Question

I have a rather small amount of state in my Android app that I wish to ensure is saved persistently. It’s tiny, clocking in at about 50 characters.

Looking at the Android Storage Options page, they give a few choices, the simplest which seems to be shared preferences. Now these aren’t actually application preferences, more a short indication of where the application is at.

My worry is that preferences are expected to change infrequently and the likely use case may not therefore handle a lot of changes. By a lot, I mean peaking at about once per second, depending on user activity.

So I want to make sure of two things.

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The first is that the sample code on that page linked above has the preferences being written out in the onStop method. Are we guaranteed that this will run when the application exits, no matter what?

If so, I can maintain the state in-memory and use it to save persistently, then this frequency-of-updates problem disappears.

Basically, I want to ensure there’s no way my application can stop without having saved its state (short of catastrophic failure of course).

Otherwise, I’d rather save this state whenever it changes so as not to lose anything.

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Second, and this is important only if I have to save state every time it changes, are there any downsides to saving preferences up to once per second? As mentioned, I’m not sure if it’s designed for frequent updates.

Answer 1

If the application exits normally, onStop() is called. If the application is backgrounded (ie with the home button/getting called) onPause() is called, which calls onStop(). If the application exits (ie with the back button), onPause() is called, which calls onStop(), which calles onDestroy(). However, when the application crashes (ie through a nullpointerException, onStop() is not called, the application just crashes).

Personally, I mostly use a settingsContainerObject of some sort, and just write my preferences to the internal storage in the onStop() method. Works pretty well.

In short: yes, onStop() is always called when the application is backgrounded or exit.

You can also stop the application in try/catch blocks, to always save your settings.