200,paint);
        paint.setColor(Color.YELLOW);
        paint.setTextSize(80);
        canvas.drawText(array,0,4,100,200,paint);

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                downX=event.getX();
                paint.setColor(Color.BLUE);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                break;
            case MotionEvent.ACTION_UP:
                float upX=event.getX();
                if (upX>100&&upX<200){
                    paint.setColor(Color.RED);
                    postInvalidate();
                }
                break;
        }
        return super.onTouchEvent(event);
    }
    public void setPaint(){
        paint.setColor(Color.BLACK);
        invalidate();
    }
}

Layout布局

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.view1.MainActivity">

     <com.example.view1.CustemView
         android:id="@+id/cv"
         android:layout_width="match_parent"
         android:layout_height="match_parent" />
</RelativeLayout>

实现效果图