Sunday, May 30, 2010

The limitations of AsyncTask

AsyncTask is a fine API, it's been said that it "holds your hand", and makes performing background operations painless. It pulls this off so well in fact, that I see people overusing it in situations where it's not really appropriate.

It's particularly unsuited for situations when you have a multiple tasks to perform concurrently. Imagine an Activity that needs to download about 30 small images from a remote server, and update the UI as these become available. AsyncTask uses a static internal work queue with a hard-coded limit of 10 elements. That means if you were to create an AsyncTask instance for each image, the work queue would quickly overflow and many of your tasks would get rejected. The best solution in this case is to create your own ThreadPoolExecutor instance that uses a queue that's large enough to hold all your tasks, if you need an unbounded queue, a LinkedBlockingQueue will work just fine.

Another severe limitation is that an AsyncTask can't survive your Activity being torn down and recreated on the other side.  Even if you pass it to the new instance via onRetainNonConfigurationState, the internal Handler inside the AsyncTask is still going to be stale and it's not going to behave correctly.  This is important to consider, and the Android documentation makes no mention of it all.  I've already blogged this scenario in detail, so I won't flog a dead horse here.

There are a few other minor issues, such as the fact that you can't change the background threads execution priority. It's hard-coded to a low priority, which granted, is the sensible default.  Also, exception handling is not very well supported.

So just to reiterate, AsyncTask is a nice API, but you should understand it's limitations and apply it appropriately.  If you're serious about writing mobile apps, then you're going to need a few more tools in the toolbox to get the job done.

Saturday, May 22, 2010

Android 2.2 Highlights, A developer's perspective

Android 2.2 is the most significant upgrade to the Android OS that I can remember. Chock full of new goodies for both users and developers.

I thought I'd just do a quickie post to highlight the features for developers I'm most interested in.

1) Android Cloud to Device Messaging - this is one feature that's been sorely lacking from the Android platform and it's a welcomed addition. The demo at Google IO demonstrated how you can push a message to a device in real time, and have it fire off an Intent which can kick off any number of interesting and yet to be dreamed of services. This also opens up a new way for devices to communicate with each other in real time over the internet, I expect interesting things to come of this.

2) Android Application Error Reports - logging exceptions remotely that occur on a users device is a very useful thing and every mobile company I've worked at has done this in some form or another. The typically way to do it is to override the default exception handler and replace it with something that will log the exception. I myself just started building a new  GAE project for this a few weeks ago The initial usefulness of this will be somewhat limited, due to the fact that many devices are going to be running older versions of android for the foreseeable future.

3) Stagefright media framework - I don't know much about Stagefright, I'll admit that right off the bat, what I do know is that I'm dreadfully tired with the limitations of the current MediaPlayer framework, and I'm really hopeful this can serve as an improvement / replacement for the API's we've been using up til now.

And lastly, I'd also like to mention the new support for Android Library Projects that are available in the newest SDK tools (SDK Tools r6). We will finally be able to share common resources between multiple projects, and since it's just build time magic, these new library projects are going to be backwards compatible with older platform versions.

Wrapping up, I'm really excited about android 2.2.  Google has really raised the bar with this release, and it's going to be interesting to see how other mobile technologies evolve in response to this.