How to trigger and wait for the result of a notification from a call back function of an event listener

public Task<UserInfo> KickOffWarmUpCache(string id)
{

    //If Status = Progress
    if (cache.Get(id).Status == "Progress")
    {
        var ret = new TaskCompletionSource<UserInfo>();
        NotifyOnceCacheIsUpdated(id,(result)=>
        {
            //complete the task.
            ret.SetResult(result.userInfo);
        });
        //return a Task...which the SetResult will complete.
        return ret.Task;
    }

    //Return a result synchronously
    return Task.FromResult(Cache.Get(id));
}

public void openFileExplorer() {
    File mPath = new File(Environment.getExternalStorageDirectory() + "/");
    fileDialog = new FileDialog(this, mPath);
    fileDialog.addFileListener(new FileDialog.FileSelectedListener() {
        public void fileSelected(File file) {
            Log.d(getClass().getName(), "selected file " + file.toString());
            chosenFile = file.toString();
generateMFCC(chosenFile);

        }
    });
    fileDialog.showDialog();
}

function checkFlag() {
    if(flag == false) {
       window.setTimeout(checkFlag, 100); /* this checks the flag every 100 milliseconds*/
    } else {
      /* do something*/
    }
}
checkFlag();

window.addEventListener("message", function(e) {
    if (/* check `e.origin` and make sure you're happy with it*/) {
        if (e.data === "onWidgetDisplayed") {
            onWidgetDisplayed.call(this, e);
        }
    }
}, false);