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 {
  //圆的初始位置
  private int x = 100;
  private int y = 100;
  Context context;
  
  /**
   * 有style资源文件时调用
   * @param context
   * @param attrs
   * @param defStyle
   */
  public DrawCircle(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    this.context = context;
  }
  /**
   * xml创建时调用
   * @param context
   * @param attrs
   */
  public DrawCircle(Context context, AttributeSet attrs) {
    super(context, attrs);
    this.context = context;
  }
  /**
   * java代码创建时调用
   * @param context
   */
  public DrawCircle(Context context) {
    super(context);
    this.context = context;
  }
  
  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    // 画笔
    Paint paint = new Paint();
    paint.setColor(Color.RED);

    //绘制圆  
    //cx :圆心的x坐标
    //cy :圆心的y坐标  
    //radius ：圆的半径
    //paint ：画笔
    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:
        // 获取当前触摸点的x,y坐标
          
          x = (int) event.getX();
          y = (int) event.getY();
        

        break;
    }
    //获取屏幕宽高
    WindowManager manager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
    int width = manager.getDefaultDisplay().getWidth();
    int heigh = manager.getDefaultDisplay().getHeight();
    
    //重新绘制圆 ,控制小球不会被移出屏幕
    if(x>=20 && y>=20 && x<=width-20 && y<=heigh-90){
      invalidate();
    }
      // 自己处理触摸事件
      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">
  
  <!-- 自定义控件的全类名 -->
  <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);
    
    //获取容器
    RelativeLayout container = (RelativeLayout) findViewById(R.id.rl);
    
    //创建自定义控件
    DrawCircle circle = new DrawCircle(this);
    
    //添加到容器
    container.addView(circle);
  }

}