Xamarin.Forms PopModalAsync：未观察到任务的异常

Question

我有以下代码：System.Threading.Tasks.Task appointmentEndTask = App.ArdaBusinessLogic.AppointmentEnd(_appointment);System.Threading.Tasks.Task appointmentEndCompletedTask = appointmentEndTask.ContinueWith(    async task =>    {        _appointmentDetailPage.IsDirty = true;        await App.MasterNavigationPage.Navigation.PopModalAsync();    },     System.Threading.CancellationToken.None,     System.Threading.Tasks.TaskContinuationOptions.OnlyOnRanToCompletion,     System.Threading.Tasks.TaskScheduler.FromCurrentSynchronizationContext());System.Threading.Tasks.Task appointmentEndFaultedTask = appointmentEndTask.ContinueWith(    async task =>    {        await App.MasterNavigationPage.Navigation.PopModalAsync();        await App.ShowErrorPageAsync(task.Exception);    },     System.Threading.CancellationToken.None,     System.Threading.Tasks.TaskContinuationOptions.OnlyOnFaulted,     System.Threading.Tasks.TaskScheduler.FromCurrentSynchronizationContext());因此，如果“AppointmentEnd”任务完成，则应关闭当前模态页面。有时（并非总是！）我在崩溃日志中收到以下错误。在本例中，第 139 行是“_appointmentDetailPage.IsDirty = true”之后的“await App.MasterNavigationPage.Navigation.PopModalAsync()”。不幸的是，我不明白为什么会出现这个错误。你能帮助我吗？

Answer 1

首先，为什么要使用如此复杂的语法而不是利用异步等待？

public async void EndAppointement()

{

    try 

    {

        await App.ArdaBusinessLogic.AppointmentEnd(_appointment);

        _appointmentDetailPage.IsDirty = true;

        await App.MasterNavigationPage.Navigation.PopModalAsync();

    }

    catch (Exception exception)

    {

        await App.MasterNavigationPage.Navigation.PopModalAsync();

        await App.ShowErrorPageAsync(exception);

    }

}

二、看XF源码：

protected override async Task<Page> OnPopModal(bool animated)

{

    Page modal = ModalStack[ModalStack.Count - 1];

    if (_owner.OnModalPopping(modal))

    {

        _owner.OnPopCanceled();

        return null;

    }

    Page result = await base.OnPopModal(animated);

    result.Parent = null;

    _owner.OnModalPopped(result);

    return result;

}

您的模态堆栈似乎混乱了：这意味着您正在尝试弹出不在堆栈上的页面。您确定您处于模态页面吗？也许使用PopAsync而不是PopModalAsync.