Catch ChangeEvents from ObservableDictionary in MVVM C# WPF

Question

I have a software (with GUI) containing metainformation about products. Part of that information are some ratings of components of that product.

I want to display this ratings somehow like this for everey product someone clicked on:

Part a: 100%
Part b: 20%
Part c: -
Part d: 40%

It can be lots of values and i don't want to create an own variable for every possible rating.

My problem is that i have lots of missing values but i want the GUI allways to look the same so i thought of an observable dictionary to store my data (coming from a webinterface on the fly for the product someone clicks on) and bind wpf elements to the values of that dictionary.

I found an implementation of an ObservableDictionary in this stackoverflow post which contains a link to the Implementation.

Here are some parts of my code to show where my problem is:

My View implementation looks like this:

<TextBlock Text="{Binding Path=Ratings, Converter={StaticResource ObservableDictionaryConverter}, ConverterParameter=Key}"/>

Where Ratings is my Observable Dictionary, Key is my hardcoded Component (e.g. "Part a") and ObservableDictionaryConverter is a converter returning the rating if it exists or "-" if it doesn't.

My ModelView Implementation looks like this

public class Foo : Bindable
{
    ObservableDictionary<string, ComponentRatings> _ratings
    public ObservableDictionary<string, ComponentRatings> Ratings
        {
            get {return _ratings; }
            set
            {
                _ratings = value;
                OnPropertyChanged();
            }
        }
}

And the Model:

ViewModel.Ratings= new ObservableDictionary<string, ComponentRatings>();
foreach (var e in foo)
{
    ViewModel.Ratings.Add(e.Drive.ToString(), e);
}

1.) If i run my code the gui only updates itself when the Dictionary is instantiated due to the OnPropertyChanged() in my ModelView property. But when i Add new Elements to the Dict, my GUI should update itself and show them. So how do i catch the CollectionChanged events from the Observable Dictionary in my ModelView?

2.) The hardcoded Keys in the XAML are not really nice but i couldn't come up with a better way to allways have all my ratings in the GUI even if they don't exist. Is there some better, more flexible way to implement something like this?

`TextBlock` doesn't listen to `CollectionChanged`. Maybe this can help you: https://stackoverflow.com/questions/36661607/binding-list-to-textbox-wont-update-if-observablecollection — Gabriel Prá, Feb 01 '18 at 15:29
Thanks !! That was exactly the problem. Make it an answer and you get your 15 reputation ;-) — Florian H, Feb 02 '18 at 08:47

score 1 · Accepted Answer · answered Feb 02 '18 at 15:22

The problem is that TextBlock doesn't listen to CollectionChanged events. So you need to either manually listen for the changes and update the TextBlock in the code behind or, better yet, raise an event that the TextBlock can subscribe to from your ObservableDictionary.

You can find an example where this is done with a List here.

Catch ChangeEvents from ObservableDictionary in MVVM C# WPF

1 Answers1