I have been studying MVVM for the last few weeks, and I get most of it. I do have one question.

I have downloaded this example. Scroll down a bit to the class diagram. I understand 99% of what's happening here.

What I don't get is why the Set assesor for the FirstName, LastName, and Age properties on the Person model dont' fire until the SavePersonCommand delegate is executed.

If the controls in the View are bound to the Person object reference on the PersonViewModel, then why don't the property assesors fire when the user makes a change to a property?

[UPDATE]

I figured it out. I had typed something into the LastName field, then clicked the Save button. The property is updated when the field
loses focus, not during the save.