Xamarin: repeating ScheduledJob causes crash when app is force closed

Question

I have a Xamarin app which runs a repeating ScheduledJob.

If the app is force closed, when the scheduled job next runs, the app crashes.

Why is the scheduled job still being run after a force close?

How can I stop it happening?

Here is how I start the job, in MainActivity.OnCreate

            serviceComponent = new ComponentName(this, Java.Lang.Class.FromType(typeof(ScheduledJob)));
            // Start service
            var startServiceIntent = new Intent(this, typeof(ScheduledJob));
            StartService(startServiceIntent);
            var builder = new JobInfo.Builder(1, serviceComponent);
            builder.SetPeriodic(App.PollFrequencyMinutes * 60000);
            builder.SetRequiredNetworkType(App.WifiOnly ? NetworkType.Unmetered : NetworkType.Any);
            var tm = (JobScheduler)GetSystemService(Context.JobSchedulerService);
            var status = tm.Schedule(builder.Build());

Here is the job itself:

[Service(Exported = true, Permission = "android.permission.BIND_JOB_SERVICE")]
public class ScheduledJob : JobService {
    JobParameters parameters;

    public override bool OnStartJob(JobParameters args) {
        try {
            Utils.Debug("Scheduled job starting");
            // New code to see why previous code crashed
            if (Xamarin.Forms.Application.Current == null)
                Utils.Debug("Application.Current == null");
            // End of new code
            if (!App.LoggedIn)
                return false;
            parameters = args;
            new Task(doJob).Start();
            return true;
        } catch(Exception ex) {
            Utils.Report(ex);
            return false;
        }
    }

    async void doJob() {
        Utils.Debug("Scheduled job running");
        bool result = await App.Database.SafeSync();
        Utils.Debug("Scheduled job complete - done={0}", result);
        JobFinished(parameters, !result);
    }

    public override bool OnStopJob(JobParameters args) {
        return true;
    }

}

Here is the logcat output for the crash

android.runtime.JavaProxyThrowable: System.InvalidOperationException: You MUST call Xamarin.Forms.Init(); prior to using it.
  at Xamarin.Forms.Device.get_PlatformServices () [0x00007] in <a7f3b60e25304b56ab3b4f0d8c85dcc3>:0 
  at Xamarin.Forms.Device.GetAssemblies () [0x00000] in <a7f3b60e25304b56ab3b4f0d8c85dcc3>:0 
  at Xamarin.Forms.DependencyService.Initialize () [0x00008] in <a7f3b60e25304b56ab3b4f0d8c85dcc3>:0 
  at Xamarin.Forms.DependencyService.Get[T] (Xamarin.Forms.DependencyFetchTarget fetchTarget) [0x00000] in <a7f3b60e25304b56ab3b4f0d8c85dcc3>:0 
  at DefectReport.App.get_Database () [0x0000e] in <697090e2ae9b42aab8ce5fb8297f8295>:0 
  at DefectReport.Utils.Report (System.Exception ex) [0x00016] in <697090e2ae9b42aab8ce5fb8297f8295>:0 
  at DefectReport.Droid.ScheduledJob.OnStartJob (Android.App.Job.JobParameters args) [0x00058] in <697090e2ae9b42aab8ce5fb8297f8295>:0 
  at Android.App.Job.JobService.n_OnStartJob_Landroid_app_job_JobParameters_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native__params) [0x0000f] in <c82a099136944d8aa96281cf061cbc12>:0 
  at (wrapper dynamic-method) System.Object:e8eee233-e03f-4af6-8407-39201ccf2589 (intptr,intptr,intptr)
    at md5a3827847cc73d14b3bdf7eb5b536dab7.ScheduledJob.n_onStartJob(Native Method)
    at md5a3827847cc73d14b3bdf7eb5b536dab7.ScheduledJob.onStartJob(ScheduledJob.java:30)
    at android.app.job.JobService$JobHandler.handleMessage(JobService.java:126)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Before I inserted the new code, it crashed in App.LoggedIn - this attempts to read some Properties with code similar to this (simplified for the purposes of this question)

static bool LoggedIn() {
        if (Current.Properties.TryGetValue(LoggedIn, out object result) && result != null) {
            return Convert.ToBool(result);
        }
        return false;
}

In this case, the crash log was:

System.NullReferenceException: Object reference not set to an instance of an object
  at DefectReport.App.get_LoggedIn () [0x00001] in <697090e2ae9b42aab8ce5fb8297f8295>:0 
  at DefectReport.Droid.ScheduledJob.OnStartJob (Android.App.Job.JobParameters args) [0x00024] in <697090e2ae9b42aab8ce5fb8297f8295>:0 

Answer 1

The Android OS is responsible for instancing/calling JobService once you schedule it and the OS becomes the client.

If this Android Service needs to always run, it should be designed to be independent of having a Xamarin.Forms-based Application init'ed (and also the hosting Activity).

Otherwise you would need to cancel the future jobs, you could do this in an MainActivity.OnPause override:

Jobscheduler.CancelAll();