Using MySurfaceView with main.xml (android)

i have a simple program on android which is drawing on user touch.

here's main activity:

public class DrawingActivity extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    //setContentView(new MySurfaceView(this)); - this way it works, but 
                                          // i'm interseted in previous
}
}

this is main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">

<com.shchurov.MySurfaceView  
android:layout_width="wrap_content" 
android:layout_height="wrap_content"
android:id="@+id/msv" 
/>
</LinearLayout>

and here's MySurfaceView class:

public class MySurfaceView extends SurfaceView 
            implements SurfaceHolder.Callback {

private SurfaceHolder surfaceHolder;
Canvas canvas=null;
Paint paint=new Paint();


public MySurfaceView(Context context) {
    super(context);
    getHolder().addCallback(this);
    paint.setColor(Color.GREEN);
    paint.setStyle(Style.FILL);
}

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

@Override
public void surfaceCreated(SurfaceHolder holder) {
    this.surfaceHolder=holder;
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
}

public boolean onTouchEvent(MotionEvent event)
{
    canvas=surfaceHolder.lockCanvas(null);
    canvas.drawCircle(event.getX(), event.getY(), 10, paint);
    surfaceHolder.unlockCanvasAndPost(canvas);
    return true;
}

}

packages are right, but for some reason it doesn't work, can anyone help?

Update: if i change MySurfaceView to this for example, then it works:

public class MySurfaceView extends View 
{
Paint paint;
public SomeView(Context context, AttributeSet attrs)
{
    super(context, attrs);
    paint = new Paint();
    paint.setColor(Color.YELLOW);
    paint.setStyle(Style.FILL);
}

@Override
protected void onDraw(Canvas canvas)
{
    canvas.drawCircle(20, 20, 20, paint);
}
}

回答1:

layout_width="fill_parent"

layout_height="fill_parent"

within the surfaceview tag, would solve your problem.

回答2:

You need to/should implement all three constructors a view can have:

//Simple constructor to use when creating a view from code.
View(Context context)

//Constructor that is called when inflating a view from XML.
View(Context context, AttributeSet attrs)

//Perform inflation from XML and apply a class-specific base style.
View(Context context, AttributeSet attrs, int defStyle)

This was just copied from the View class constructor details in the documentation.