alwaysRetainTaskState doesn't work as expected. Singleton class is losing its state

Issue

On the first time my app is running, on the root activity, the user is required to select a certain options that determines which data would be downloaded for him from a database.

Once he had picked that option, the data is downloaded and kept on a singleton class that should hold that data as long as the application is running. (Even in the background)

My problem is that sometimes after the user exits my application for a while (using the home button), Android apparently kills some of the activities in my app, and somehow with them, it resets the important singleton class. Causing my app to receive all kinds of null reference exception once I try to access the data that is supposed to be kept on the singleton.

I understand that the Android OS can sometimes choose to kill an application, but allow the user to return to it from his last visited activity. Sometimes it just kills the whole application and forces the user to begin from the start.

I was looking for a solution, and I found out about "android:alwaysRetainTaskState" attribute that I can apply on my root activity. Apparently with it, whenever android decides to kill my app, it would at least force the user to begin from the first activity where I can re-download the data, instead of allowing the user to begin with a more advanced activity causing him to get null exceptions.

However when I applied it on my application it did not work, and when I returned to my application I was beginning from an advanced activity, with an empty singleton instance.

I also tried "android:clearTaskOnLaunch" and it worked, but it's an overkill, since I don't want that every time the user would return to the app, he would have to start over. I want it to happen only if android decided to kill the app.

Is there something I'm doing wrong? Is there any chance maybe there's a better solution for keeping the singleton alive?

Thanks!

Solution

Nope. There is not better solution. This is how it works. Android can kill your application's process whenever it wants to (as long as it is in the background). You can keep data in a singleton, but it is always possible that Android kills your process anyway. When the user returns to your application Android will create a new process and then create a new instance of the activity that the user is resuming. You need to put code in onCreate() of all your activities that recognizes that your singleton is gone and then does an appropriate thing (either redirects to the root activity of your application or reinitializes your singleton.

It is pretty easy to tell if your process has been killed and recreated. Just add a static boolean variable to your singleton that you set to true when it has initialized. In onCreate() of all your activities, check the state of that variable and do something intelligent if the variable is false.

Answered By - David Wasser