There is a small improvement that should prevent showing a wrong bitmap in the recycled view

[Vitaly-Olshevsky]

Hi,

Sometimes when I scroll a ListView, wrong images appear in recycled views. These images appear just for a moment until actual images are loaded. It happens quite rarely, but it is still very irritating.

Here is the simple scheme of what is actually happening in the code:

1. Some image is completely loaded in a background thread, it is checked for actuality and a new DisplayBitmapTask is posted to the UI.
2. Some views are recycled. Among them is a view that is still linked with the image from item 1. At this point the image becomes not actual.
3. The DisplayBitmapTask from item 1 is finally executed and unfortunately it shows the wrong image.

So I added a simple actuality check into the DisplayBitmapTask. Therefore there is no single chance for an inactual image to be shown. Please review this fix and apply it if you want.

[nostra13]

But there is still exists possibility that task becomes not actual after

if (memoryCacheKey.equals(currentCacheKey)) {

but before bitmap was set into ImageView in BitmapDisplayer :) Especially if BitmapDisplayer.display(...) do some long work before imageView.setImage... (like RoundedBitmapDisplayer).
But I agree it can reduce unexpected images displaying count.

Did you mention in your app that this improvement helps?

[Vitaly-Olshevsky]

But there is still exists possibility that task becomes not actual after

if (memoryCacheKey.equals(currentCacheKey)) {

but before bitmap was set into ImageView in BitmapDisplayer :) Especially if BitmapDisplayer.display(...) do some long work before imageView.setImage... (like RoundedBitmapDisplayer).

No, there is no such a possibility unless a client starts a new background thread in BitmapDisplayer.display(...). But it is a client's synchronization problem, so it doesn't matter here. At least, a client may be sure that parameters of the display(...) method are still actual at this point.

I'm telling that there is no such a possibility, because BitmapDisplayer.display(...) runs in the UI thread as well as an adapter recycles its views in this thread. There is no chance for actions to overlap.

Did you mention in your app that this improvement helps?

Yeah, I've checked that this improvement helps. I've reproduced the issue on the example app as well, but only with logging. It is not that noticeable in the example.

[nostra13]

I'm telling that there is no such a possibility, because BitmapDisplayer.display(...) runs in the UI thread as well as an adapter recycles its views in this thread. There is no chance for actions to overlap.

You're right :) Forgot that BitmapDisplayer runs on UI thread.
Thank you, Vitaly, for this improvement.