Instantiation of a GameObject not working for a threading reason

Question

I receive the following error

"INTERNAL_CALL_Internal_InstantiateSingle can only be called from the main thread." "Constructors and field initializers will be executed from the loading thread when loading a scene." "Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function."

whenever I try to instantiate some GameObject's using this function:

public void DisplayCalls () {
    for (int i = 0; i < calls [0].Count; i++) {
        for (int j = 0; j < impFields.Length; j++) {
            GameObject textClone = Instantiate (textBox, Vector3.zero, Quaternion.Euler(Vector3.zero)) as GameObject;
            textClone.transform.SetParent (canvas.transform);
            Text text = textClone.GetComponent <Text> ();
            text.text = calls[impFields [j]][i];
            text.alignment = TextAnchor.MiddleCenter;
            textClone.transform.localPosition = new Vector3 (-262.5f + (175f * j), (182f + (30f * i)), 0f);
        }
    }
}

The function is called using this other function:

public void GetPanelInfo () {
    string file = "getpanelinfo.php";
    string hash = Md5Sum (mostRecentID.ToString () + SecretKey1 + SecretKey2);
    string parameters = "?mostrecentid=" + mostRecentID.ToString () + "&hash=" + hash;
    string fullURL = baseURL + file + parameters;
    Uri fullURI = new Uri (fullURL);
    WebClient client = new WebClient ();

    string jsonString = string.Empty;
    client.DownloadStringCompleted += (sender, e) => {
        if (!e.Cancelled && e.Error == null) {
            jsonString = e.Result;
            JSONNode json = JSON.Parse (jsonString);
            for (int i = 0; i < json["calls"].Count; i++) {
                for (int j = 0; j < calls.Length; j++) {
                    calls[j].Add (json["calls"][i][names [j]]);
                }
                mostRecentID = json["calls"][i]["callID"].AsInt;
            }
        } else if (e.Cancelled && e.Error == null) {
            Debug.Log ("Cancelled");
        } else {
            Debug.Log ("Error: " + e.Error);
        }
        loading = false;
        LogCalls ();
        DisplayCalls ();

    };
    client.DownloadStringAsync (fullURI);
}

Which is called once using a button.

I cannot seem to find the problem, but as stated above, the error says that it is a threading problem and that I cannot call instantiate from another thread other than the main one. I do not know how to change the instantiation to another thread, but any help to solve this problem would be much appreciated.

Answer 1

Looking at the WebClient docs, the method is described as:

Downloads a string from a resource, without blocking the calling thread.

and given the error message:

INTERNAL_CALL_Internal_InstantiateSingle can only be called from the main thread

It's clear that you need to download the data and store it, and only call DisplayCells, in the main thread, once the download is complete. This is not unique as in Cocoa, for example, you can only call UI-related API methods on the main thread.

It took me some hunting on Google to find any decent info on this (I am a beginner when it comes to Unity/C#), but I think this will do it:

Create a boolean that is shows there is data to display:

private bool _callsToDisplay = false;

Set that when the data has downloaded:

client.DownloadStringCompleted += (sender, e) => {
    ...
    loading = false;
    LogCalls ();
    _callsToDisplay = true;
};

and then in the Update() method (which runs on the main thread), call DisplayCalls() if that boolean is set:

void Update()
{
    if (_callsToDisplay) {
        DisplayCalls();
        _callsToDisplay = false;
    }
}

Answer 2

I think this is happening because you are trying to Instantiate() in a class that does not derive from MonoBehaviour.

Ensure that the class you use to "Instantiate" gameobjects, derive from the MonoBehaviour base class, otherwise, you will be getting this error.