android - What does postInvalidate() do?

Question

What is the use of the postInvalidate() function in Android? I have been seeing this function in a lot of places. When I Googled, I came out with this much:

postInvalidate-- post an invalidate request on the UI-thread

I don't know what the phrase "invalidate request" there means. Can someone explain in detail what is happening here?

Answer 1

Each class which is derived from the View class has the invalidate and the postInvalidate method. If invalidate gets called it tells the system that the current view has changed and it should be redrawn as soon as possible. As this method can only be called from your UIThread another method is needed for when you are not in the UIThread and still want to notify the system that your View has been changed. The postInvalidate method notifies the system from a non-UIThread and the View gets redrawn in the next eventloop on the UIThread as soon as possible. It is also briefly explained in the SDK documentation.

Just compare invalidate and postInvalidate.