surfaceview

/**
* Provides a dedicated drawing surface embedded inside of a view hierarchy.
* You can control the format of this surface and, if you like, its size; the
* SurfaceView takes care of placing the surface at the correct location on the
* screen
*
* <p>The surface is Z ordered so that it is behind the window holding its
* SurfaceView; the SurfaceView punches a hole in its window to allow its
* surface to be displayed. The view hierarchy will take care of correctly
* compositing with the Surface any siblings of the SurfaceView that would
* normally appear on top of it. This can be used to place overlays such as
* buttons on top of the Surface, though note however that it can have an
* impact on performance since a full alpha-blended composite will be performed
* each time the Surface changes.
*
* <p> The transparent region that makes the surface visible is based on the
* layout positions in the view hierarchy. If the post-layout transform
* properties are used to draw a sibling view on top of the SurfaceView, the
* view may not be properly composited with the surface.
*
* <p>Access to the underlying surface is provided via the SurfaceHolder interface,
* which can be retrieved by calling {@link #getHolder}.
*
* <p>The Surface will be created for you while the SurfaceView's window is
* visible; you should implement {@link SurfaceHolder.Callback#surfaceCreated}
* and {@link SurfaceHolder.Callback#surfaceDestroyed} to discover when the
* Surface is created and destroyed as the window is shown and hidden.
*
* <p>One of the purposes of this class is to provide a surface in which a
* secondary thread can render into the screen. If you are going to use it
* this way, you need to be aware of some threading semantics:
*
* <ul>
* <li> All SurfaceView and
* {@link SurfaceHolder.Callback SurfaceHolder.Callback} methods will be called
* from the thread running the SurfaceView's window (typically the main thread
* of the application). They thus need to correctly synchronize with any
* state that is also touched by the drawing thread.
* <li> You must ensure that the drawing thread only touches the underlying
* Surface while it is valid -- between
* {@link SurfaceHolder.Callback#surfaceCreated SurfaceHolder.Callback.surfaceCreated()}
* and
* {@link SurfaceHolder.Callback#surfaceDestroyed SurfaceHolder.Callback.surfaceDestroyed()}.
* </ul>
*/


   @Override
    public void draw(Canvas canvas) {
        if (mWindowType != WindowManager.LayoutParams.TYPE_APPLICATION_PANEL) {
            // draw() is not called when SKIP_DRAW is set
            if ((mPrivateFlags & PFLAG_SKIP_DRAW) == 0) {
                // punch a whole in the view-hierarchy below us
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }
        }
        super.draw(canvas);
    }