重力感应控制画布小球问题

import android.app.Activity;

import android.content.Context;

import android.content.pm.ActivityInfo;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.hardware.Sensor;

import android.hardware.SensorEvent;

import android.hardware.SensorEventListener;

import android.hardware.SensorManager;

import android.os.Bundle;

import android.view.SurfaceHolder;

import android.view.SurfaceView;

import android.view.Window;

import android.view.WindowManager;

import android.view.SurfaceHolder.Callback;

public class SurfaceViewAcitvity extends Activity {

MyView mAnimView = null;  

@Override  
public void onCreate(Bundle savedInstanceState) {  
super.onCreate(savedInstanceState);  
// 全屏显示窗口  
requestWindowFeature(Window.FEATURE_NO_TITLE);  
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,  
    WindowManager.LayoutParams.FLAG_FULLSCREEN);  
//强制横屏   
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);  

// 显示自定义的游戏View  
mAnimView = new MyView(this);  
setContentView(mAnimView);  
}  

public class MyView extends SurfaceView implements Callback,Runnable ,SensorEventListener{  

 /**每50帧刷新一次屏幕**/    
public static final int TIME_IN_FRAME = 50;   

/** 游戏画笔 **/  
Paint mPaint = null;  
Paint mTextPaint = null;  
SurfaceHolder mSurfaceHolder = null;  

/** 控制游戏更新循环 **/  
boolean mRunning = false;  

/** 游戏画布 **/  
Canvas mCanvas = null;  

/**控制游戏循环**/  
boolean mIsRunning = false;  

/**SensorManager管理器**/  
private SensorManager mSensorMgr = null;      
Sensor mSensor = null;      

/**手机屏幕宽高**/  
int mScreenWidth = 0;  
int mScreenHeight = 0;  

/**小球资源文件越界区域**/  
private int mScreenBallWidth = 0;  
private int mScreenBallHeight = 0;  

/**游戏背景文件**/  
private Bitmap mbitmapBg;  

/**小球资源文件**/  
private Bitmap mbitmapBall;  

/**小球的坐标位置**/  
private float mPosX = 200;  
private float mPosY = 0;  

/**重力感应X轴 Y轴 Z轴的重力值**/  
private float mGX = 0;  
private float mGY = 0;  
private float mGZ = 0;  

public MyView(Context context) {  
    super(context);  
    /** 设置当前View拥有控制焦点 **/  
    this.setFocusable(true);  
    /** 设置当前View拥有触摸事件 **/  
    this.setFocusableInTouchMode(true);  
    /** 拿到SurfaceHolder对象 **/  
    mSurfaceHolder = this.getHolder();  
    /** 将mSurfaceHolder添加到Callback回调函数中 **/  
    mSurfaceHolder.addCallback(this);  
    /** 创建画布 **/  
    mCanvas = new Canvas();  
    /** 创建曲线画笔 **/  
    mPaint = new Paint();  
    mPaint.setColor(Color.WHITE);  
    /**加载小球资源**/  
    mbitmapBall = BitmapFactory.decodeResource(this.getResources(), R.drawable.ball);  
    /**加载游戏背景**/  
    mbitmapBg = BitmapFactory.decodeResource(this.getResources(), R.drawable.bg);  

    /**得到SensorManager对象**/  
    mSensorMgr = (SensorManager) getSystemService(SENSOR_SERVICE);     
    mSensor = mSensorMgr.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);     
    // 注册listener，第三个参数是检测的精确度    
        //SENSOR_DELAY_FASTEST 最灵敏 因为太快了没必要使用  
        //SENSOR_DELAY_GAME    游戏开发中使用  
        //SENSOR_DELAY_NORMAL  正常速度  
        //SENSOR_DELAY_UI          最慢的速度  
    mSensorMgr.registerListener(this, mSensor, SensorManager.SENSOR_DELAY_GAME);    
}  

private void Draw() {  

    /**绘制游戏背景**/  
    mCanvas.drawBitmap(mbitmapBg,0,0, mPaint);  
    /**绘制小球**/  
    mCanvas.drawBitmap(mbitmapBall, mPosX,mPosY, mPaint);  
    /**X轴 Y轴 Z轴的重力值**/  
    mCanvas.drawText("X轴重力值 ：" + mGX, 0, 20, mPaint);  
    mCanvas.drawText("Y轴重力值 ：" + mGY, 0, 40, mPaint);  
    mCanvas.drawText("Z轴重力值 ：" + mGZ, 0, 60, mPaint);  
}  

@Override  
public void surfaceChanged(SurfaceHolder holder, int format, int width,  
    int height) {  

}  

@Override  
public void surfaceCreated(SurfaceHolder holder) {  
    /**开始游戏主循环线程**/  
    mIsRunning = true;  
    new Thread(this).start();  
    /**得到当前屏幕宽高**/  
    mScreenWidth = this.getWidth();  
    mScreenHeight = this.getHeight();  

    /**得到小球越界区域**/  
    mScreenBallWidth = mScreenWidth - mbitmapBall.getWidth();  
    mScreenBallHeight = mScreenHeight - mbitmapBall.getHeight();  
}  

@Override  
public void surfaceDestroyed(SurfaceHolder holder) {  
    mIsRunning = false;  
}  

@Override  
public void run() {  
    while (mIsRunning) {  

    /** 取得更新游戏之前的时间 **/  
    long startTime = System.currentTimeMillis();  

    /** 在这里加上线程安全锁 **/  
    synchronized (mSurfaceHolder) {  
        /** 拿到当前画布 然后锁定 **/  
        mCanvas = mSurfaceHolder.lockCanvas();  
        Draw();  
        /** 绘制结束后解锁显示在屏幕上 **/  
        mSurfaceHolder.unlockCanvasAndPost(mCanvas);  
    }  

    /** 取得更新游戏结束的时间 **/  
    long endTime = System.currentTimeMillis();  

    /** 计算出游戏一次更新的毫秒数 **/  
    int diffTime = (int) (endTime - startTime);  

    /** 确保每次更新时间为50帧 **/  
    while (diffTime <= TIME_IN_FRAME) {  
        diffTime = (int) (System.currentTimeMillis() - startTime);  
        /** 线程等待 **/  
        Thread.yield();  
    }  

    }  

}  

@Override  
public void onAccuracyChanged(Sensor arg0, int arg1) {  
    // TODO Auto-generated method stub  

}  

@Override  
public void onSensorChanged(SensorEvent event) {  
    mGX = event.values[SensorManager.DATA_X];  
    mGY= event.values[SensorManager.DATA_Y];  
    mGZ = event.values[SensorManager.DATA_Z];  

    //这里乘以2是为了让小球移动的更快  
    mPosX -= mGX * 2;  
    mPosY += mGY * 2;  

    //检测小球是否超出边界  
    if (mPosX < 0) {  
    mPosX = 0;  
    } else if (mPosX > mScreenBallWidth) {  
    mPosX = mScreenBallWidth;  
    }  
    if (mPosY < 0) {  
    mPosY = 0;  
    } else if (mPosY > mScreenBallHeight) {  
    mPosY = mScreenBallHeight;  
    }  
}  
}  

}

这时用重力感应器控制小球移动的一个demo，但是现在我想自己画一个圆充当小球，而不是用图片替代，我该怎么改这段代码？求指教！

主要是修改Draw()方法，drawBitmap(...)需要改为drawCircle(...)
以下是修改的代码，仅供参考：

import android.app.Activity;

import android.content.Context;

import android.content.pm.ActivityInfo;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.hardware.Sensor;

import android.hardware.SensorEvent;

import android.hardware.SensorEventListener;

import android.hardware.SensorManager;

import android.os.Bundle;

import android.view.SurfaceHolder;

import android.view.SurfaceView;

import android.view.Window;

import android.view.WindowManager;

import android.view.SurfaceHolder.Callback;

public class SurfaceViewAcitvity extends Activity {
MyView mAnimView = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

// 全屏显示窗口
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN);
//强制横屏
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

// 显示自定义的游戏View
mAnimView = new MyView(this);
setContentView(mAnimView);
}

    public class MyView extends SurfaceView implements Callback,Runnable ,SensorEventListener{

        /**每50帧刷新一次屏幕**/
        public static final int TIME_IN_FRAME = 50;

        /** 游戏画笔 **/
        Paint mPaint = null;
        Paint mTextPaint = null;
        SurfaceHolder mSurfaceHolder = null;

        /** 控制游戏更新循环 **/
        boolean mRunning = false;

        /** 游戏画布 **/
        Canvas mCanvas = null;

        /**控制游戏循环**/
        boolean mIsRunning = false;

        /**SensorManager管理器**/
        private SensorManager mSensorMgr = null;
        Sensor mSensor = null;

        /**手机屏幕宽高**/
        int mScreenWidth = 0;
        int mScreenHeight = 0;

        /**小球资源文件越界区域**/
        private int mScreenBallWidth = 0;
        private int mScreenBallHeight = 0;

        /**游戏背景文件**/
        //private Bitmap mbitmapBg;

        private int mCircleRadius = 48;

        private int mCircleColor = Color.CYAN;
        private Paint mCirclePaint = new Paint();

        private int mBackgroundColor = Color.DKGRAY;

        /**小球资源文件**/
       // private Bitmap mbitmapBall;

        /**小球的坐标位置**/
        private float mPosX = 200;
        private float mPosY = 0;

        /**重力感应X轴 Y轴 Z轴的重力值**/
        private float mGX = 0;
        private float mGY = 0;
        private float mGZ = 0;



        public MyView(Context context) {
            super(context);
            /** 设置当前View拥有控制焦点 **/
            this.setFocusable(true);
            /** 设置当前View拥有触摸事件 **/
            this.setFocusableInTouchMode(true);
            /** 拿到SurfaceHolder对象 **/
            mSurfaceHolder = this.getHolder();
            /** 将mSurfaceHolder添加到Callback回调函数中 **/
            mSurfaceHolder.addCallback(this);
            /** 创建画布 **/
            mCanvas = new Canvas();
            /** 创建曲线画笔 **/
            mPaint = new Paint();
            mPaint.setColor(Color.WHITE);

            mCirclePaint.setColor(mCircleColor);
            /**加载小球资源**/
            //mbitmapBall = BitmapFactory.decodeResource(this.getResources(), R.mipmap.ic_launcher);
            /**加载游戏背景**/
           // mbitmapBg = BitmapFactory.decodeResource(this.getResources(),  R.drawable.switch_thumb_pressed_holo_light);

            /**得到SensorManager对象**/
            mSensorMgr = (SensorManager) getSystemService(SENSOR_SERVICE);
            mSensor = mSensorMgr.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
            // 注册listener，第三个参数是检测的精确度
            //SENSOR_DELAY_FASTEST 最灵敏 因为太快了没必要使用
            //SENSOR_DELAY_GAME    游戏开发中使用
            //SENSOR_DELAY_NORMAL  正常速度
            //SENSOR_DELAY_UI          最慢的速度
            mSensorMgr.registerListener(this, mSensor, SensorManager.SENSOR_DELAY_GAME);
        }

        private void Draw() {

            /**绘制游戏背景**/
            //mCanvas.drawBitmap(mbitmapBg,0,0, mPaint);
            mCanvas.drawColor(mBackgroundColor);
            /**绘制小球**/
            mCanvas.drawCircle(mPosX,mPosY,mCircleRadius,mCirclePaint);
            //mCanvas.drawBitmap(mbitmapBall, mPosX,mPosY, mPaint);
            /**X轴 Y轴 Z轴的重力值**/
            mCanvas.drawText("X轴重力值 ：" + mGX, 0, 20, mPaint);
            mCanvas.drawText("Y轴重力值 ：" + mGY, 0, 40, mPaint);
            mCanvas.drawText("Z轴重力值 ：" + mGZ, 0, 60, mPaint);
        }

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                                   int height) {

        }

        @Override
        public void surfaceCreated(SurfaceHolder holder) {
            /**开始游戏主循环线程**/
            mIsRunning = true;
            new Thread(this).start();
            /**得到当前屏幕宽高**/
            mScreenWidth = this.getWidth();
            mScreenHeight = this.getHeight();

            /**得到小球越界区域**/

/* mScreenBallWidth = mScreenWidth - mbitmapBall.getWidth();
mScreenBallHeight = mScreenHeight - mbitmapBall.getHeight();*/
mScreenBallWidth = mScreenWidth - mCircleRadius;
mScreenBallHeight = mScreenHeight - mCircleRadius;
}

        @Override
        public void surfaceDestroyed(SurfaceHolder holder) {
            mIsRunning = false;
        }

        @Override
        public void run() {
            while (mIsRunning) {

                /** 取得更新游戏之前的时间 **/
                long startTime = System.currentTimeMillis();

                /** 在这里加上线程安全锁 **/
                synchronized (mSurfaceHolder) {
                    /** 拿到当前画布 然后锁定 **/
                    mCanvas = mSurfaceHolder.lockCanvas();
                    Draw();
                    /** 绘制结束后解锁显示在屏幕上 **/
                    mSurfaceHolder.unlockCanvasAndPost(mCanvas);
                }

                /** 取得更新游戏结束的时间 **/
                long endTime = System.currentTimeMillis();

                /** 计算出游戏一次更新的毫秒数 **/
                int diffTime = (int) (endTime - startTime);

                /** 确保每次更新时间为50帧 **/
                while (diffTime <= TIME_IN_FRAME) {
                    diffTime = (int) (System.currentTimeMillis() - startTime);
                    /** 线程等待 **/
                    Thread.yield();
                }

            }

        }

        @Override
        public void onAccuracyChanged(Sensor arg0, int arg1) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onSensorChanged(SensorEvent event) {
            mGX = event.values[SensorManager.DATA_X];
            mGY= event.values[SensorManager.DATA_Y];
            mGZ = event.values[SensorManager.DATA_Z];

            //这里乘以2是为了让小球移动的更快
            mPosX -= mGX * 2;
            mPosY += mGY * 2;

            //检测小球是否超出边界
            if (mPosX < mCircleRadius) {
                mPosX = mCircleRadius;
            } else if (mPosX > mScreenBallWidth) {
                mPosX = mScreenBallWidth;
            }
            if (mPosY < mCircleRadius) {
                mPosY = mCircleRadius;
            } else if (mPosY > mScreenBallHeight) {
                mPosY = mScreenBallHeight;
            }
        }
    }
}