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Bind to total of an items field in an ObservableCollection and update when values change

I have an ObservableCollection that populates a datagrid in WPF. I need to bind to the total of the "Hours" column, and have that total update when a value in the "Hours" column is changed. I can achieve this by listening to the "LostFocus" event and running a function, but would like to try my hand at binding.

The issue I am running into, is the NotifyPropertyChanged event will not fire when an items property in the collection is changed.

The sortie class NotifyPropertyChanged will fire, but the collection doesn't interpret that as its own property changing. How can I listen to the sortie PropertyChanged from the collection in the missions class?

My Models

public class Mission : INotifyPropertyChanged

private ObservableCollection<Sortie> sorties;
public ObservableCollection<Sortie> Sorties

get return this.sorties;
set


if (this.sorties != value)

this.sorties = value;
this.NotifyPropertyChanged("Sorties");



public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged(string propName)

if (this.PropertyChanged != null)

this.PropertyChanged(this, new PropertyChangedEventArgs(propName));




public class Sortie : INotifyPropertyChanged

private double hours;
public double Hours

get return this.hours;
set

if (this.hours != value)

this.hours = value;
this.NotifyPropertyChanged("Hours");



public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged(string propName)

if (this.PropertyChanged != null)

this.PropertyChanged(this, new PropertyChangedEventArgs(propName));





I didn't bother posting my XAML or View Model, as I am confident I can solve the issue once I learn how to trigger a PropertyChanged event for the collection and I wanted to spare you having to read through massive amounts of code. If you believe it is needed however, let me know.

2 Answers
2

Write a readonly property in the parent viewmodel that calculates the value.

public double SortieHours => Sorties.Sum(x => x.Hours);


Parent viewmodel handles PropertyChanged on each item in Sorties, and CollectionChanged on Sorties. In CollectionChanged on Sorties, you have to add/remove PropertyChanged handlers from Sortie instances as they're added and removed. When you get a new Sorties collection (you might want to make that setter private for this reason), you need to toss out all the old handlers and add new ones.

PropertyChanged

Sorties

CollectionChanged

Sorties

CollectionChanged

Sorties

PropertyChanged

Sortie

Sorties

Now, whenever a Sortie is added or removed, or its Hours changes, or somebody hands you a new Sorties collection, raise PropertyChanged:

Sortie

Hours

Sorties

PropertyChanged

OnPropertyChanged(nameof(SortieHours));


And bind that property to whatever you like in the XAML.

This looks horrible (because it is), but what else are you going to do?

A lot of people would advise you to give Sortie an HoursChanged event. PropertyChanged is annoying for this type of case because it can get raised for multiple different properties and you have to check which one. And it's a magic string thing.

Sortie

HoursChanged

PropertyChanged

Above is C#6. For C#5,

public double SortieHours get return Sorties.Sum(x => x.Hours);

OnPropertyChanged("SortieHours");


I found this link to be a big help. It creates a base class for ObservableCollections to "AutoMagicly" add the PropertyChanged events of the collection items to the CollectionChanged event.

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