Question

我有一个类似静态的（发布者生命周期=应用程序生命周期）事件，我需要从视图中订阅。我无法可靠地确定何时导航视图（在Xamarin.Forms NavigationPage中按下导航栏后退按钮就是一个示例），因此我无法确定视图何时应取消订阅observable。（我知道可以在OnAppearing / OnDisappearing中订阅/取消订阅，但这会带来一系列问题我不会在这里详细介绍。）

因此，我发现自己需要让视图对事件略微订阅，即允许视图被垃圾收集而不必取消订阅事件。理想情况下，我喜欢可以在bold或function preloader(){ $('.preloader').delay(800).fadeOut('slow'); } $(preloader); $(document).on("turbolinks:load", preloader);或简称myObj.myEvent |> Observable.AsWeak |> Observable.Subscribe ...的行中使用的内容。

不幸的是我不知道如何实现这一点。我听说过System.WeakReference课程，但这对我来说都是全新的，我不知道如何正确使用它 - 我见过的大多数例子对于我尝试过的事情似乎过于复杂要做，这意味着要么我想要一些不同的东西，要么表面下面还有比我怀疑的更多的陷阱。

如何在允许订阅者进行垃圾收集而不取消订阅的情况下订阅F＃中的事件/可观察对象？

类似但不重复的问题：

Do F# observable events obviate, mediate, or are not related to the need for weak references?该问题询问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。

Answer 1

我已经找到了一个看似正常工作的相对简单的功能，虽然我不知道自己在做什么，所以我已经把它放在Code Review SE。它基于Samuel Jack的Weak Events in .Net, the easy way信息以及CodeProject Weak Events in C#中的解决方案4。

实施

module Observable =
    open System

    // ('a -> 'b -> unit) -> 'a -> IObservable<'b>
    let subscribeWeakly callback target source = 

        let mutable sub:IDisposable = null
        let mutable disposed = false
        let wr = new WeakReference<_>(target)

        let dispose() =
            lock (sub) (fun () -> 
                if not disposed then sub.Dispose(); disposed <- true)

        let callback' x =
            let isAlive, target = wr.TryGetTarget()
            if isAlive then callback target x else dispose()

        sub <- Observable.subscribe callback' source
        sub

用法示例

请参阅下面的WeakSubscriber类型。

重要

您必须使用回调＆＃39; me参数来调用相关方法。如果您在回调中使用this，则由于上述文章中描述的原因，您仍然会得到强有力的参考。出于同样的原因（我猜？），你不能调用＆＃34;普通＆＃34;使用let定义的类中的函数。（但是，您可以将方法定义为private。）

测试

助手班：

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

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

输出：

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

如何弱地订阅事件/可观察对象

1 个答案:

实施

用法示例

重要

测试