Creating a weak subscription to an IObservable

Question

What I want to do is ensure that if the only reference to my observer is the observable, it get's garbage collected and stops receiving messages.

Say I have a control with a list box on it called Messages and this code behind:

//Short lived display of messages (only while the user's viewing incoming messages)
public partial class MessageDisplay : UserControl
{
    public MessageDisplay()
    {
        InitializeComponent();
        MySource.IncomingMessages.Subscribe(m => Messages.Items.Add(m));
    }
}

Which is connecting to this source:

//Long lived location for message store
static class MySource
{
    public readonly static IObservable<string> IncomingMessages = new ReplaySubject<string>;
}

What I don't want is to have the Message Display being kept in memory long after it's no longer visible. Ideally I'd like a little extension so I can write:

MySource.IncomingMessages.ToWeakObservable().Subscribe(m => Messages.Items.Add(m));

I also don't want to rely on the fact that MessageDisplay is a user control as I will later want to go for an MVVM setup with MessageDisplayViewModel which won't be a user control.

Answer 1

You can subscribe a proxy observer to the observable that holds a weak reference to the actual observer and disposes the subscription when the actual observer is no longer alive:

static IDisposable WeakSubscribe<T>(
    this IObservable<T> observable, IObserver<T> observer)
{
    return new WeakSubscription<T>(observable, observer);
}

class WeakSubscription<T> : IDisposable, IObserver<T>
{
    private readonly WeakReference reference;
    private readonly IDisposable subscription;
    private bool disposed;

    public WeakSubscription(IObservable<T> observable, IObserver<T> observer)
    {
        this.reference = new WeakReference(observer);
        this.subscription = observable.Subscribe(this);
    }

    void IObserver<T>.OnCompleted()
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnCompleted();
        else this.Dispose();
    }

    void IObserver<T>.OnError(Exception error)
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnError(error);
        else this.Dispose();
    }

    void IObserver<T>.OnNext(T value)
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnNext(value);
        else this.Dispose();
    }

    public void Dispose()
    {
        if (!this.disposed)
        {
            this.disposed = true;
            this.subscription.Dispose();
        }
    }
}

Answer 2

Ran across this thread a couple years later...just wanted to point forward to the solution identified on Samuel Jack's blog which adds an extension method to IObservable called WeaklySubscribe. It uses an approach of adding a shim between the subject and observer that tracks the target with a WeakReference. That is similar to solutions offered by others for the problem of strong references in event subscriptions, such as in this article or this solution by Paul Stovell. Having for awhile used something based on Paul's approach I like Samuel's solution to weak IObservable Subscribes.

Answer 3

There is another option using the weak-event-patterns

Essentially System.Windows.WeakEventManager has you covered.

Using MVVM when your ViewModel relies on services with events you can weakly subscribe to those services allowing your ViewModel to be collected with the view without the event subscription keeping it alive.

using System;
using System.Windows;

class LongLivingSubject
{ 
    public event EventHandler<EventArgs> Notifications = delegate { }; 
}

class ShortLivingObserver
{
    public ShortLivingObserver(LongLivingSubject subject)
    { 
        WeakEventManager<LongLivingSubject, EventArgs>
            .AddHandler(subject, nameof(subject.Notifications), Subject_Notifications); 
    }

    private void Subject_Notifications(object sender, EventArgs e) 
    { 
    }
}

Answer 4

this is my implementation (quit simple one)

public class WeakObservable<T>: IObservable<T>
{
    private IObservable<T> _source;

    public WeakObservable(IObservable<T> source)
    {
        #region Validation

        if (source == null)
            throw new ArgumentNullException("source");

        #endregion Validation

        _source = source;
    }

    public IDisposable Subscribe(IObserver<T> observer)
    {
        IObservable<T> source = _source;
        if(source == null)
            return Disposable.Empty;
        var weakObserver = new WaekObserver<T>(observer);
        IDisposable disp = source.Subscribe(weakObserver);
        return disp;
    }
}
    public class WaekObserver<T>: IObserver<T>
{
    private WeakReference<IObserver<T>> _target;

    public WaekObserver(IObserver<T> target)
    {
        #region Validation

        if (target == null)
            throw new ArgumentNullException("target");

        #endregion Validation

        _target = new WeakReference<IObserver<T>>(target);
    }

    private IObserver<T> Target
    {
        get
        {
            IObserver<T> target;
            if(_target.TryGetTarget(out target))
                return target;
            return null;
        }
    }

    #region IObserver<T> Members

    /// <summary>
    /// Notifies the observer that the provider has finished sending push-based notifications.
    /// </summary>
    public void OnCompleted()
    {
        IObserver<T> target = Target;
        if (target == null)
            return;

        target.OnCompleted();
    }

    /// <summary>
    /// Notifies the observer that the provider has experienced an error condition.
    /// </summary>
    /// <param name="error">An object that provides additional information about the error.</param>
    public void OnError(Exception error)
    {
        IObserver<T> target = Target;
        if (target == null)
            return;

        target.OnError(error);
    }

    /// <summary>
    /// Provides the observer with new data.
    /// </summary>
    /// <param name="value">The current notification information.</param>
    public void OnNext(T value)
    {
        IObserver<T> target = Target;
        if (target == null)
            return;

        target.OnNext(value);
    }

    #endregion IObserver<T> Members
}
    public static class RxExtensions
{
    public static IObservable<T> ToWeakObservable<T>(this IObservable<T> source)
    {
        return new WeakObservable<T>(source);
    }
}
        static void Main(string[] args)
    {
        Console.WriteLine("Start");
        var xs = Observable.Interval(TimeSpan.FromSeconds(1));
        Sbscribe(xs);

        Thread.Sleep(2020);
        Console.WriteLine("Collect");
        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();
        Console.WriteLine("Done");
        Console.ReadKey();
    }

    private static void Sbscribe<T>(IObservable<T> source)
    {
        source.ToWeakObservable().Subscribe(v => Console.WriteLine(v));
    }

Answer 5

The key is to recognize that you are going to have to pass in both the target and a two-parameter action. A one-parameter action will never do it, because either you use a weak reference to your action (and the action gets GC'd), or you use a strong reference to your action, which in turn has a strong reference to the target, so the target can't get GC'd. Keeping that in mind, the following works:

using System;

namespace Closures {
  public static class WeakReferenceExtensions {
    /// <summary> returns null if target is not available. Safe to call, even if the reference is null. </summary>
    public static TTarget TryGetTarget<TTarget>(this WeakReference<TTarget> reference) where TTarget : class {
      TTarget r = null;
      if (reference != null) {
        reference.TryGetTarget(out r);
      }
      return r;
    }
  }
  public static class ObservableExtensions {

    public static IDisposable WeakSubscribe<T, U>(this IObservable<U> source, T target, Action<T, U> action)
      where T : class {
      var weakRef = new WeakReference<T>(target);
      var r = source.Subscribe(u => {
        var t = weakRef.TryGetTarget();
        if (t != null) {
          action(t, u);
        }
      });
      return r;
    }
  }
}

Sample Observable:

using System;
using System.Reactive.Subjects;

namespace Closures {
  public class Observable {
    public IObservable<int> ObservableProperty => _subject;
    private Subject<int> _subject = new Subject<int>();
    private int n;
    public void Fire() {
      _subject.OnNext(n++);
    }
  }
}

Usage:

Class SomeClass {

 IDisposable disposable;

 public void SomeMethod(Observable observeMe) {
   disposable = observeMe.ObservableProperty.WeakSubscribe(this, (wo, n) => wo.Log(n));
 }

  public void Log(int n) {
    System.Diagnostics.Debug.WriteLine("log "+n);
  }
}

Answer 6

The code below is inspired by dtb's original post. The only change is that it returns a reference to the observer as part of the IDisposable. This means that the reference to the IObserver will be kept alive as long as you keep a reference to the IDisposable that you get out at the end of the chain (assuming all disposables keep a reference to the disposable before them). This allows the usage of the extension methods such as Subscribe(M=>DoSomethingWithM(M)) because we keep a reference to the implicitly constructed IObserver but we don't keep a strong reference from the source to the IObserver (which would produce a memory leek).

using System.Reactive.Linq;

static class WeakObservation
{
    public static IObservable<T> ToWeakObservable<T>(this IObservable<T> observable)
    {
        return Observable.Create<T>(observer =>
            (IDisposable)new DisposableReference(new WeakObserver<T>(observable, observer), observer)
            );
    }
}

class DisposableReference : IDisposable
{
    public DisposableReference(IDisposable InnerDisposable, object Reference)
    {
        this.InnerDisposable = InnerDisposable;
        this.Reference = Reference;
    }

    private IDisposable InnerDisposable;
    private object Reference;

    public void Dispose()
    {
        InnerDisposable.Dispose();
        Reference = null;
    }
}

class WeakObserver<T> : IObserver<T>, IDisposable
{
    private readonly WeakReference reference;
    private readonly IDisposable subscription;
    private bool disposed;

    public WeakObserver(IObservable<T> observable, IObserver<T> observer)
    {
        this.reference = new WeakReference(observer);
        this.subscription = observable.Subscribe(this);
    }

    public void OnCompleted()
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnCompleted();
        else this.Dispose();
    }

    public void OnError(Exception error)
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnError(error);
        else this.Dispose();
    }

    public void OnNext(T value)
    {
        var observer = (IObserver<T>)this.reference.Target;
        if (observer != null) observer.OnNext(value);
        else this.Dispose();
    }

    public void Dispose()
    {
        if (!this.disposed)
        {
            this.disposed = true;
            this.subscription.Dispose();
        }
    }
}