Custom yield instruction to wait inside a Unity coroutine until a callback is called.

Example.txt

public static class UrlDownloader
{
	public static void DownloadText(string url, Action<string> callback);
}

// Brute force way of doing it
IEnumerator MyAsyncFunction()
{
	var done = false;
	var result = default(string);
	UrlDownloader.DownloadText("http://google.com", r => 
  { 
    result = r; 
    done = true; 
  });
	while (done == false)
	{
		yield return result;
	}
	Debug.Log("downloaded: " + result);
}

// Using the custom yield instruction
IEnumerator MyAsyncFunction()
{
	var download = new WaitForCallback<string>(
		done => UrlDownloader.DownloadText("http://google.com", text => done(text))
	);
	yield return download;
	Debug.Log("downloaded: " + download.Result);
}

WaitForCallback.cs

using UnityEngine;
 
// Custom yield instruction to wait until a callback is called
// by Jackson Dunstan, http://JacksonDunstan.com/articles/3678
// License: MIT
public class WaitForCallback<TResult> : CustomYieldInstruction
{
	public TResult Result { get; private set; }
	
	private bool done;
 
	public WaitForCallback(Action<Action<TResult>> callCallback)
	{
		callCallback(r => 
		{ 
		  Result = r; 
		  done = true; 
	  	});
	}
 
	override public bool keepWaiting { get { return done == false; } }
}