Waiting for a value to return from another script using coroutines?

For web requests you could use something like this:

 // C#
 // WebRequest.cs
 public class WebRequest : MonoBehaviour
 {
     private static WebRequest m_Instance = null;
     public static WebRequest Instance
     {
         get
         {
             if (m_Instance == null)
             {
                 m_Instance = (WebRequest)FindObjectOfType(typeof(WebRequest));
                 if (m_Instance == null)
                     m_Instance = (new GameObject("WebRequest")).AddComponent<WebRequest>();
                 DontDestroyOnLoad(m_Instance.gameObject);
             }
             return m_Instance;
         }
     }
     public static Coroutine Get(string aURL, System.Action<bool, string> aCallback)
     {
         return Instance.StartCoroutine(_GetRequest(aURL, aCallback));
     }
     public static Coroutine Post(string aURL, WWWForm aForm, System.Action<bool, string> aCallback)
     {
         return Instance.StartCoroutine(_PostRequest(aURL, aForm, aCallback));
     }
 
     private static IEnumerator _GetRequest(string aURL, System.Action<bool, string> aCallback)
     {
         WWW request = new WWW(aURL);
         yield return request;
         if (string.IsNullOrEmpty(request.error))
         {
             if (aCallback != null)
                 aCallback(true, request.text);
         }
         else
         {
             if (aCallback != null)
                 aCallback(false, request.error);
         }
     }
     private static IEnumerator _PostRequest(string aURL, WWWForm aForm, System.Action<bool, string> aCallback)
     {
         WWW request = new WWW(aURL, aForm);
         yield return request;
         if (string.IsNullOrEmpty(request.error))
         {
             if (aCallback != null)
                 aCallback(true, request.text);
         }
         else
         {
             if (aCallback != null)
                 aCallback(false, request.error);
         }
     }
 }

With this class you can start a webrequest and you can pass a callback to the function which is called when the request has finished. The callback has two parameters, a bool and a string. The bool indicates success and the string is either the response or the error text.

This class can be used from everywhere since it's a singleton. Just do this:

 WebRequest.Get("http://www.google.com", (success, text) =>{
     if (success)
     {
         Debug.Log("Success: " + text);
         // Do something with text
     }
 });

Since the function returns the Coroutine object you can also wait in a coroutine like this:

 IEnumerator Start()
 {
     string result = "";
     yield return WebRequest.Get("http://www.google.com", (success, text) =>result = success?text:"");
     Debug.Log("Done: " + result);
 }

Note: I just rewrote the class here in UA so i didn't test the class, but i use a similar implementation in my projects ;)

Because of the asynchronous nature of this behavior you're trying to model, I would think it's best to use events. However, Unity has a similar system, without the coding hassle; SendMessage. If you have numerous, or even lots of this kind of behavior, I would suggest using events, as SendMessage is quite costly and could cause lag.

I'm thinking something along these lines for your program logic: as soon as your Script B gets its value, it sends a message to Script A notifying it's ok to read the value.

Hmm. I'm wondering if you could use a different method. Maybe you could do

 while (foo == null){
      yield return new WaitForSeconds(0.2f);
 }

The problem with this is that it could loop infinitely if foo remains null. So perhaps instead use a for loop, and after a few loops, return an error.