[英]Implementing IObservable<T> from scratch
Reactive Extensions帶有許多輔助方法,用於將現有事件和異步操作轉換為可觀察對象,但是如何從頭開始實現IObservable <T>?
IEnumerable有一個可愛的yield關鍵字,使其實現起來非常簡單。
實現IObservable <T>的正確方法是什么?
我需要擔心線程安全嗎?
我知道有人支持在特定的同步上下文中回調,但這是否是我作為IObservable <T>作者需要擔心或者以某種方式內置的內容?
更新:
這是Brian的F#解決方案的C#版本
using System;
using System.Linq;
using Microsoft.FSharp.Collections;
namespace Jesperll
{
class Observable<T> : IObservable<T>, IDisposable where T : EventArgs
{
private FSharpMap<int, IObserver<T>> subscribers =
FSharpMap<int, IObserver<T>>.Empty;
private readonly object thisLock = new object();
private int key;
private bool isDisposed;
public void Dispose()
{
Dispose(true);
}
protected virtual void Dispose(bool disposing)
{
if (disposing && !isDisposed)
{
OnCompleted();
isDisposed = true;
}
}
protected void OnNext(T value)
{
if (isDisposed)
{
throw new ObjectDisposedException("Observable<T>");
}
foreach (IObserver<T> observer in subscribers.Select(kv => kv.Value))
{
observer.OnNext(value);
}
}
protected void OnError(Exception exception)
{
if (isDisposed)
{
throw new ObjectDisposedException("Observable<T>");
}
if (exception == null)
{
throw new ArgumentNullException("exception");
}
foreach (IObserver<T> observer in subscribers.Select(kv => kv.Value))
{
observer.OnError(exception);
}
}
protected void OnCompleted()
{
if (isDisposed)
{
throw new ObjectDisposedException("Observable<T>");
}
foreach (IObserver<T> observer in subscribers.Select(kv => kv.Value))
{
observer.OnCompleted();
}
}
public IDisposable Subscribe(IObserver<T> observer)
{
if (observer == null)
{
throw new ArgumentNullException("observer");
}
lock (thisLock)
{
int k = key++;
subscribers = subscribers.Add(k, observer);
return new AnonymousDisposable(() =>
{
lock (thisLock)
{
subscribers = subscribers.Remove(k);
}
});
}
}
}
class AnonymousDisposable : IDisposable
{
Action dispose;
public AnonymousDisposable(Action dispose)
{
this.dispose = dispose;
}
public void Dispose()
{
dispose();
}
}
}
編輯:如果調用Dispose兩次,不要拋出ObjectDisposedException
官方文檔不贊成自己實現IObservable的用戶。 相反,用戶應該使用工廠方法Observable.Create
如果可能,通過組合現有運算符來實現新運算符。 否則使用Observable.Create實現自定義運算符
碰巧Observable.Create是Reactive的內部類AnonymousObservable
一個簡單的包裝器:
public static IObservable<TSource> Create<TSource>(Func<IObserver<TSource>, IDisposable> subscribe)
{
if (subscribe == null)
{
throw new ArgumentNullException("subscribe");
}
return new AnonymousObservable<TSource>(subscribe);
}
我不知道他們為什么不公開他們的實施,但是,嘿,無論如何。
老實說,我不確定這一切是多么'正確',但如果根據我迄今為止的經驗感覺相當不錯。 它是F#代碼,但希望你能感受到它的味道。 它允許您“新建”一個源對象,然后可以調用Next / Completed / Error on,它管理訂閱並在源或客戶端執行錯誤操作時嘗試Assert。
type ObservableSource<'T>() = // '
let protect f =
let mutable ok = false
try
f()
ok <- true
finally
Debug.Assert(ok, "IObserver methods must not throw!")
// TODO crash?
let mutable key = 0
// Why a Map and not a Dictionary? Someone's OnNext() may unsubscribe, so we need threadsafe 'snapshots' of subscribers to Seq.iter over
let mutable subscriptions = Map.empty : Map<int,IObserver<'T>> // '
let next(x) = subscriptions |> Seq.iter (fun (KeyValue(_,v)) -> protect (fun () -> v.OnNext(x)))
let completed() = subscriptions |> Seq.iter (fun (KeyValue(_,v)) -> protect (fun () -> v.OnCompleted()))
let error(e) = subscriptions |> Seq.iter (fun (KeyValue(_,v)) -> protect (fun () -> v.OnError(e)))
let thisLock = new obj()
let obs =
{ new IObservable<'T> with // '
member this.Subscribe(o) =
let k =
lock thisLock (fun () ->
let k = key
key <- key + 1
subscriptions <- subscriptions.Add(k, o)
k)
{ new IDisposable with
member this.Dispose() =
lock thisLock (fun () ->
subscriptions <- subscriptions.Remove(k)) } }
let mutable finished = false
// The methods below are not thread-safe; the source ought not call these methods concurrently
member this.Next(x) =
Debug.Assert(not finished, "IObserver is already finished")
next x
member this.Completed() =
Debug.Assert(not finished, "IObserver is already finished")
finished <- true
completed()
member this.Error(e) =
Debug.Assert(not finished, "IObserver is already finished")
finished <- true
error e
// The object returned here is threadsafe; you can subscribe and unsubscribe (Dispose) concurrently from multiple threads
member this.Value = obs
我會對任何有關這里好壞的想法感興趣; 我還沒有機會從devlabs看到所有新的Rx東西......
我自己的經歷表明:
如果人們能夠沿着這些方向展示更具體的建議,我會很好奇。
是的,yield關鍵字很可愛; 也許IObservable(OfT)會有類似的東西? [編輯:在Eric Meijer的PDC '09談話中,他說“是的,看這個空間”,以產生可觀察的聲明產量。
對於接近的東西(而不是自己動手),請查看“ (尚未)101 Rx Samples ”維基的底部 ,團隊建議使用Subject(T)類作為“后端”來實現IObservable( OFT)。 這是他們的例子:
public class Order
{
private DateTime? _paidDate;
private readonly Subject<Order> _paidSubj = new Subject<Order>();
public IObservable<Order> Paid { get { return _paidSubj.AsObservable(); } }
public void MarkPaid(DateTime paidDate)
{
_paidDate = paidDate;
_paidSubj.OnNext(this); // Raise PAID event
}
}
private static void Main()
{
var order = new Order();
order.Paid.Subscribe(_ => Console.WriteLine("Paid")); // Subscribe
order.MarkPaid(DateTime.Now);
}
裂開反射器並看看。
觀看一些C9視頻 - 這個視頻展示了如何'衍生'選擇'組合'
秘訣是創建AnonymousObservable,AnonymousObserver和AnonymousDisposable類(這些只是因為你無法實例化接口而解決)。 當您使用Actions和Funcs傳遞它時,它們包含零實現。
例如:
public class AnonymousObservable<T> : IObservable<T>
{
private Func<IObserver<T>, IDisposable> _subscribe;
public AnonymousObservable(Func<IObserver<T>, IDisposable> subscribe)
{
_subscribe = subscribe;
}
public IDisposable Subscribe(IObserver<T> observer)
{
return _subscribe(observer);
}
}
我會讓你解決剩下的問題......這是一個非常好的理解練習。
這里有一個很好的小線程,有相關的問題。
關於這個實現只有一個評論:
在.net fw 4中引入並發集合之后,最好使用ConcurrentDictioary而不是簡單的字典。
它節省了集合上的處理鎖。
ADI公司。
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