Passing references to Activity Intent

Question

For quite some time I've had troubles passing variables from one Activity to another, and I've usually had to resolve to some pretty ugly Static-class-hacks to make it work.

Generally something along the lines of a static method that I call with the type of the Activity, along with the variables the Activity requires. These gets stored in a static variable, and retrieved in the constructor of said activity.

Like I said, pretty ugly. And there's no such thing as "myActivity.StartActivity(new Activity);". All of the overloads for StartActivity takes either an Intent, or a typeof(MyOtherActivity).

So my question is, have I completely misunderstood the concept of Activities, or am I simply missing a completely obvious way to pass arguments to them?

@Edit: The reason I want to pass an actual reference to an object, instead of simply a copy of the object, is because I'm trying to pass a View Model from an overlying Activity, down to the new Activity. And of course any changes made to this view model, should be reflected on the parent activity, which will only be possible if the the two activy's viewmodels points to the same instance.

I'm writing the app using Xamarin.Android, but the code is nearly identical between C# and Java, so answers in either those languages is fine.

Answer 1

The problem is that Android can kill the process hosting your app at any time (if it is in the background). When the user then returns to your app, Android will create a new process to host your app and will recreate the Activity at the top of the stack. In order to do this, Android keeps a "serialized" version of the Intent so that it can recreate the Intent to pass it to the Activity. This is why all "extras" in an Intent need to be Parcelable or Serializable.

This is also why you cannot pass a reference to an object. When Android recreates the process, none of these objects will exist anymore.

Another point to consider is that different activities may run in different processes. Even activities from the same application may be in different processes (if the manifest specifies this). Since object references don't work across process boundaries, this is another reason why you cannot pass a reference to an object in an Intent.

Answer 2

You can also use The Application class to store objects globally and retrieve them:

using Android.Runtime;

namespace SomeName
{
[Application]
public class App : Application
{
public string Name { get; set;}

public App (IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer)
{
}

public override void OnCreate ()
{
    base.OnCreate ();

    Name = "";
}
}
}

And you can access the data with:

App application = (App)Application.Context;
application.Name = "something";

I choose to do this on the Application calss because this class is called on the App startup so you don't have to initiate it manually.
Keep in mind that variables which are scoped to the Application have their lifetime scoped to the application by extension.
This class will be Garbage Collected if the Android feels it is necessary so you have to modify the code to include this case also.
You can use SharedPreferences or a Database to save your variables in case they get deleted and retrieve them from the App class for faster results.

Don't be overly wasteful in how you use this approach though, as attaching too much information on this class it can lead to a degradation in performance. Only add information that you know will be needed by different parts of the application, and where the cost of retrieving that information exceeds the cost of storing it as an application variable.

Investigate which information you need to hold as application wide state, and what information can simply be retrieved straight from your database. There are cost implications for both and you need to ensure you get the balance right.

And don't forget to release resources as needed on OnStop and OnDestroy
I rarely use intents, i find this way better.