Events 如何弱订阅事件/观测值

我需要从视图订阅一个类似静态（发布者生存期=应用程序生存期）的事件。我无法可靠地确定视图何时被导航离开（Xamarin.Forms NavigationPage中按下的navbar back按钮就是一个例子），因此我无法确定视图何时应该从observable取消订阅。（我知道在OnAppearing/OnDisappearing中订阅/取消订阅是可能的，但这也带来了一系列问题，我在这里不详细介绍。）

因此，我发现自己需要视图弱订阅事件，即允许视图被垃圾收集而不必取消订阅事件。理想情况下，我想要的东西可以按照

myObj.myEvent |>Observable.AsWeak |>Observable.Subscribe…

，或者

myObj.myEvent |>Observable.subscribebevely…

，或者简单地

myObj.myEvent.subscribebevent…

不幸的是，我不知道如何实现这一点。我听说过这门课，但这对我来说是全新的，我不知道如何正确地使用它——我所看到的大多数例子对于我试图做的事情来说似乎过于复杂，这意味着要么我想要一些不同的东西，要么表面下的陷阱比我想象的要多得多

我如何在F#中订阅事件/观察对象，同时允许订阅方在不取消订阅的情况下进行垃圾收集

类似但不重复的问题：

• 问题是在F#中是否需要弱引用，而不是如何实现上述功能。这个问题的唯一答案在这方面也没有帮助

---

我已经找到了一个相对简单的函数，它似乎可以正常工作，尽管我真的不知道我在做什么，所以我把它放在了网站上。它基于Samuel Jack的信息以及CodeProject中的解决方案4

实施 实际测试：

[<EntryPoint>]
let main argv = 

    let pub = Publisher()

    let doGc() =
        System.GC.Collect()
        System.GC.WaitForPendingFinalizers()
        System.GC.Collect()
        printfn "\nGC completed\n"

    let someScope() =
        let strong = StrongSubscriber()
        let weak = WeakSubscriber()
        strong.Subscribe(pub)
        weak.Subscribe(pub)

        doGc() // should not remove weak subscription since it's still in scope
        printfn "All subscribers should still be triggered:"
        pub.Trigger(1)

    someScope()

    doGc() // should remove weak subscriptions
    printfn "Weak subscribers should not be triggered:"
    pub.Trigger(2)

    System.Console.ReadKey() |> ignore

    0

---

你不能直接“重写OnParentSet（）”和“if（Parent==null）”在那里取消订阅吗。只要点击“后退”按钮弹出页面时，视图从屏幕上移除/就像父视图一样，它就会一直被调用。这很简单，但感谢您的提示。（顺便说一句，OnAppearing的主要问题是，在iOS上，它是在视图完全出现后调用的。）@Funk的可能重复在发布之前，我读了好几遍这个问题及其答案，但没有帮助，也不是重复。我已经更新了这个问题来解释这一点。如果你把它作为一个一般的.NET问题来处理，并把你发现的任何东西翻译成F#，可能会有所帮助。我在这里没有现成的答案，但有一种被很好地描述的模式（在WPF中使用）是使用中介

WeakEventManager

（请参阅）。如果Xamarin已经有了相应的解决方案，我不会感到惊讶。

type Publisher() =
    let myEvent = new Event<_>()
    [<CLIEvent>] member this.MyEvent = myEvent.Publish
    member this.Trigger(x) = myEvent.Trigger(x)


type StrongSubscriber() =

    member this.MyMethod x = 
        printfn "Strong: method received %A" x

    member this.Subscribe(publisher:Publisher) =
        publisher.MyEvent |> Observable.subscribe this.MyMethod
        publisher.MyEvent |> Observable.subscribe 
                             (fun x -> printfn "Strong: lambda received %A" x)


type WeakSubscriber() =

    member this.MyMethod x = 
        printfn "Weak: method received %A" x

    member this.Subscribe(publisher:Publisher) =
        publisher.MyEvent |> Observable.subscribeWeakly
                             (fun (me:WeakSubscriber) x -> me.MyMethod x) this
        publisher.MyEvent |> Observable.subscribeWeakly
                             (fun _ x -> printfn "Weak: lambda received %A" x) this

[<EntryPoint>]
let main argv = 

    let pub = Publisher()

    let doGc() =
        System.GC.Collect()
        System.GC.WaitForPendingFinalizers()
        System.GC.Collect()
        printfn "\nGC completed\n"

    let someScope() =
        let strong = StrongSubscriber()
        let weak = WeakSubscriber()
        strong.Subscribe(pub)
        weak.Subscribe(pub)

        doGc() // should not remove weak subscription since it's still in scope
        printfn "All subscribers should still be triggered:"
        pub.Trigger(1)

    someScope()

    doGc() // should remove weak subscriptions
    printfn "Weak subscribers should not be triggered:"
    pub.Trigger(2)

    System.Console.ReadKey() |> ignore

    0

GC completed

All subscribers should still be triggered:
Strong: method received 1
Strong: lambda received 1
Weak: method received 1
Weak: lambda received 1

GC completed

Weak subscribers should not be triggered:
Strong: method received 2
Strong: lambda received 2