How to make a Xamarin.Forms app load its methods on appearing without using Task.Run() in the ctor and block threads

Question

I've read many explanations but none of them made sense to me.

I'm doing this in Xamarin.Forms:

public class SomeClass
{
    public SomeClass
    {
        var request = new GeolocationRequest(GeolocationAccuracy.High, TimeSpan.FromSeconds(10));
        var cts = new CancellationTokenSource();
        var location = Task.Run<Location>(async () => await Geolocation.GetLocationAsync(request, cts.Token));
        var userLat = location.Latitude; // doesn't work
        var userLon = location.Longitude; // doesn't work
    }
}

The reason I'm doing this is that I'm trying to load all the methods I need such as getting the user's location, show it on the map, load up some pins etc. as soon as the Xamarin.Forms.Maps map appears.

I know it's bad practice from what you guys answered so I'm working on changing that but I'm still having a hard time understanding how to do it differently in the sense of it's confusing. But I'm reading your articles and links to make sure I understand.

I tried to run Task.Run(async () => await) on many methods, and tried to save their values in variables but I can't get the returned value and that's what made me post this question, I need to change my code.

I know could get the returned value using Task.Result() but I've read that this was bad.

How to get the UI to load and wait on the ViewModel to do what it has to do, and then tell the UI to then use whatever the ViewModel is giving him when it is ready ?

Answer 1

I think it could be this:

public async Task SomeMethodAsync()
{
    var request = new GeolocationRequest(GeolocationAccuracy.High, TimeSpan.FromSeconds(10));
    var cts = new CancellationTokenSource();
    var location = await Geolocation.GetLocationAsync(request, cts.Token);
    var userLat = location.Latitude;
    var userLon = location.Longitude;
}

Not that methods which contain await calls should be marked as async. If you need to return something from this method then the return type will be Task<TResult>.

Constructors cannot be marked with async keyword. And it's better not to call any async methods from constructor, because it cannot be done the right way with await. As an alternative please consider Factory Method pattern. Check the following article of Stephen Cleary for details: Async OOP 2: Constructors.

Answer 2

You mentioned in a comment to another answer that you can't do it async because this code is in a constructor. In that case, it is recommended to move the asynchronous code into a separate method:

public class MyClass
{
   public async Task Init()
   {
      var request = new GeolocationRequest(GeolocationAccuracy.High, TimeSpan.FromSeconds(10));
      var cts = new CancellationTokenSource();
      var location = await Geolocation.GetLocationAsync(request, cts.Token);
      var userLat = location.Latitude; 
      var userLon = location.Longitude;
  }
}

You can use it the following way:

var myObject = new MyClass();
await myObject.Init();

The other methods of this class could throw an InvalidOperationException if Init() wasn't called yet. You can set a private boolean variable wasInitialized to true at the end of the Init() method. As an alternative, you can make your constructor private an create a static method that creates your object. By this, you can assure that your object is always initialized correctly:

public class MyClass
    {
       private MyClass() { }
       public async Task<MyClass> CreateNewMyClass()
       {
          var result = new MyClass();
          var request = new GeolocationRequest(GeolocationAccuracy.High, TimeSpan.FromSeconds(10));
          var cts = new CancellationTokenSource();
          var location = await Geolocation.GetLocationAsync(request, cts.Token);
          result.userLat = location.Latitude; 
          result.userLon = location.Longitude; 
          return result;
       }
    }

Answer 3

The normal way to retrieve results from a Task<T> is to use await. However, since this is a constructor, there are some additional wrinkles.

Think of it this way: asynchronous code make take some time to complete, and you can never be sure how long. This is in conflict with the UI requirements; when Xamarin creates your UI, it needs something to show the user right now.

So, your UI class constructor must complete synchronously. The normal way to handle this is to (immediately and synchronously) create the UI in some kind of "loading..." state and start the asynchronous operation. Then, when the operation completes, update the UI with that data.

I discuss this in more detail in my article on async data binding.

Answer 4

Do the asynchronous work before constructing the object and pass in the data to the constructor. I suggest a factory.

class MyFactory : IMyFactory
{
    public async Task<MyClass> GetMyClass()
    {
        var request = new GeolocationRequest(GeolocationAccuracy.High, TimeSpan.FromSeconds(10));
        var cts = new CancellationTokenSource();
        var location = Task.Run<Location>(async () => await Geolocation.GetLocationAsync(request, cts.Token));
        return new MyClass(location);   
    }
}

class MyClass
{
    public MyClass(Location location)
    {
        var userLat = location.Latitude; 
        var userLon = location.Longitude; 
        //etc....
    }
}

To create an instance, instead of

var x = new MyClass();

you'd call

var factory = new MyFactory();
var x = await factory.GetMyClass();

The nice thing about this approach is that you can mock the factory (e.g. for unit tests) in a way that does not depend on the external service.