package com.dc.customview.view;

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 android.view.WindowManager;

public class DrawCircle extends View {
 //圓的初始位置
 private int x = 100;
 private int y = 100;
 Context context;
 
 /**
  * 有style資源文件時(shí)調(diào)用
  * @param context
  * @param attrs
  * @param defStyle
  */
 public DrawCircle(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  this.context = context;
 }
 /**
  * xml創(chuàng)建時(shí)調(diào)用
  * @param context
  * @param attrs
  */
 public DrawCircle(Context context, AttributeSet attrs) {
  super(context, attrs);
  this.context = context;
 }
 /**
  * java代碼創(chuàng)建時(shí)調(diào)用
  * @param context
  */
 public DrawCircle(Context context) {
  super(context);
  this.context = context;
 }
 
 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  // 畫筆
  Paint paint = new Paint();
  paint.setColor(Color.RED);

  //繪制圓 
  //cx :圓心的x坐標(biāo)
  //cy :圓心的y坐標(biāo) 
  //radius ：圓的半徑
  //paint ：畫筆
  canvas.drawCircle(x, y, 20, paint);
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
  switch (event.getAction()) {
   case MotionEvent.ACTION_DOWN:

   case MotionEvent.ACTION_MOVE:

   case MotionEvent.ACTION_UP:
    // 獲取當(dāng)前觸摸點(diǎn)的x,y坐標(biāo)
     
     x = (int) event.getX();
     y = (int) event.getY();
    

    break;
  }
  //獲取屏幕寬高
  WindowManager manager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
  int width = manager.getDefaultDisplay().getWidth();
  int heigh = manager.getDefaultDisplay().getHeight();
  
  //重新繪制圓 ,控制小球不會被移出屏幕
  if(x>=20 && y>=20 && x<=width-20 && y<=heigh-90){
   invalidate();
  }
   // 自己處理觸摸事件
   return true;
  }
 
}

<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:id="@+id/rl">
 
 <!-- 自定義控件的全類名 -->
 <com.dc.customview.view.DrawCircle
  android:id="@+id/circle"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content" >
 </com.dc.customview.view.DrawCircle>

</RelativeLayout>

package com.dc.customview;

import com.dc.customview.view.DrawCircle;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.RelativeLayout;

public class MainActivity extends Activity {

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  
  //獲取容器
  RelativeLayout container = (RelativeLayout) findViewById(R.id.rl);
  
  //創(chuàng)建自定義控件
  DrawCircle circle = new DrawCircle(this);
  
  //添加到容器
  container.addView(circle);
 }

}