更新 Xamarin.Android 中的前台通知文本

Question

我想更新前台通知的 ContentText，以便 ContentText 显示最新产品的日期。

我使用两个全局变量来保留对原始 Builder 和 notificationManager 的引用。 从另一个 class 调用 UpdateNotificationContent()，我已将更新的文本提供给 SetContentText 并调用 notificationManager.notify 以尝试更新构建器。

以下行（UpdateNotificationContent 的最后一行）出现错误： notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());

Java.Lang.NullPointerException: 'Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference'

完整代码在这里。

class MyService : Service
{
        private Handler handler;
        private Action runnable;
        private bool isStarted;
        private int DELAY_BETWEEN_LOG_MESSAGES = 5000;
        private int NOTIFICATION_SERVICE_ID = 1001;
        private int NOTIFICATION_AlARM_ID = 1002;
        private string NOTIFICATION_CHANNEL_ID = "1003";
        private string NOTIFICATION_CHANNEL_NAME = "Update Notifications";
        private NotificationCompat.Builder notificationBuilder;
        private NotificationManager notificationManager

        private void DispatchNotificationThatServiceIsRunning()
        {
            Intent intent = new Intent(this, typeof(CustomReceiver));
            PendingIntent pendingIntent = PendingIntent.GetBroadcast(
                    this,
                    1,
                    intent,
                    PendingIntentFlags.UpdateCurrent
            );

            string contextText = App.latestProduct.ProductSaveTime.ToString();
            }

            var notification = new Notification.Builder(this, default)
           .SetChannelId("location_notification")
           .SetContentTitle("AppName")
           .SetContentText(contextText)
           .SetSmallIcon(Resource.Drawable.icon)
           .SetVisibility(NotificationVisibility.Secret)
           .SetContentIntent(pendingIntent)
           .SetOngoing(true)
           .Build();

            // Enlist this instance of the service as a foreground service
            StartForeground(NOTIFICATION_SERVICE_ID, notification);

            var notificationManager = (NotificationManager)GetSystemService(NotificationService);
            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notification);
        }

        // every 5 seconds push a notificaition
        private void DispatchNotificationThatAlarmIsGenerated()
        {
            Intent intent = new Intent(this, typeof(CustomReceiver));
            PendingIntent pendingIntent = PendingIntent.GetBroadcast(
                    this,
                    1,
                    intent,
                    PendingIntentFlags.UpdateCurrent
            );

           string contextText = App.latestProduct.ProductSaveTime.ToString();

            notificationBuilder = new NotificationCompat.Builder(this, "default")
                .SetAutoCancel(false)
                .SetSmallIcon(Resource.Drawable.icon)
                .SetContentIntent(pendingIntent)
                .SetContentTitle("AppName")
                .SetContentText(contextText);

            notificationManager = (NotificationManager)GetSystemService(NotificationService);
            notificationManager.Notify(NOTIFICATION_AlARM_ID, notificationBuilder.Build());
        }


        public void UpdateNotificationContent()
        {  
            string contextText = App.latestProduct.ProductSaveTime.ToString();

            notificationBuilder = new NotificationCompat.Builder(this, "default")
                .SetContentText(contextText);

            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());
        }
}

Answer 1

不要创建新的 NotificationCompat.Builder。 如果您只想更新内容，请使用具有相同 ID（ NOTIFICATION_SERVICE_ID ）的相同NotificationCompat.Builder设置来覆盖。

改变：

 string contextText = App.latestProduct.ProductSaveTime.ToString();

        notificationBuilder = new NotificationCompat.Builder(this, "default")
            .SetContentText(contextText);

        notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder.Build());

至：

  string contextText = "update";//I do not have you contextText, so i use string instead

            notificationBuilder .SetContentText(contextText);

            notificationManager.Notify(NOTIFICATION_SERVICE_ID, notificationBuilder .Build());

Answer 2

notificationManager 也必须只声明一次

notification_Manager = (NotificationManager)GetSystemService(NotificationService);
if (Build.VERSION.SdkInt >  BuildVersionCodes.O)
{
    '
    '
    notification_Manager.CreateNotificationChannel(channel);
}

然后为您所做的每个通知更新使用相同的变量notification_Manager.notify()和notificationBuilder 。