How do different Android app components with different lifecycles share objects?

Question

I am new to android development and I see that there are different components (activities, services, etc.) where each component has a different life cycle.

I am encountering a situation where I have an object that should be accessed by different components. Namely, I have a list of objects that changes dynamically, and that list should be accessed by:

1. An activity that displays the list in a ListView
2. A broadcast receiver, that responds to alarm events scheduled in the AlarmManager.

The broadcast receiver may kick in hours after the activity was used. At this time, the activity may or may not be still alive in memory.

Is there anywhere I can put the list so it will be accessible to all app parts? My idea was to serialize the objects I need to share, write them to shared preferences and then recreate them whenever needed. I'm not sure it's a good idea, and even if it is - how would I design it? when would writing to shared preferences (or somewhere else) occur? when would reading occur (it would be preferable to read the object from memory when possible)?

Answer 1

You can put it in SharedPreferences if it a compatible data type, or serialize the list to a file in the internal or external storage (see saving files, you probably want internal storage for your situation).

You can read the list in the onResume() method of the Activity or the onReceive() method of the receiver. Writing will occur whenever a new element needs to be added to the list, you will deserialize the list, add the element and re-serialize it. You may need to place the reading and writing in a synchronized block or method to prevent simultaneous access.

If the list may be added to outside of the Activity when the Activity is resumed you may need to provide a broadcast receiver or other mechanism for the Activity to be alterted and reload the list.

As Gabe pointed out, you could store it in a static variable somewhere, but the way you described the situation serializing sounds like a better option. Otherwise the list is gone if your process is terminated by the Android system.

As kcoppock pointed out, you could use an SQLite database to store the data, but this sounds like overkill from what you described. You will be in for some additional coding without much benefit.

Answer 2

What you are looking for is the Android Shared Preferences. You can access Sharepreferences using getApplicationContext() from an Activity or a Service.

Hope this helps

Answer 3

If you don't want it permanently stored, put it in a static variable somewhere (anywhere they both agree to- a separate class would be a good idea). Then you can access the list unless Android kicks the class out of memory- which it should only do it its terminating the app due to lack of memory, in which case it doesn't matter much anyway.

If you do want it permanently stored, the filesystem is the best way. You'll need to synchronize access to it of course.