I have existing code from a sample app written for Xamarin.Android (not Forms). I need to use this in my new app written for .NET MAUI, the successor of all Xamarin. It uses a BroadcastReceiver to know when a USB device was detached from the phone. But this doesn't work in MAUI.

Here's the old code:

public partial class MainPage : ContentPage
{
    private BroadcastReceiver detachedReceiver;

    public MainPage()
    {
        InitializeComponent();
    }

    protected override void OnAppearing()
    {
        base.OnAppearing();

        OnScanClicked(this, EventArgs.Empty);

        // TODO: Fix for MAUI
        //register the broadcast receivers
        detachedReceiver = new UsbDeviceDetachedReceiver(this);
        RegisterReceiver(detachedReceiver, new IntentFilter(UsbManager.ActionUsbDeviceDetached));
    }

    protected override void OnDisappearing()
    {
        base.OnDisappearing();

        CloseDevice();

        // TODO: Fix for MAUI
        var temp = detachedReceiver; // copy reference for thread safety
        if (temp != null)
            UnregisterReceiver(temp);
    }

    private async void OnScanClicked(object sender, EventArgs args)
    {
        // Here's some code
    }

    class UsbDeviceDetachedReceiver : BroadcastReceiver
    {
        readonly MainPage mainPage;

        public UsbDeviceDetachedReceiver(MainPage mainPage)
        {
            this.mainPage = mainPage;
        }

        public override void OnReceive(Context context, Intent intent)
        {
            mainPage.OnScanClicked(null, null);
        }
    }
}

The code below the two TODO comments doesn't compile. I cannot figure out how this should be written for MAUI. Any ideas?

CodePudding user response：

In MAUI, code that in Xamarin.Android you would have put in Activity lifecycle events, can be done inside #if ANDROID, as part of the App Builder. This is documented in Platform Lifecycle Events.

Example from doc:

using Microsoft.Maui.LifecycleEvents;

namespace PlatformLifecycleDemo
{
    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .ConfigureLifecycleEvents(events =>
                {
#if ANDROID
                    events.AddAndroid(android => android
                        .OnActivityResult((activity, requestCode, resultCode, data) => LogEvent("OnActivityResult", requestCode.ToString()))
                        .OnStart((activity) => LogEvent("OnStart"))
                        .OnCreate((activity, bundle) => LogEvent("OnCreate"))
                        .OnBackPressed((activity) => LogEvent("OnBackPressed"))
                        .OnStop((activity) => LogEvent("OnStop")));
#endif
                    static void LogEvent(string eventName, string type = null)
                    {
                        System.Diagnostics.Debug.WriteLine($"Lifecycle event: {eventName}{(type == null ? string.Empty : $" ({type})")}");
                    }
                });

            return builder.Build();
        }
    }
}

In Xamarin.Android docs, we find Broadcast Receivers in Xamarin.Android.

Its example of using a custom Broadcast Receiver:

// --- This is Xamarin.Android code, not MAUI code. ---
[Activity(Label = "MainActivity", MainLauncher = true, Icon = "@mipmap/icon")]
public class MainActivity: Activity
{
    MySampleBroadcastReceiver receiver;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        receiver = new MySampleBroadcastReceiver();

        // Code omitted for clarity
    }

    protected override void OnResume()
    {
        base.OnResume();
        RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));
        // Code omitted for clarity
    }

    protected override void OnPause()
    {
        UnregisterReceiver(receiver);
        // Code omitted for clarity
        base.OnPause();
    }
}

To keep MAUI's App Builder code concise, I would define methods that are called from the builder code:

public static class MauiProgram
...
#if ANDROID
private static void MyOnCreate(Activity activity, Bundle bundle)
{
    receiver = new MySampleBroadcastReceiver();
}
private static void MyOnResume(Activity activity)
{
    activity.RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));
}
private static void MyOnPause(Activity activity)
{
    activity.UnregisterReceiver(receiver);
}
#endif

So that the builder code is always:

    ...
    builder
        .UseMauiApp<App>()
        .ConfigureLifecycleEvents(events =>
        {
#if ANDROID
            events.AddAndroid(android => android
                .OnCreate((activity, bundle) => MyOnCreate(activity, bundle))
                .OnResume((activity) => MyOnResume(activity))
                .OnPause((activity) => MyOnPause(activity)));
#endif
        });

In your case, place your old OnAppearing code into MyOnResume.

NOTE: It is possible to use .net 6 to write an Android app, without any mention of MAUI. That would be almost identical to writing a Xamarin.Android app, except for namespace changes.

To create a ".Net 6 Android" app, in VS 2022 (Preview), when "Create a new project", choose "Android Application (Preview)".