Good alternatives to share a complex tree of objects between activities in Android?

Question

This is a question I've now had for a few different apps I've built, and I have yet to be satisified with any of the solutions I've come up with. I thought I'd put it out there to the community to see other solutions there might be.

Let's say you have an Activity that downloads a complex tree of data (in this case via json, but it could be anything), unmarshalls that data to a set of java objects (in this case using gson, but again, could be whatever), then spawns additional activities to view different parts of that data. There might be one activity to view Trips in your response, and another to view Flights in those trips, and maybe another to view Passengers of those flights.

My initial implementation of this app was to unmarshall all the Trips in the first activity, then pass them by value (as an extra in the intent) to the TripActivity. The TripActivity then passes individual flights to the FlightActivity, and so on.

The problem with this is that there's a noticeable pause between activities while the app serializes and deserializes the data. We're talking several seconds. The pause is quite noticeable when my tree uses Serialization or Parcelable to pass data around. Initial performance testing with using google's Parcelable instead show a roughly 30% speedup over serialization, but Parcelable is difficult to work with and doesn't seem to handle circular object references well like Serialization does, and besides it still pauses for almost as many seconds, so I've put that experiment on the backburner while I try other things.

So then I tried moving the tree of objects directly into the Application class. Each activity just gets the tree directly from the app whenever it needs it. This makes performance quite snappy, but handling corner cases like unexpected activity start/stops (either due to activity crashes or because the activity has been closed temporarily to make more memory available, or whatever other cause) seems tricky. Perhaps it's no more than implementing onSaveInstanceState(), I'm not sure, but the solution seems a bit hacky so I haven't investigated further yet.

So in search of a less cobbled-together solution, I tried creating a custom ContentProvider to store and retrieve my objects. Since ContentProviders can be configured to run in-process using multiprocess=true, I thought that would be an excellent way to avoid serialization costs while doing something more "standard" than storing data in the Application object. However, ContentProviders were clearly not intended to return arbitrary object types -- they only support types such as numbers, strings, booleans, etc. It appears I can finagle one to store arbitrary objects by using ContentResolver.getContentProviderClient().getLocalContentProvider() and accessing my custom class directly, but I'm not sure that's less hacky than storing data in the Application object.

Surely someone must have a good solution to this problem. What am I doing wrong?

Answer 1

In addition to fiXedd's solution, another one is to use a local service. Have the service "own" the objects, with activities calling service APIs to get whatever it needs. The service can also be responsible for fetching and parsing the data, encapsulating that bit of logic.

The Application object is the "red-headed step-child" of Android components. Members of the core Android team have come out against the practice of creating custom Application subclasses, though it is certainly supported by the API. Having engineered one ADC2 200 application that leveraged a custom Application subclass, I can say that I should have gone with a service in my case as well. Live and learn...

By using the local binding pattern, your service will automatically be created and destroyed as needed, so you don't have to worry about that. And, by definition, a local service runs in the same process/VM as your activities, so you don't have to worry about marshaling overhead like you would in the ContentProvider scenario.

Answer 2

The way I'm handling this in one of my apps is downloading the data then shoving it into a database. This way I don't have to carry all those objects around (which, IIRC, eat about 1kb each just for the object instantiation) and I can easily pull just the data that I need. I don't know if this will work for you, but it worked for my use-case.