获取"PushAsync在iOS上不支持全局.“在使用NavigationPage时

Question

艾略尔，我一直得到以下信息：

System.InvalidOperationException with "PushAsync在iOS上不支持全局，请使用NavigationPage“。

尽管我使用的是一个NavigationPage，而且从我所能知道的情况来看，我正在正确地使用它。我的F#技能并不令人惊讶，所以我肯定认为我在这里做错了什么，但令人惊讶的是，在F#中很难找到预生成Navigation.PushAsync的例子。我确实找到了这段代码，并一直在寻找一个正确外观的例子。

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

在这个示例中，我看到了包装在async{}中的代码

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

所以我想我离这里不远了。这么说，我做错什么了？

AssemblyVersions

• 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

我也尝试过这一点，并收到了同样的错误。我怀疑我的问题在于异步部分，但它似乎也抛出了相同的异常而没有异步。

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()

我开始觉得这是个窃听器。我已经验证了在调用Navigation.PushAsync之前，页面和导航页是在单独的步骤中实例化的，并且仍然得到相同的异常。

错误

"PushAsync在iOS上不受全局支持，请使用NavigationPage。“

StackTrace

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$Eclipsefest>.$MenuPage.xaml+Handle_Tapped@17.Invoke (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

Answer 1

来自Xamarin的人的回答。

在您显示的代码中，您正在包装要在NavigationPage中推送的新页面。但是，您正在推送的页面需要在导航页中。 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，只需直接推送新页面，而不将它们包装在另一个NavigationPage中，因为这样做会导致其他问题。看看这个：

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()))

希望这能有所帮助！

它做了~