Xamarin.Android Architecture Components: Not getting callbacks for lifecycle events

问题

I'm trying to use the architecture components package for detecting when the application enters background or foreground state. The problem is that the callbacks are not being invoked. In the sample code below, the methods onApplicationForegrounded and onApplicationBackgrounded are not invoked:

namespace POC.Droid
{
    [Application]
    public class MyApp : Application, ILifecycleObserver
    {
        static readonly string TAG = "MyApp";

        public MyApp(IntPtr handle, Android.Runtime.JniHandleOwnership ownerShip) : base(handle, ownerShip)
        {
        }

        public override void OnCreate()
        {
            base.OnCreate();
            ProcessLifecycleOwner.Get().Lifecycle.AddObserver(this);
        }

        [Lifecycle.Event.OnStop]
        public void onAppBackgrounded()
        {
            Log.Debug(TAG, "App entered background state.");
        }

        [Lifecycle.Event.OnStart]
        public void onAppForegrounded()
        {
            Log.Debug(TAG, "App entered foreground state.");
        }
    }
}

My Xamarin version is 8.2.0.16 (Visual Studio Community) and Xamarin.Android.Arch.Lifecycle.Extensions version is 1.0.0. I'm using a Nougat device (7.0) for testing.

回答1:

TL;DR Please annotate your lifecycle callbacks with [Export]

Here a more detailed description:

Generally, to get the methods of a lifecycle observer be invoked, please make sure that the related packages are present. Here is a part of my packages.config:

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Xamarin.Android.Arch.Core.Common" version="26.1.0" targetFramework="monoandroid81" />
  <package id="Xamarin.Android.Arch.Core.Runtime" version="1.0.0.1" targetFramework="monoandroid81" />
  <package id="Xamarin.Android.Arch.Lifecycle.Common" version="26.1.0" targetFramework="monoandroid81" />
  <package id="Xamarin.Android.Arch.Lifecycle.Extensions" version="1.0.0.1" targetFramework="monoandroid81" />
  <package id="Xamarin.Android.Arch.Lifecycle.Runtime" version="1.0.3.1" targetFramework="monoandroid81" />

This is how this looks in Visual Studio:

To be able to set a lifecycle observer, we need a lifecycle owner. On the application level this can be ProcessLifecycleOwner, just like the original poster showed.

Here is a slightly modified version:

using System;
using Android.App;
using Android.Arch.Lifecycle;
using Android.Util;
using Java.Interop;

namespace Stopwatch_AAC
{
    [Application]
    public class MyApp : Application, ILifecycleObserver
    {
        const string TAG = "MyApp";

        public MyApp(IntPtr handle, Android.Runtime.JniHandleOwnership ownerShip) : base(handle, ownerShip)
        {
        }

        public override void OnCreate()
        {
            base.OnCreate();
            ProcessLifecycleOwner.Get().Lifecycle.AddObserver(this);
        }

        [Lifecycle.Event.OnStop]
        [Export]
        public void Stopped()
        {
            Log.Debug(TAG, "App entered background state.");
        }

        [Lifecycle.Event.OnStart]
        [Export]
        public void Started()
        {
            Log.Debug(TAG, "App entered foreground state.");
        }
    }
}

As you can see, you annotate your lifecycle methods with for example [Lifecycle.Event.OnStop]. Also, please note that you need to use [Export]. Please make sure that Mono.Android.Export is referenced in your project as shown in the following screenshot.

If you want to have lifecycle observers for an activity, I suggest to extend AppCompatActivity as it is a lifecycle owner:

using Android.App;
using Android.Arch.Lifecycle;
using Android.OS;
using Android.Support.V7.App;
using Android.Util;
using Java.Interop;

namespace Stopwatch_AAC
{

    [Activity(Label = "Minimal", Exported = true, MainLauncher = true)]
    public class Minimal : AppCompatActivity, ILifecycleObserver
    {
        const string TAG = "Stopwatch_AAC";

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            Lifecycle.AddObserver(this);
            Log.Debug(TAG, Lifecycle.CurrentState.ToString());
        }

        [Lifecycle.Event.OnAny]
        [Export]
        public void Hello()
        {
            Log.Debug(TAG, Lifecycle.CurrentState.ToString());
        }
    }
}

回答2:

if you need it in the activities here the events:

protected override void OnStart(){
            base.OnStart();
            Log.Debug(logTag, "MainActivity.OnStart() called, the activitiy is active");
        }

        protected override void OnPause()
        {
            base.OnPause();
            Log.Debug(logTag, "MainActivity.OnPause() called, the activity in background");
        }

        protected override void OnStop()
        {
            base.OnStop();
            Log.Debug(logTag, "MainActivity.OnStop() called, the activity is in background because of other activiy or app");
        }

        protected override void OnResume()
        {
            base.OnResume();
            Log.Debug(logTag, "MainActivity.OnResume() called, the activity stated");
        }

        protected override void OnRestart()
        {
            base.OnRestart();
            Log.Debug(logTag, "MainActivity.OnRestart() called,  the activity is startet");
        }

        protected override void OnDestroy()
        {
            base.OnDestroy();
            Log.Debug(logTag, "MainActivity.OnDestroy() called, activity is destroyed");
        }

for Xamarin Forms you will find in app.xaml.cs the event which are needed for the apps.

protected override void OnStart ( ) {
            // Handle when your app starts
        }

        protected override void OnSleep ( ) {
            // Handle when your app sleeps
        }

        protected override void OnResume ( ) {
            // Handle when your app resumes
        }

回答3:

I have used that package in the past, however I much prefer the implementation by James Montemagno which can be found as a nuget package called "Plugin.CurrentActivity". It creates an application class and implements the ILifecycle events for you.

From the description:

Provides a simple solution for getting access to the current Activity of the application when developing a Plugin for Xamarin. This will lay down a base "application" class for developers in their Android application with boilerplate code to get them started. Can be used with Android API 14+

* I am making the assumption that you're not using Xamarin.Forms. This works perfectly for a native Xamarin Android project.

Link to the Github page

来源：https://stackoverflow.com/questions/49644709/xamarin-android-architecture-components-not-getting-callbacks-for-lifecycle-eve

标签

Xamarin

xamarin.android

android-architecture-components