在集合的多个实例中查找项目总数，并将其绑定到Textblock

Question

As the title states, I am trying to find the total number of items in multiple instances of an ObservableCollection and bind the result to a Textblock . I managed to get the correct number via LINQ...

public WorldViewModel(GameContainer container)
{
    GlobalPopulation = container.Regions
            .SelectMany(x => x.Settlements)
            .SelectMany(x => x.Population)
            .Distinct()
            .Count();
}

public int GlobalPopulation { get; set; }

... but LINQ doesn't work well with XAML binding. The initial value displays properly but doesn't update as the count changes. If I create a new instance of my viewmodel, the new number displays since the constructor is being called again. My viewmodel inherits from MVVM's ViewModelBase and implements INPC via Kind of Magic. I have other properties that are bound properly and behave as expected, so that's not the issue.

I have tried using tools like OLinq and BindableLinq with no luck. Right now it's looking like I'll have to do this without LINQ, but unfortunately I don't know how to, without LINQ, retrieve the same value and bind it to the Textblock . If anybody could either show me a way to make the LINQ binding work in this situation or how to achieve the same result without LINQ, I would greatly appreciate it.

Answer 1

It's easy to forget that you can subscribe to the changes in your ObservableCollection yourself:

// not production-quality code
public WorldViewModel(GameContainer container)
{
    foreach (var region in container.Regions)
    {
        region.CollectionChanged += (s,e) => RecalculateCount(container);
        foreach(var settlement in region.Settlements)
        {
            settlement.CollectionChanged += (s,e) => RecalculateCount(container);
            foreach( var population in settlement.Population)
            {
                population.CollectionChanged += (s,e) => RecalculateCount(container);
            }        
        }
    }

    RecalculateCount();
}

private static void RecalculateCount(GameContainer container)
{
    GlobalPopulation = container.Regions
                .SelectMany(x => x.Settlements)
                .SelectMany(x => x.Population)
                .Distinct()
                .Count();
}

So long as GlobalPopulation is firing property changed events, changes to the count of settlements will be propagated to the UI.

(You'll probably want to store the GameContainer instance on your view model and then reference it from a non-static version of RecalculateCount, rather than capturing it in the event handling lambda; that code is intended to be a drop-in demo, but probably has strange object lifetime issues hiding in it.)

Answer 2

Change this:

public int GlobalPopulation { get; set; }

to this:

private int _globalPopulation;
public int GlobalPopulation 
{ 
    get { return _globalPopulation; }
    set
    {
        _globalPopulation = value;
        NotifyPropertyChange("GlobalPopulation");
    }
}