invalidate() and postInvalidate()

/**
* Invalidate the whole view. If the view is
visible,
* {@link #onDraw(android.graphics.Canvas)} will be
called at some point in
* the future. This must be called
from a UI thread. To call from a non-UI thread,
* call
{@link #postInvalidate()}.
*/

invalidate():This must be called from a UI thread

postInvalidate():This must be called from a non-UI
thread