在使用 NavigationPage 时出现“在 iOS 上全局不支持 PushAsync…”的错误提示

Question

大家好，我一直遇到以下问题：

System.InvalidOperationException with "PushAsync在iOS上不被支持，请使用NavigationPage。"

尽管我正在使用NavigationPage并且从我所能看到的来说使用得很好。我的F#技能并不是很好，所以我肯定认为我在这里做错了什么，但是惊讶地发现在F#中执行Navigation.PushAsync的示例很难找到。我找到了这段代码，并一直在寻找它作为正确示例的例子。

https://github.com/fsprojects/Tamarin/blob/master/samples/Todo/TodoListMvc.fs

在这个例子中，我看到这段代码被包裹在一个async{}中。

 do! this.Navigation.PushAsync(page = downcast view.Root) |> Async.AwaitIAsyncResult |> Async.Ignore

所以我认为我在这里并没有太离谱。那么，我做错了什么？

程序集版本

• Xamarin.Forms 版本 2.3.5.256-pre6
• FSharp.Core 版本 4.0.0.1

代码

namespace Eclipsefest

open System
open Xamarin.Forms
open Xamarin.Forms.Xaml

type MenuItem = 
{ Title : string
  TargetType : System.Type }

type MenuPage() = 
inherit ContentPage()
let _ = base.LoadFromXaml(typeof<MenuPage>)
member this.Handle_Tapped(sender : Object, arg : EventArgs) = 
    let navItem = sender :?> TextCell
    async { 
        match navItem.Text with
        | "Information" -> 
            do! this.Navigation.PushAsync(page = new NavigationPage(new InfoPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "Events" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new EventsPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "BusRoute" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new BusRoutePage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "Glasses" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new GlassesPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "Merch" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new MerchPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "Day" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new DayPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | "About" -> 
            do! this.Navigation.PushAsync(new NavigationPage(new AboutPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
        | _ -> 
            do! this.Navigation.PushAsync(new NavigationPage(new EclipsefestPage()))
                |> Async.AwaitIAsyncResult
                |> Async.Ignore
    }
    |> Async.RunSynchronously

代码更新

open System
open Xamarin.Forms
open Xamarin.Forms.Xaml

type MenuItem = 
    { Title : string
      TargetType : System.Type }

type MenuPage() = 
    inherit ContentPage()
    let _ = base.LoadFromXaml(typeof<MenuPage>)
    member this.Handle_Tapped(sender : Object, arg : EventArgs) = 
        this.Navigation.PushAsync(new NavigationPage(new EclipsefestPage())) |> ignore

我也尝试了这一部分，但收到了相同的错误。我怀疑我的问题在于异步部分，但似乎没有异步也会抛出相同的异常。

代码更新2

open System
open Xamarin.Forms
open Xamarin.Forms.Xaml

type MenuItem = 
    { Title : string
      TargetType : System.Type }

type MenuPage() = 
    inherit ContentPage()
    let _ = base.LoadFromXaml(typeof<MenuPage>)
    member this.Handle_Tapped(sender : Object, arg : EventArgs) = 
        let page = new EclipsefestPage()
        let navpage = new NavigationPage(page)
        let result = this.Navigation.PushAsync(navpage)
        result.RunSynchronously()

我开始有种感觉，这可能是一个bug。我已经验证了在调用Navigation.PushAsync之前，页面和导航页面都已经实例化了，但仍然出现相同的异常。

错误信息

"在iOS上全局不支持PushAsync，请使用NavigationPage。"

堆栈跟踪

System.InvalidOperationException: PushAsync is not supported globally on iOS, please use a NavigationPage.
  at Xamarin.Forms.Platform.iOS.Platform.Xamarin.Forms.INavigation.PushAsync (Xamarin.Forms.Page root, System.Boolean animated) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.iOS\Platform.cs:154
  at Xamarin.Forms.NavigationProxy.OnPushAsync (Xamarin.Forms.Page page, System.Boolean animated) [0x00022] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:190
  at Xamarin.Forms.NavigationProxy.PushAsync (Xamarin.Forms.Page root, System.Boolean animated) [0x00013] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:109
  at Xamarin.Forms.NavigationProxy.OnPushAsync (Xamarin.Forms.Page page, System.Boolean animated) [0x00022] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:190
  at Xamarin.Forms.NavigationProxy.PushAsync (Xamarin.Forms.Page root, System.Boolean animated) [0x00013] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:109
  at Xamarin.Forms.NavigationProxy.OnPushAsync (Xamarin.Forms.Page page, System.Boolean animated) [0x00022] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:190
  at Xamarin.Forms.NavigationProxy.PushAsync (Xamarin.Forms.Page root, System.Boolean animated) [0x00013] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:109
  at Xamarin.Forms.NavigationProxy.PushAsync (Xamarin.Forms.Page root) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\NavigationProxy.cs:102
  at <StartupCode$Eclipsefest>.$[e-mail] (Microsoft.FSharp.Core.Unit unitVar) [0x00090] in /Users/terrancesmith/Projects/Eclipsefest/Eclipsefest/MenuPage.xaml.fs:19
  at Microsoft.FSharp.Control.AsyncBuilderImpl+callA@851[b,a].Invoke (Microsoft.FSharp.Control.AsyncParams`1[T] args) [0x00047] in <5584acdb904cf4daa7450383dbac8455>:0
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Library/Frameworks/Xamarin.iOS.framework/Versions/10.10.0.36/src/mono/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:151
  at Microsoft.FSharp.Control.AsyncBuilderImpl.commit[a] (Microsoft.FSharp.Control.AsyncBuilderImpl+Result`1[T] res) [0x00030] in <5584acdb904cf4daa7450383dbac8455>:0
  at Microsoft.FSharp.Control.CancellationTokenOps.RunSynchronously[a] (System.Threading.CancellationToken token, Microsoft.FSharp.Control.FSharpAsync`1[T] computation, Microsoft.FSharp.Core.FSharpOption`1[T] timeout) [0x000a3] in <5584acdb904cf4daa7450383dbac8455>:0
  at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T] (Microsoft.FSharp.Control.FSharpAsync`1[T] computation, Microsoft.FSharp.Core.FSharpOption`1[T] timeout, Microsoft.FSharp.Core.FSharpOption`1[T] cancellationToken) [0x00071] in <5584acdb904cf4daa7450383dbac8455>:0
  at Eclipsefest.MenuPage.Handle_Tapped (System.Object sender, System.EventArgs arg) [0x00008] in /Users/terrancesmith/Projects/Eclipsefest/Eclipsefest/MenuPage.xaml.fs:16
  at Xamarin.Forms.Cell.OnTapped () [0x00008] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\Cells\Cell.cs:106
  at Xamarin.Forms.TextCell.OnTapped () [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\Cells\TextCell.cs:82
  at Xamarin.Forms.TableView+TableSectionModel.OnRowSelected (System.Object item) [0x00007] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\TableView.cs:214
  at Xamarin.Forms.TableModel.RowSelected (System.Object item) [0x0001a] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\TableModel.cs:65
  at Xamarin.Forms.TableModel.RowSelected (System.Int32 section, System.Int32 row) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\TableModel.cs:57
  at Xamarin.Forms.Platform.iOS.TableViewModelRenderer.RowSelected (UIKit.UITableView tableView, Foundation.NSIndexPath indexPath) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.iOS\Renderers\TableViewModelRenderer.cs:85
  at at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Users/builder/data/lanes/4691/d2270eec/source/xamarin-macios/src/UIKit/UIApplication.cs:79
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Users/builder/data/lanes/4691/d2270eec/source/xamarin-macios/src/UIKit/UIApplication.cs:63
  at Eclipsefest.iOS.Main.main (System.String[] args) [0x00001] in <59853c0a6da8a60ca74503830a3c8559>:0

- Terrance

---

这是 https://dev59.com/eVcO5IYBdhLWcg3w5lQM 的副本吗？（显然是不同的语言） - N_A

请注意，我已经使用“new NavigationPage()”作为现有问题的推荐解决方案，但这并没有解决问题。 - Terrance

你找到的例子可行吗？如果是这样，我认为最简单的方法是慢慢将工作正常的示例转换为上述代码，并查看何时出现错误。 - N_A

嗯，我刚意识到我找到的搜索结果是用C#写的。不过你应该能够将其移植到F#。 - N_A

错误已提交： https://bugzilla.xamarin.com/show_bug.cgi?id=58603 - Terrance

Answer 1

来自 Xamarin 团队的回答。

在您展示的代码中，您正在使用 NavigationPage 包装要推送的新页面。但是，需要在导航页中包含要从中推送的页面。

最简单的方法是将初始页面包装在 NavigationPage 中，然后将其设置为 MainPage。然后，您只需直接推送新页面，而无需将它们包装在另一个 NavigationPage 中，因为这样做会引起其他问题。请查看以下内容：

https://gist.github.com/jimmgarrido/d0b702c1282faa5bb91d0ed14e2d7e1b

namespace FForms

open System
open Xamarin.Forms

type TestPage() as this = 
    inherit ContentPage()

    let nextBtn = Button(Text = "Next Page")
    let layout = StackLayout()

    let handler = EventHandler(fun sender args ->
        let newPage = ContentPage(BackgroundColor = Color.Red)
        do this.Navigation.PushAsync(newPage))

    do nextBtn.Clicked.AddHandler handler
    do layout.Children.Add nextBtn
    do base.Content <- layout

type App() = 
    inherit Application(MainPage = NavigationPage(TestPage()))

希望这有所帮助！
它确实有用~

- Terrance