How to use Xamarin.AndroidX.Lifecycle interfaces to determine when app transitions from foreground to background

Richard Jones 136 Reputation points
2021-05-28T08:42:32.84+00:00

I have a complex Xamarin.Android app that has many Activities, few of which could be considered base activities, and many of which could be considered "modal" activities that temporarily gain foreground focus, with the base activity running behind it. All works well enough, but I've encountered a problem trying to play music across the lifecycle of these activities, where the ~20ms delay between when an activity is paused and another resumed causes my music to artifact in an undesirable way.

The best solution I can find from researching without rewriting my entire app to be fragment based (which is a whole other issue in itself) is to create an Application class, and utilize the interfaces within the AndroidX.Lifecycle namespace to hook into callbacks for when the app transitions from foreground to background, and vice versa. I know for a fact that this is possible in native Android, as I've encountered various answers on StackOverflow as well as the Android Developer documentation saying exactly how to do it in Java/Kotlin.

The problem is that none of the properties that I need exist. Various answers refer to a property called ProcessLifecycleOwner that I cannot seem to find anywhere. I don't know if I've simply not added a reference to the correct NuGet package (there are about 8 with the AndroidX.Lifecycle header alone), or if the bindings were never completed, or what. Any help would be appreciated, as I'm pretty stumped at this point.

Developer technologies | .NET | Xamarin
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  1. Richard Jones 136 Reputation points
    2021-05-31T08:50:59.193+00:00

    Okay, I finally found something I could use. It looks like the missing ProcessLifecycleOwner property I needed was hidden away within the Xamarin.AndroidX.Lifecycle.Extensions NuGet package. Why it is there is anyone's guess but after adding a reference to that, I was able to successfully subscribe my custom Application class to app-level lifecycle events.

    I also needed to add the necessary attributes to my callback methods, including the [Export] attribute for some reason. This was documented in a StackOverflow answer and nowhere else.

    Finally, before the project would compile with these changes, I needed to add a .dll reference (using the references modal, not NuGet, don't get tricked) to Mono.Droid.Export.dll, which was found under the Assemblies tree. To be fair, the Error output window told me what to do here so this one should be an easy fix if you forget to do it.

    After making those changes, I was able to successfully compile the app, and get appropriate callbacks for when the app was backgrounded and foregrounded, which was exactly what I needed. See the below code for an example of what the custom Application class should look like to accomplish this.

    using System;
    using Android.App;
    using Android.Runtime;
    using AndroidX.Lifecycle;
    using Java.Interop;
    
    
    namespace MyAndroidApp
    {
        [Application]
        public class MyApp : Application, ILifecycleObserver
        {
            public MyApp() { }
            public MyApp(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer) { }
    
            public override void OnCreate()
            {
                base.OnCreate();
    
                ProcessLifecycleOwner.Get().Lifecycle.AddObserver(this);
            }
    
            [Export, Lifecycle.Event.OnStop]
            public void OnAppBackgrounded()
            {
                // Handle background logic
            }
    
            [Export, Lifecycle.Event.OnStart]
            public void OnAppForegrounded()
            {
                // Handle foreground logic
            }
        }
    }
    
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