CODEWITHSUNDEEP
androidandroid-fragments
neonDion
neonDion

Reputation: 2358

Setting data on Android retained Fragment before onCreate() has been called

I have an Activity that creates a retained Fragment in order to persist a long running network operation in the event that the Activity goes through any configuration changes during the long running operation. So basically I'm trying to use the retained Fragment like a Singleton that lives until Activity's onDestroy() is called.

My goal is to have Activity's onCreate() trigger the long running network operation because I only want it to run when the activity is created not every time it starts again (otherwise I'd put it in onStart()).

To do this I first create a retained fragment in Activity's onCreate, then use FragmentManager to add the retained fragment then I kick off the network call in Activity's onCreate method and pass the networking object to the retained Fragment to hold onto.

This works, however I'm concerned because if I log what's going on I can see that the Activity first sets data on the retained Fragment and then the retained Fragment's onCreate() method is called. This looks wrong and seems out of order, however it works.

Is it bad practice to utilize the retained Fragment instance before the fragment has run its onCreate() method?

EDIT After reading the responses and thinking a bit more about this, I'm realizing that initiating the network call from Activity onCreate() although convenient, is risky to do. As noted in the responses there's a chance that the long running operation returns very quickly and attempts to manipulate the Activity's view which may not have been initialized yet. So for my specific case I am resorting to initiating the long running operation from Activity's onStart() method and then using the retainedFragment to cache the response. This way even if onStart() is called multiple times and attempts to kick off the long running operation again, the result from the first attempt will be cached and can be returned.

Upvotes: 2

Views: 396

Answers (2)

Rafal Zawadzki
Rafal Zawadzki

Reputation: 901

As long as you are not relying on Activity/Fragment views being available while Fragment's onCreate, you will be fine:

Note that this can be called while the fragment's activity is still in the process of being created. As such, you can not rely on things like the activity's content view hierarchy being initialized at this point.

(https://developer.android.com/reference/android/app/Fragment.html#onCreate(android.os.Bundle)

Still, you need to secure possible edge case that long-running operation may be finished before Activity and Fragment are fully created (if you depend on their views, this might be an issue). Therefore, think about what you will do when long-running operation has finished and the results need to be presented somewhere.

Upvotes: 1

DeeV
DeeV

Reputation: 36035

I will admit that's unusual, but I can't think of a reason off the top of my head why it would be bad. The advantage of a retained Fragment is the reference to the Fragment is not destroyed so the references it holds on to are also retained. onCreate() and onDestroy() are called once throughout the lifetime of the Fragment (when the Fragment is added and removed respectively).

The danger might be that the asynchronous operation finishes before onCreate() is called. Likewise, the operation could finish after onDestroy() is called and when you expected the Fragment to be running. There are some methods like Fragment#setArguments() that can not be called during some parts of the Fragment's lifecycle. If you were to call these methods when you expected the Fragment to be running, then you will get an Exception thrown. So you end up having to put in a bunch of checks like if(isAttached()) { /* do this */ }. Putting the operation in onCreate() will ensure that it at least started before the operation finished.

But if you are not actually relying on any of the Fragment's functionality, then it should be fine. The lifecycles are only meant to tell you what's going on with it.

Upvotes: 2

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