android - Xamarin:强制关闭应用程序时，重复 ScheduledJob 会导致崩溃

Question

我有一个运行重复 ScheduledJob 的 Xamarin 应用。

如果应用被强制关闭，当计划的作业下次运行时，应用会崩溃。

为什么强制关闭后计划的作业仍在运行？

我怎样才能阻止它发生？

这是我在 MainActivity.OnCreate

中开始工作的方式

            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());

这是作业本身:

[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;
    }

}

这是崩溃的 logcat 输出

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)

在我插入新代码之前，它在 App.LoggedIn 中崩溃了 - 这会尝试使用类似于此的代码读取一些属性(为了这个问题的目的而简化)

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

在这种情况下，崩溃日志是:

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

Android 操作系统负责实例化/调用 JobService，一旦您安排它并且操作系统成为客户端。

如果此 Android Service 需要始终运行，则应将其设计为独立于基于 Xamarin.Forms 的应用程序初始化(以及托管 Activity )。

否则您将需要取消 future 的作业，您可以在 MainActivity.OnPause 覆盖中执行此操作:

Jobscheduler.CancelAll();