c# - 应用程序在后台时如何正确通知当前视图模型有关推送通知

Question

我正在使用 Xamarin 和 MvvmCross 为 iOS 和 Android 平台开发手机应用程序。在 Android 版本中，我可以在应用程序处于前台时和应用程序关闭时接收推送通知，并在这两种情况下导航到所需的视图模型。

当应用程序处于后台并由于用户点击推送通知而恢复时，我无法从初始屏幕导航到特定视图模型。

如果设置了该标志并执行导航，我可以保留一个标志，然后检查恢复的每个视图模型，但我不太喜欢该解决方案，并且希望像应用程序处于前台时那样使用事件。问题是当应用程序处于后台并从推送通知恢复时，我的事件仍未分配给它的处理程序。

这是我的代码。闪屏：

public class SplashScreen : MvxSplashScreenAppCompatActivity
{
    private bool _setPushNotificationHint;
    private Dictionary<string, string> _pushNotificationData;

    public SplashScreen() : base(Resource.Layout.SplashScreenLayout)
    {
    }
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        if (Intent.Extras != null)
        {
            _pushNotificationData = new Dictionary<string, string>();
            foreach (var key in Intent.Extras.KeySet())
            {
                var value = Intent.Extras.Get(key);
                if(value != null)
                {
                    _pushNotificationData.Add(key, value.ToString());
                }                   
            }
            _setPushNotificationHint = true;
        }

        //avoid showing splash screen when resuming app pressing app icon or when tapping on push notification
        if (!IsTaskRoot)
        {
            //deliver notification only when app is started and is in background. When it's closed we'll use GetAppStartHint
            if (_setPushNotificationHint)
            {
                PushNotificationManager.OnMessageReceivedAppInBackground(this, _pushNotificationData);
            }
            Finish();
            return;
        }
        //just initialize essentials when first start of the app, not when splash screen is shown
        Xamarin.Essentials.Platform.Init(this, bundle);
    }

这是我获取推送通知信息并触发我的 PushNotificationManager 的地方，它应该触发事件。这是它的摘录：

public class PushNotificationManager : IPushNotification
{
    public const string controlIdKey = "controlId";

    private static PushNotificationDataEventHandler _onNotificationReceived;
    public event PushNotificationDataEventHandler OnNotificationReceived
    {
        add
        {
            _onNotificationReceived += value;
        }
        remove
        {
            _onNotificationReceived -= value;
        }
    }

    private static PushNotificationDataEventHandler _onNotificationReceivedWhileForeground;
    public event PushNotificationDataEventHandler OnNotificonReceivedWhileForegroundationReceived
    {
        add
        {
            _onNotificationReceivedWhileForeground += value;
        }
        remove
        {
            _onNotificationReceivedWhileForeground -= value;
        }
    }

    /// <summary>
    /// Handles an incoming push notification in Android when app is in Foreground.
    /// </summary>
    /// <param name="remoteMessage">User info.</param>
    public static void OnMessageReceived(RemoteMessage remoteMessage)
    {
        var notification = CreateNotificationFromRemoteMessage(remoteMessage);
        _onNotificationReceivedWhileForeground?.Invoke(CrossPushNotification.Instance, new PushNotificationDataEventArgs(notification));
    }

    /// <summary>
    /// When app is in background in Android, we get to the splashscreen, who will call this method to publish a NotificationMessage to the interested ViewModels
    /// </summary>
    /// <param name="sender">Sender.</param>
    /// <param name="parameters">Parameters.</param>
    public static void OnMessageReceivedAppInBackground(object sender, Dictionary<string, string> parameters)
    {
        var notification = CreateNotificationFromIntentExtras(parameters);
        //we need to publish a message instead of rising an event because this is triggered from the splash screen and the view model 
        //Mvx.IoCProvider.Resolve<IMvxMessenger>().Publish(notificationMessage);
        //_onNotificationReceived?.Invoke(CrossPushNotification.Instance, new PushNotificationDataEventArgs(notification));
        //todo find a way to properly notify the current view to navigate to the device details view model
    }

    /// <summary>
    /// In Android plataform, we create the notification from the parameters received in the splashscreen as intent extras
    /// </summary>
    /// <returns>The notification from intent extras.</returns>
    /// <param name="parameters">Parameters.</param>
    private static Notification CreateNotificationFromIntentExtras(Dictionary<string, string> parameters)
    {
        parameters.TryGetValue(controlIdKey, out var controlId);

        return new Notification
        {
            ControlId = controlId
        };
    }

然后在 BaseViewModel.cs 中附加应该从 PushNotificationManager 触发的事件的处理程序：

public override void ViewAppeared()
    {
        base.ViewAppeared();
        //_token = Mvx.IoCProvider.Resolve<IMvxMessenger>().Subscribe<NotificationMessage>(OnNotificaitonMessage);
        CrossPushNotification.Instance.OnNotificationReceived += NotificationReceived;
        CrossPushNotification.Instance.OnNotificonReceivedWhileForegroundationReceived += NotificationInForegroundReceived;
    }

    public override void ViewDisappeared()
    {
        base.ViewDisappeared();
        //Mvx.IoCProvider.Resolve<IMvxMessenger>().Unsubscribe<NotificationMessage>(_token);
        CrossPushNotification.Instance.OnNotificationReceived -= NotificationReceived;
        CrossPushNotification.Instance.OnNotificonReceivedWhileForegroundationReceived -= NotificationInForegroundReceived;
    }

    public override void ViewDestroy(bool viewFinishing = true)
    {
        //Mvx.IoCProvider.Resolve<IMvxMessenger>().Unsubscribe<NotificationMessage>(_token);
        CrossPushNotification.Instance.OnNotificationReceived -= NotificationReceived;
        CrossPushNotification.Instance.OnNotificonReceivedWhileForegroundationReceived -= NotificationInForegroundReceived;
        base.ViewDestroy(viewFinishing);
    }

但是当应用程序来自后台时，这些事件处理程序为空，因为视图尚未出现。我无法在构造函数中附加事件的处理程序，因为当视图消失时我无法删除它。只有当前的视图模型应该处理事件并导航到我想要的视图模型。

我也尝试过 MvxMessenger 插件但没有成功。

所以我的问题是，当应用程序在后台时，如何正确通知我的视图模型推送通知？

标签： c#xamarinpush-notificationxamarin.androidmvvmcross