How to stack a canvas and a button?

I want to write the following application. There is a Canvas and a Button stacked vertically in a LinearView. When the button is pressed the first time a 开发者_运维技巧circle is drawn in the canvas, then if pressed again the circle disappears. The circle must appear centered in its space.

Any ideas?

Thanks,

JG

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This should work

Custom View class

public class DrawView extends View {
    private Canvas viewCanvas;

    public DrawView(Context context) {
        super(context);
        setLayoutParams(new LayoutParams(LinearLayout.LayoutParams.FILL_PARENT
            ,LinearLayout.LayoutParams.FILL_PARENT)); 
    }

    public DrawView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override 
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint p = new Paint();
        p.setColor(Color.RED);
        canvas.drawCricle(getWidth()/2,getHeight()/2,50,null);
        viewCanvas = canvas;
    }

    public clearCircle(){
        viewCanvas.drawColor(Color.BLACK);
    }
}

Activity class should look like this

public class KeyboardTopDemo extends Activity {
    private FrameLayout container;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.ss);

        container = (FrameLayout)findViewById(R.id.sc);        
        container.addView(new DrawView(this));
    }

    public void clearHandler(View target){
        container.getChildAt(0).clearCircle();
    }
}

This is the layout xml file

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="fill_parent" android:layout_height="fill_parent" 
    android:orientation="vertical">
    <FrameLayout android:orientation="vertical" android:layout_width="fill_parent" 
        android:layout_height="0dip" android:id="@+id/sc" 
        android:scrollbars="horizontal" android:layout_weight="1.0">
        <ImageView android:layout_width="fill_parent" 
            android:layout_height="fill_parent" android:src="@drawable/chips"/>
    </FrameLayout>
    <Button android:layout_width="fill_parent" android:layout_height="wrap_content" 
        android:text="Clear" android:onClick="clearHandler"/>     
</LinearLayout>