Android Google maps - custom tile overlay

Been working on a custom tile overlay implementation for an android app. I'm having trouble with threading the image downloads in the background.

Quick overview: I have a class (MapOverlay) that extends Overlay. The draw method basically calculates the images (bitmaps) needed and the x,y position to draw them on the screen. I then have an AsyncTask that loops through each tiles and download the images if it doesn't exist on the storage card (local caching) then once the AsyncTask has completed I then loop through the images and draw them to the canvas object that is passed into the draw method of the MapOverlay. But the bitmaps are never getting drawn.

I have done some 开发者_开发问答rudimentary debugging by logging the width and the height of the canvas element at the time the AsyncTask completes but it is returning 0 for them so I'm guessing that the canvas object is no longer available to the code running in AsyncTask's onPostExecute() after the draw method has completed

Without the threading I had this working pretty well, except for the "freezing" while panning while the image downloading blocks the UI thread. Now I'm totally suck trying to get threading to work

Have you tried calling postInvalidate() when the AsyncTask done downloading? That way the call in MapView.draw() will get and draw the downloaded overlay (that is if your caching system works).