Unity - need to return value only after coroutine finishes

Question

I'm working on a game in unity and encountered an issue which I cannot solve. I'm connecting to a web server via standard WWW object and using a coroutine in order to execute a POST request.

The code in itself works, but I need to update a variable value and return that variable once the coroutine finishes, which I'm not able to do.

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;
}

private IEnumerator WaitForRequest(WWW www)
{

    yield return www;
    if (www.error == null)
    {

            if(www.text.Contains("user exists"))
            {

                success_fail = 2;
            }
            else
            {
                success_fail=1;
            }
    } else {
        success_fail=0;
    }    
}

The coroutine updates the value of 'success_fail' with the relevant value. But the 'return success_fail;' line in the POST method runs before the coroutine finishes, which causes it to return a false value.

I've tried to use an additional coroutine but unsuccessfully, suppose that I had a error there as well. How I can return the 'success_fail' value only after the coroutine finishes?

Thanks.

Answer 1

Functions don't wait for coroutines before return, however you could use an Action to give some kind of return.

Put this in your Start function

WWW www = new WWW("http://google.com");

StartCoroutine(WaitForRequest(www,(status)=>{
    print(status.ToString());
}));

and add this.

private IEnumerator WaitForRequest(WWW www,Action<int> callback) {
    int tempInt = 0;
    yield return www;
    if (string.IsNullOrEmpty(www.error)) {
        if(!string.IsNullOrEmpty(www.text)) {
            tempInt = 3;
        }
        else {
            tempInt=2;
        }
    } else {
        print(www.error);
        tempInt=1;
    }
    callback(tempInt);
}

Give this a try, although the function can change a value it doesn't return a value and only has a single parameter. So in essence this isn't a solution for returning your coroutine however once received the int from the coroutine we are then able to justify what to do with it and even call other functions from within the callback.

StartCoroutine(WaitForRequest(www,(status)=>{
    print(status.ToString());
    Awake(); // we can call other functions within the callback to use other codeblocks and logic.
    if(status != 0)
        print("yay!");
    }
));

This might be of use to you. http://answers.unity3d.com/questions/744888/returning-an-ienumerator-as-an-int.html

Answer 2

Only a coroutine can wait for another coroutine. Since you need to wait for the coroutine you started (WaitForRequest), it means you have to convert POST to be a coroutine and it won't be able to return int.

It looks like success_fail is a member variable, so if that's exposed to whoever is starts POST (as a coroutine), you wouldn't need to return it anyway.

public int success_fail

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

    WWW www = new WWW(url, form);

    yield return StartCoroutine(WaitForRequest(www));
}

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

Basically, if you want your code to "wait", it has to be a coroutine. You can't make a call that waits without blocking the whole engine (without some type of loop hack).

This thread gives a way where you could return the int from your coroutine if you really need to, but POST still can't be a blocking call...

http://answers.unity3d.com/questions/24640/how-do-i-return-a-value-from-a-coroutine.html