CODEWITHSUNDEEP
androidlistview
JPvdMerwe
JPvdMerwe

Reputation: 3353

ListView asynchronous image loading strategy

I currently have a ListView with a custom adapter that gets information describing the content of the rows asynchronously. Part of each row is an image URL, that I'm planning to download asynchronously and then display.

My current plan for a strategy to download these images is:

I am also planning to kill pending downloads when the Activity object owning the ListView's onDestroy()-method (Or possibly even in the onPause()-method) is called, but most importantly I want to kill the download of pending images when the row goes off screen. I might only actually cancel the download after a short delay, so it can be resumed without wasting bandwidth if the row comes on-screen quickly again.

I, however, am unsure about a few things:

Also any comments on the whole strategy would be appreciated.

Upvotes: 8

Views: 3695

Answers (3)

Travis Biehn
Travis Biehn

Reputation: 116

I use the getFirstVisible and getLastVisible AdapterView properties to detect the visible rows, and put requests in a fixed size stack.

My project is open source and has a most permissive license, if you want to use it: https://github.com/tbiehn/Android-Adapter-Image-Loader

-Travis

Upvotes: 3

Jarek Potiuk
Jarek Potiuk

Reputation: 20047

I don't think calling notifyDataSetChanged() is really needed... I would do it like that:

  • store URL as Tag in the view when created/updated
  • register a listener in downloader thread (async task???) for download keeping reference to the view and the URL
  • whenever image is downloaded asynchronously, I check TAG in the view and if it matches - i would update the ImageView (important to do it in UI thread, but when using async task, it is given). The image should also be stored on SD card (and every time you request URL you should check if it is not already downloaded).
  • every time when getView() reuses the view (passed view is not empty) I would check the Tag (old URL), replace it with the new URL and cancel the download of the oldURL.

I think it would be pretty much it (some corner cases might happen)...

Upvotes: 8

Cody Caughlan
Cody Caughlan

Reputation: 32748

I found the remote resource managing / fetching in the Foursquared source code to be pretty helpful:

http://code.google.com/p/foursquared/source/browse/main/src/com/joelapenna/foursquared/util/RemoteResourceManager.java

It caches images on disk and handles all 3 of your feature requests. See an adapter for how to use it.

As for canceling a download when a row goes off screen you'll have to handle that yourself

Upvotes: 0

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