Android 自定义View画圆

package com.jiyun.dell.myview;import android.os.Bundle;import android.support.v7.app.AppCompatActivity;import android.widget.LinearLayout;public class MainActivity extends AppCompatActivity {    private LinearLayout linear;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        //绑定控件        linear = (LinearLayout) findViewById(R.id.ll);        //创建画画的类        MyView myview=new MyView(MainActivity.this);        //进行配置        myview.invalidate();//        把画画的这个类（View）添加到布局中        linear.addView(myview);    }}

<?xml version="1.0" encoding="utf-8"?><LinearLayout    android:id="@+id/ll"    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="com.jiyun.dell.myview.MainActivity"></LinearLayout>

package com.jiyun.dell.myview;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.view.View;/** * Created by DELL zhanghuirong on 2017/8/14. */public class MyView extends View {    //这是一个参数的构造方法    public MyView(Context context) {        super(context);    }    @Override//重写画圆的方法    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        //创建画笔工具        Paint paint=new Paint();        //给画笔设置颜色        paint.setColor(Color.GREEN);//        在画布上画圆  括号里的参数，前两个是偏移量也就是离圆心的距离，50是半径，painit是画笔        canvas.drawCircle(100,100,50,paint);    }}