Using AsyncTask to preload data for a new Activity

Question

I have my MainActivity which gives the user a selection of pages to open, all of which involve downloading some data from the internet and displaying it. To save the user waiting when they choose their page I've made an AsyncTask as a subclass of MainActivity which produces an object DATAwhen the download is complete.

How would I pass DATA on to the SecondActivity in the following circumstances:

• The user chooses the SecondActivity before the AsyncTask download has completed.
• The download completes before the user chooses the SecondActivity.

the AsyncTask doesn't have to be a sub-class of MainActivity its just been tidy to do it that way so far,

thanks for the help!

Answer 1

Here's one way to do this:

Create a reference to your data in your Application. The Android Application is a good place to store global data. Next, populate the data via your AsyncTask (Watch out for the pitfalls of using an AsyncTask). You can now access your data via a call similar to this: ((MyApplication)getApplication).mydata

As you mentioned, two scenarios can come up. Either the data has been populated, or not. To handle this, use an observer that observes changes to the data. Have SecondActivity register as an observer when the data is null. When the data is available your SecondActivity's update method will get called and you can do whatever you please with it. Finally, make sure to unregister from being an observer. Hope this helps.

Answer 2

Passing information directly between activities works only if it is Parcellable (via Intent). Almost anything could be made Parcellable but it is not always a good idea especially when the amount of data is large.

The next problem is that your AsyncTask most likely keeps the Context of your first activity alive when it is running longer than the activity lasts. Activity instances are quite often recreated when you rotate the device and naive implementations tend to start another asynctask in the new instance and end up with multiple tasks that download the same data. You would need to pass the reference of a running task between instances of the same Activity.

The simplest solution is probably to create a singleton (or a Service) accessible from both activities that hosts the AsyncTask & loads the data. If it requires a Context use getApplicationContext() since that's safe to use outside the lifetime of Activites. Activities could register themselves as listeners for "data loaded" events while they are active.

Answer 3

I've recently struggled with AsyncTask and had difficulty having the UI behave while the task was running in the background. While there are comments around that services aren't really appropriate for the sort of thing you're describing, I've found them much easier to work with. You might check intentService as a middle ground. Good tut's can be found here and, specifically concerning intentService, here.