イベントパターンと注意点（メモリリーク）

注意点の概要

リークを防ぐために購読を解除する

using System;

public class Publisher
{
  public event EventHandler Tick;
  public void Fire()
  {
    EventHandler h = Tick;
    if (h != null) h(this, EventArgs.Empty);
  }
}

public class Subscriber
{
  public void Handle(object s, EventArgs e)
  {
    Console.WriteLine("Handling tick");
  }

  public void Subscribe(Publisher p) { p.Tick += Handle; }
  public void Unsubscribe(Publisher p) { p.Tick -= Handle; } // required to avoid leaks
}

public class Program
{
  public static void Main(string[] args)
  {
    Publisher p = new Publisher();
    Subscriber s = new Subscriber();

    s.Subscribe(p);
    p.Fire();                // prints once

    s.Unsubscribe(p);        // detach when done
    s = null;                // now eligible for GC in a real app
    p.Fire();                // no output
  }
}

ハンドラーの参照を保持する

using System;

public class Button
{
  public event EventHandler Click;
  public void Simulate()
  {
    EventHandler h = Click;
    if (h != null) h(this, EventArgs.Empty);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Button b = new Button();

    // BAD: cannot easily remove this later
    // b.Click += (s, e) => Console.WriteLine("inline");

    // GOOD: store the handler, attach, later detach the same instance
    EventHandler saved = (s, e) => Console.WriteLine("saved");
    b.Click += saved;

    b.Simulate();           // prints "saved"
    b.Click -= saved;       // can remove because we kept the reference
    b.Simulate();           // no output
  }
}

破棄可能な購読

using System;

public class Ticker
{
  public event EventHandler Tick;

  public IDisposable Subscribe(EventHandler handler)
  {
    Tick += handler;
    return new Unsubscriber(this, handler);
  }

  private sealed class Unsubscriber : IDisposable
  {
    private Ticker _t;
    private EventHandler _h;
    private bool _done;

    public Unsubscriber(Ticker t, EventHandler h) { _t = t; _h = h; }

    public void Dispose()
    {
      if (!_done)
      {
        _t.Tick -= _h;
        _done = true;
      }
    }
  }

  public void Fire()
  {
    EventHandler h = Tick;
    if (h != null) h(this, EventArgs.Empty);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Ticker t = new Ticker();
    using (t.Subscribe((s, e) => Console.WriteLine("tick")))
    {
      t.Fire(); // prints once
    }
    t.Fire();   // nothing after disposal
  }
}

try/finally で解除する

using System;

public class Alarm
{
  public event EventHandler Ring;
  public void Trigger()
  {
    EventHandler h = Ring;
    if (h != null) h(this, EventArgs.Empty);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Alarm a = new Alarm();
    EventHandler h = (s, e) => Console.WriteLine("ring");

    a.Ring += h;
    try
    {
      a.Trigger(); // do work with subscription active
    }
    finally
    {
      a.Ring -= h; // always detach
    }

    a.Trigger(); // no output
  }
}

リークを防ぐヒント

イベントによるリークの原因

まとめ