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１．バックグラウンドフェッチを有効にする

２．AppDelegateの記述

private NotificationService _nService;
public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
    global::Xamarin.Forms.Forms.Init();
    LoadApplication(new App());

    //バックグラウンドフェッチの間隔を設定
    UIApplication.SharedApplication.SetMinimumBackgroundFetchInterval(UIApplication.BackgroundFetchIntervalMinimum);

    return base.FinishedLaunching(app, options);
}

public override void PerformFetch(UIApplication app, Action<UIBackgroundFetchResult> completionHandler)
{
    try
    {
        System.Diagnostics.Debug.WriteLine("●PerformFetch start");
        this.BackgroundService();
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine + ex.StackTrace);
    }
    finally
    {
        completionHandler(UIBackgroundFetchResult.NewData);
    }
}

private async void BackgroundService()
{
    await Task.Run(() =>
    {
        try
        {
            System.Diagnostics.Debug.WriteLine("●BackgroundService.Thread.Start");
            while (true)
            {
                InvokeOnMainThread(delegate
                {
                    if (_nService == null)
                    {
                        //通知機能の認証
                        _nService = new NotificationService();
                        _nService.Regist();
                    }
                    _nService.On("PerformFetchTitle", "PerformFetchBody time:" + UIApplication.SharedApplication.BackgroundTimeRemaining.ToString());
                });
                System.Threading.Thread.Sleep(60000);
            }
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine + ex.StackTrace);
        }
    }).ConfigureAwait(false);
}