C# Parse how to wait until Query returns a value

Question

I'm trying to retrieve user data from Parse (xamarin.ios using c#). I'm using an async method with await. My challenge is,each time I navigate to the tableView in the app, which should populate the user data in question,the table is always empty.

I would like to wait until the results have been returned before proceeding with the other portion of code.I have tried to use the ContinueWith() function but constantly ran into a build error -

Cannot implicitly convert type 'void' to System.Collections.Generic.IEnumerable<Parse.ParseObject>

My Questions:

1. Is this the best way to wait for the result?
2. How do I solve the build error?

Here is my current implementation:

public async void retrieveData(string username)
    {
        try
        {

            this.requests.ClearRequests();
            refreshed = false;
            var query = ParseObject.GetQuery("Requests").WhereEqualTo("username", username);

            IEnumerable<ParseObject> results = await query.FindAsync().ContinueWith(t =>{

                if(results != null)
                {
                    foreach(ParseObject parseObject in results)
                    {
                        UserRequest request = new UserRequest();
                        request.objectId = parseObject.ObjectId;
                        request.make = parseObject.Get<string> ("item1");
                        request.model = parseObject.Get<string> ("item2");
                        request.year = parseObject.Get<string> ("item3");
                        request.userName = parseObject.Get<string> ("username");
                        this.requests.addRequest (request);
                    }
                    refreshed = true;
                }



            });
}
        catch(ParseException e) {
            Console.WriteLine (e.Message + e.StackTrace);
        }
    }

Answer 1

You shouldn't need a ContinueWith...that's what the await should handle.

await waits on a Task and then brings back the result with the proper return type. ContinueWith returns a Task, so you would have to grab the Result from the task to make it usable.

For more on this type of thing, you may want to check out Difference between await and ContinueWith

You can try something like this.

public async void retrieveData(string username, )
    {
        try
        {

            this.requests.ClearRequests();
            refreshed = false;
            var query = ParseObject.GetQuery("Requests").WhereEqualTo("username", username);

            IEnumerable<ParseObject> results = await query.FindAsync();

            if(results != null)
            {
                foreach(ParseObject parseObject in results)
                {
                    UserRequest request = new UserRequest();
                    request.objectId = parseObject.ObjectId;
                    request.make = parseObject.Get<string> ("item1");
                    request.model = parseObject.Get<string> ("item2");
                    request.year = parseObject.Get<string> ("item3");
                    request.userName = parseObject.Get<string> ("username");
                    this.requests.addRequest (request);
                }
                refreshed = true;
            }
            //This is your refresh method for your TableView
            this.RefreshTableView();
            //or, if in iOS
            NSNotificationCenter.DefaultCenter.PostNotificationName("resultsRetrieved", null);
        }
        catch(ParseException e) {
            Console.WriteLine (e.Message + e.StackTrace);
        }
    }

To show the results in the tableView, I would recommend moving the refreshing of the tableView to a separate method that gets triggered synchronously after the results have been retrieved and parsed. This is shown with the this.RefreshTableView() call above.

If in iOS on Xamarin, another option is to post a notification to the NSNotificationCenter (the Xamarin documentation for which is here). Use the PostNotificationName part seen above instead and then add an observer in the ViewControllers that you want to be dependent on the data. This is done as follows:

Make a notificationToken object:

    NSObject notificationToken;

Then in your setup method (you could put this inside of your ViewDidLoad):

void Setup ()
{
    notificationToken = NSNotificationCenter.DefaultCenter.AddObserver ("resultsRetrieved", RefreshData);
}

Make your RefeshData method:

void RefreshData (NSString notifString)
{
    this.tableView.ReloadData();
}

And then, make sure you dispose of the notification observer when you tear down the class

void Teardown ()
{
    NSNotificationCenter.DefaultCenter.RemoveObserver (notificationToken);
}

Answer 2

I had a similar issue so started using callbacks. I'm using them in Xamarin.Android, pretty sure they're available in Xamarin.iOS.

Method that starts the task method - Note I am passing in a method of this class as a parameter

private async void updatedData()
    {
        await Utils.DataTasks.getNewLiveTips(populateTipsList);
    } 

Method that calls for data from server

public class DataTasks
{
    public static async Task getAllData(Action<IEnumerable<ParseObjects>> callback) { 

        var query = new ParseQuery<ParseObjects>().OrderByDescending("updatedAt").Limit(5);
        IEnumerable<ParseObjects> parseTips = await query.FindAsync();

        foreach (var tip in parseTips)
        {
         // Save data to DB if needed 
        }

        callback(parseTips);
    }

Method I passed as parameter in the first instance is now called

private void populateTipsList(IEnumerable<ParseObjects> results)
    {
        mAdapter = new TipAdapter(this.Activity, results);
        mRecyclerView.SetAdapter(mAdapter);

        refresher.Refreshing = false;
    }