How can I fix this touch event / draw loop "deadlock"?

Question

Just want to start out by saying this seems like a great site, hope you guys can help!

I'm trying to use the structure laid out in LunarLander to create a simple game in which the user can drag some bitmaps around on the screen (the actual game is more complex, but that's not important). I ripped out the irrelevant parts of LanderLander, and set up my own bitmap drawing, something like

BoardThread (an inner class of BoardView):
run()
{
  while(mRun)
  {
    canvas = lockSurfaceHolder...
    syncronized(mSurfaceHolder)
    {
      /* drawStuff using member position fields
         in BoardView */
    }
    unlockSurfaceHolder
  }
}

My drawStuff simply walks through some arrays and throws bitmaps onto the canvas. All that works fine. Then I wanted to start handling touch events so that when the user presses a bitmap, it is selected, when the user unpresses a bitmap, it is deselected, and if a bitmap is selected during a touch move event, the bitmap is dragged. I did this stuff by listening for touch events in the BoardView's parent, BoardActivity, and passing them down into the BoardView. Something like

In BoardView
handleTouchEvent(MotionEvent e)
{
  synchronized(mSurfaceHolder)
  {
    /* Modify shared member fields in BoardView
       so BoardThread can render the bitmaps */
  }
}

This ALSO works fine. I can drag my tiles around the screen no problem.

However, every once in a while, when the app first starts up and I trigger my first touch event, the handleTouchEvent stops executing at the synchronized line (as viewed in DDMS). The drawing loop is active during this time (I can tell because a timer changes onscreen), and it usually takes several seconds or more before a bunch of touch events come through the pipeline and everything is fine again.

This doesn't seem like deadlock to me, since the draw loop is constantly going in and out of its syncronized block. Shouldn't this allow the event handling thread to grab a lock on mSurfaceHolder? What's going on here? Anyone have suggestions for improving how I've structured this?

Some other info. This "hang" only ever occurs on first touch event after activity start. This includes on orientation change after restoreState has been called. Also, I can remove EVERYTHING within the syncronized block in the event handler, and it will still get hung up at the syncronized call.

Thanks!

Answer 1

So Rob Green from rbgrn helped me get a little more info on this. I ended up using a concurrent queue to deliver touch events to my game thread, but I still had hanging issues when saveInstanceState got called, so I'm now calling Thread.sleep(16) at the bottom of every iteration of my game thread's while loop. Full discussion here.