WPF Detect Property Change In Entity In VM

[Pete OHanlon]

There's a couple of ways you could do this. One way is to hook into the property itself and call the method when you're changing the value. The other way is to use RX to observe the property and call the method whenever the property changes. There's a pretty decent primer here[^] that should give you a heads up.

[Matt T Heffron]

I'd suggest looking into the PropertyObserver class from Josh Smith's MVVM Foundation library[^]. This will let you watch for the NotifyPropertyChanged event. Specific info at: One way to avoid messy PropertyChanged event handling[^]

[pradeep surya]

Hi, Simply you can achieve this by using triggers. Write an Event Trigger with event name as Click and bind an ICommand property which is in the viewmodel. So whenever there is a click on the checkbox(either select or deselect this method triggers) Then you can write the logic according to your functionality. There are so many ways to achieve this as well.

[Kevin Marois]

Well I implemented this and it doesn't work. I registered the property, compiled, and ran it. Nothing happens. The method I registered doesn't get called. The property IS changed, but the callback doesn't get called. Too bad, it looked promising. [UPDATE] So here's what I just found.... I have a GridView bound to a list of JobAssignmentEntities and who's SelectedItem is bound to a property in my VM called SelectedJobAssignment. In the CTOR of the VM I did

propertyObserver = new PropertyObserver(this.SelectedJobAssignment)
.RegisterHandler(n => n.PhaseStart, n => this.phaseStartChanged(n));

In order to register it the property must not be null, because of a null ref check in the register method. So I moved the block above inside the SelectedJobAssignment's Getter and it works. sooo, I will need to find a way to register EACH time the SelectedJobAssignment is set.

If it's not broken, fix it until it is

[Kevin Marois]

Pete, Can you explain what you mean by 'Hook into the property'? Thanks

If it's not broken, fix it until it is

[Pete OHanlon]

It's just a case of adding a piece of logic into your property setter and calling your method if the value has changed.

[Matt T Heffron]

Yes, this makes sense. The PropertyObserver constructor attaches to the current value (this.SelectedJobAssignment) of the passed property. It does not automatically track if that property itself changes. (I.e., it doesn't notice if this.SelectedJobAssignment changes) The registration tracks if the referenced property of that value changes. (I.e., it only notices if this.SelectedJobAssignment.PhaseStart changes)

[Matt T Heffron]

Why not move it into the SelectedJobAssignment's Setter ?

[Kevin Marois]

In ref to this and your prior posting.... I see how it works, it just looks kludgy. First, I could see something like

propertyObserver.Register(typeof(theType), x => x.Property, MyHandler());

this way you can register multiple type, and only need to have this on one place. Having to re-register each time the property changes is ugly

If it's not broken, fix it until it is

[Matt T Heffron]

You just use a different property observer. One for "the class" that checks when specific properties are changed. In the handlers for that observers, you register handlers for the properties on the values of the "properties of the class". This way the PropertyObserver handles the general case.

[Kevin Marois]

Ya I know. THe idea is great, just not the cleanest implementation.

If it's not broken, fix it until it is