源码注释很清晰。（可以直接拷贝运行）

circleImageView类代码

package cc.willread.www.viewapp;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.view.View;

public class circleImageView extends View {
    private Paint paint;

    public circleImageView(Context context) {
        super(context);
        paint=new Paint();
    }

    public circleImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
        paint=new Paint();
    }
    @Override
    protected void onDraw(Canvas canvas) {
        /*
         * 方法 说明 drawCircle 绘制圆形 drawRect 绘制矩形  drawOval 绘制椭圆  drawPosText绘制文字 drawPath 绘制任意多边形
         * drawLine 绘制直线 drawPoin 绘制点
         */
        paint.setAntiAlias(true); // 是否抗锯齿
        paint.setAlpha(50); // 设置alpha不透明度，范围为0~255
        paint.setColor(Color.RED);//        给画笔设置颜色

//        设置画笔属性
        paint.setStyle(Paint.Style.FILL);//画笔属性是实心圆
      //  paint.setStyle(Paint.Style.STROKE);//画笔属性是空心圆
        paint.setStrokeWidth(4);//设置画笔粗细

        Dcircle(canvas);//drawCircle 绘制圆形
        Drect(canvas);//drawRect 绘制矩形
        Doval(canvas);//drawOval 绘制椭圆
        DposText(canvas);//drawPosText 按照既定点 绘制文本内容

        Pic(canvas);//绘制图片

    }

//  drawCircle 绘制圆形
    public void Dcircle(Canvas canvas){
        /*四个参数：
                参数一：圆心的x坐标
                参数二：圆心的y坐标
                参数三：圆的半径
                参数四：定义好的画笔
                */
        canvas.drawCircle(60, 60, 50, paint);
    }

//drawRect 绘制矩形
    public void Drect(Canvas canvas){
        /*五个参数：
                参数一：矩形距离父view左边距离
                参数二：矩形距离父view上边距离
                参数三：矩形距离父view左边距离
                参数四：矩形距离父view上边距离
                参数五：定义好的画笔
                */
        canvas.drawRect(120, 10, 220, 110, paint);// drawRect 绘制矩形
    }
    //drawOval 绘制椭圆
    public void Doval(Canvas canvas){
        /*四个参数：
                参数一：矩形距离父view左边距离
                参数二：矩形距离父view上边距离
                参数三：矩形距离父view左边距离
                参数四：矩形距离父view上边距离
                */
        //定义一个矩形区域
        RectF oval = new RectF(230, 10, 380, 110);
        //矩形区域内切椭圆
        canvas.drawOval(oval, paint);// drawOval 绘制椭圆
    }
    //drawPosText绘制文字
    public void DposText(Canvas canvas){

        canvas.drawPosText("Android", new float[]
                {10,120,//第一个字母在坐标10,10
                 20,120,//第二个字母在坐标20,20
                 30,120, //....
                 40,120,
                 50,120,
                 60,120,
                 70,120
                }, paint);
    }

    //绘制图片
    public void Pic(Canvas canvas){
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.item_a);

        // 将画布坐标系移动到画布中央
         canvas.translate(getWidth()/2,getHeight()/2); // 指定图片绘制区域(左上角的四分之一)
         Rect src = new Rect(0,0,bitmap.getWidth(),bitmap.getHeight()); // 指定图片在屏幕上显示的区域  图片 >>原矩形
         Rect dst = new Rect(0,0,200,200); // 绘制图片  屏幕 >>目标矩形
        canvas.drawBitmap(bitmap, src,dst, paint);

       // canvas.drawBitmap(bitmap, 50,160, paint);
    }


}

activity_main.xml代码

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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"
    tools:context=".MainActivity">

    <cc.willread.www.viewapp.circleImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</android.support.constraint.ConstraintLayout>

https://www.willread.cc/p/5be3e688b8bfe