surfaceview组件的初始化

在activity中onCreate里，直接操作surfaceview，是无法成功的。

像以下代码：

protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.trace);

    TraceView=(SurfaceView) findViewById(R.id.TraceView);

  TraceViewHolder=TraceView.getHolder();  

   Canvas canvas = TraceViewHolder.lockCanvas(); 

canvas.drawColor(Color.WHITE);                  //设置背景颜色  
Paint p = new Paint();
 p.setStyle(Style.STROKE);
 p.setStrokeWidth(2);
 p.setColor(Color.BLUE); 
 canvas.drawLine(10, 0,10, 400, p);
if (canvas!=null)  TraceViewHolder.unlockCanvasAndPost(canvas);

}

运行完后，surfaceview里黑乎乎一片，没有想要的线条。

要添加 surfaceviewHolder的Callback函数，才能实现。代码如下:

protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.trace);

    TraceView=(SurfaceView) findViewById(R.id.TraceView);

///三个函数缺一不可。surfaceCreated， surfaceChanged，surfaceDestroyed /

TraceView.getHolder().addCallback(new Callback() { 

public void surfaceCreated(SurfaceHolder holder) { 

  Canvas canvas = holder.lockCanvas();  
canvas.drawColor(Color.WHITE);                  //设置背景颜色  
Log.i("TraceView Callback","OK");
Paint p = new Paint();
 p.setStyle(Style.STROKE);
 p.setStrokeWidth(1);
 p.setColor(Color.BLUE); 
//  canvas.drawLine(20, 0,20, 400, p);
 DrawGrid(canvas,p,10);  
if (canvas!=null)  holder.unlockCanvasAndPost(canvas);
}

public void surfaceChanged(SurfaceHolder holder, int format, int width, int height){
// Log.i("TraceView Changed Callback","OK");
}

public void surfaceDestroyed(SurfaceHolder holder){

}



}); 

/

}