Android - Issue with lazy loading images into a ListView

Question

This is a very common scenario: displaying images in a ListView which have to be downloaded from the internet.

Right now I have a custom subclass of ArrayAdapter which I use for the ListView. In my getView() implementation of the ArrayAdapter, I spawn a separate thread to load an image. After the loading is done, it looks up the appropriate ImageView and sets the image with ImageView.setImageDrawable(). So the solution I used is kind of similar to this one: http://stackoverflow.com/questions/541966/android-how-do-i-do-a-lazy-load-of-images-in-listview

The problem I'm having is that as soon as I make the call to setImageDrawable() on the ImageView, the ListView somehow refreshes all currently visible rows in the list! This results in kind of an infinite loop:

1. getView() is called
2. thread is spawned to load image
3. image is loaded; setImageDrawable() is called on ImageView
4. ListView picks it up for some reason and refreshes itself
5. For the ListView to refresh, getView() is called for each visible row, so we go back to step 1 and the whole thing repeats itself

So as far as I can see, the solution proposed in "Android - How do I do a lazy load of images in ListView" (see link above) simply doesn't work. It might look like it does, but it will run very slow because in the background, it keeps reloading the currently visible rows.

Did anyone encounter this before and/or have a solution for this?

Answer 1

In the linked solution, fetchDrawableOnThread() should only be called if the view does not already have the correct drawable.

A view does not have a drawable if getDrawable() returns null.

If you are reusing slots, you views you need to go further and manage the state. If your views have a member variable storing the URL, and a boolean to say whether it is loaded, it'd be easy to know whether to call fetchDrawableOnThread() or not, for example.

I'd speculate that the drawable's toString() detailed the path from which the image was loaded. (If it doesn't, you could subclass the drawable returned to make it so). In this case, you could avoid the boolean outlined above and just do a comparison to determine if its the right drawable or whether to fetch a replacement.

Additionally, your getView() on a visible row should ensure that those that no longer visible get unloaded, to prevent memory exhaustion. A finesse would be to move the no longer visible images to soft references (so they are unloaded when memory is needed) as another poster on the original thread noted.

Answer 2

I have a ThumbnailAdapter that wraps up this whole pattern that may help.