Binding Issue constructor viewmodel

Question

I relatively new to mvvm, however I have a question about binding from a viewmodel. I have a viewmodel where in its constructor I want to populate a observablecollection. My constructor looks like this:

public StudySelectionViewModel() : base() {
        _studyRepository = StudyRepository.Instance;

        InitializeViewModelData();
    }

The InitializeViewModelData() looks like this:

        private void InitializeViewModelData() {
        _studyRepository.RetrieveModalityTypes();
        RaisePropertyChanged("ModalityTypes");
    }

My property ModalityTypes looks like this:

public ObservableCollection<ModalityType> ModalityTypes {
        get {
            return _studyRepository.ModalityTypes;
        }
    }

Probably needless to say that my repository makes a call to a database and retrieves the data. When debugging it looks like that the RaisePropertyChanged is executed earlier than my _studyRepository.RetrieveModalityTypes method is executed and hence it binds to an empty property.

Am I missing something here? Is my design bad? Any ideas?

Thanks in advance,

Answer 1

The binding will inspect the property when the form initialises. It will also inspect the property when the PropertyChanged notification is raised - so you may get two calls to the getter of the property

I'm assuming that your call to get data is a web service call or something? Silverlight is async, so you need to consider that web service calls may return well after your form is initialised

Answer 2

There are two things to consider here.

1. You have an ObservableCollection - it's observable, so if you initialise the collection when you initialise the view model, the add to the collection rather than replacing it with a new collection when your data service returns the view will update fine.

2. If you really must replace the collection with a new collection in the data service data ready callback, then ensure you write the collection setter thus:

    public ObservableCollection<ModalityType> ModalityTypes { get { return _studyRepository.ModalityTypes; } set { this.__studyRepository.ModalityTypes = value; RaisePropertyChanged("ModalityTypes"); { 

   }