title: ImageView: automatically recycle bitmap if ImageView is not visible (within ScrollView)
body: <p>So, I've been looking at the ImageView source for a while, but I haven't figured out a hook to do this yet. </p>
  
  <p>The problem: having, let's say, 30 400x800 images inside a ScrollView (the number of images is variable). Since they fit exactly in the screen they will take 1.3 MB of RAM each. </p>
  
  <p>What I want: to be able to load/unload bitmaps for the ImageViews that are currently visible inside a ScrollView. If the user scrolls and the bitmap is no longer visible  (within a distance threshold) the bitmap should be recycled so that the memory can be used by other bitmaps in the same ScrollView. I'm doing downsampling and all that, so no worries there. Bonus points if you do this by only extending ImageView (I would like to not mess with the ScrollView if possible).</p>
  
  <p>Summary: I can make the images load only after the ImageView becomes visible (using a clever trick), but I don't know when to unload them.</p>
  
  <p>Notes: I can't do this with a ListView due to other usability reasons.</p>