How to Properly Return Values from Unity Coroutines?

Returning Values from Coroutines in Unity

In game development using Unity, coroutines are commonly used to perform asynchronous tasks, such as sending HTTP requests. However, retrieving results from a coroutine after it completes can be challenging.

Consider the following code snippet, where a coroutine is used to execute a POST request and update a variable, success_fail:

public int POST(string username, string passw)
{
    WWWForm form = new WWWForm();
    form.AddField("usr", username);
    form.AddField("pass", passw);

    WWW www = new WWW(url, form);

    StartCoroutine(WaitForRequest(www));

    //problem is here !
    return success_fail;
}

The problem here is that the return statement executes before the coroutine completes, resulting in an incorrect value being returned. To address this, we can use the following approach:

Using Actions

Instead of returning a value directly from the function, we can use an Action parameter to provide a callback that will be invoked once the coroutine completes. Here is the modified code:

public void POST(string username, string passw, Action<int> callback)
{
    WWWForm form = new WWWForm();
    form.AddField("usr", username);
    form.AddField("pass", passw);

    WWW www = new WWW(url, form);

    StartCoroutine(WaitForRequest(www, callback));
}

In the coroutine, we set the success_fail variable and invoke the callback with its value:

private IEnumerator WaitForRequest(WWW www, Action<int> callback)
{
    yield return www;
    if (www.error == null)
    {
        if(www.text.Contains("user exists"))
        {
            success_fail = 2;
        }
        else
        {
            success_fail=1;
        }
    } else {
        success_fail=0;
    }    
    
    callback(success_fail);
}

You can call the POST function from any script and provide a callback to handle the result:

this.POST("user1", "password", (result) => {
    print("Result: " + result);
});

This approach allows you to retrieve the value from the coroutine once it has completed, even if the function has already returned.

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