/**
* 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