In MVVM, is there a standard way to automatically NotifyPropertyChanged when the value of one property depends on another?

Question

I've been using the MVVM pattern for a little while now and frequently run into a scenario where the value of one property depends on the value of another property. For instance, I have a control with a height and width, and I want to display the height and width on the control as a formatted string, "{height} x {width}". So I include the following properties in my view model:

public class FooViewModel : INotifyPropertyChanged
{
    // . . .

    private double _width;
    public double Width
    {
        get { return _width; }
        set
        {
            if(_width != value)
            {
                _width = value;
                NotifyPropertyChanged("Width");
                NotifyPropertyChanged("DisplayString"); // I had to remember to
                                                        // do this.
            }
        }
    }

    public string DisplayString
    {
        get
        {
            return string.Format("{0} x {1}", _width, _height);
        }
    }

    // . . .
}

Then I bind to content of my Label to the DisplayString property, which seems a lot more convenient than using a IMultiValueConverter to convert from the Width and Height properties. The inconvenient part is that anywhere I need to NotifyPropertyChanged for "Width" or "Height", I also have to remember to notify for "DisplayString". I can think of myriad ways to automate this, to varying degrees, but my question is whether there is a standard practice that people generally use to do this under MVVM in WPF?

There's [this](http://code.google.com/p/notifypropertyweaver/) for example. — H.B., Apr 23 '12 at 19:57
and this: http://stackoverflow.com/questions/527602/automatically-inotifypropertychanged — Phil, Apr 23 '12 at 20:00

score 2 · Accepted Answer · answered Apr 23 '12 at 20:08

No there is no standard way of doing this.

You can write a base viewmodel class which has PropertyChanged helper method. It will look at the properties of class with DependsOn attribute (which also you'll create) and fire event for all the properties that depend on the updated property.

score 1 · Answer 2 · answered Apr 23 '12 at 20:01

1

If there are many dependencies between properties in the ViewModel, you can invoke NotifyPropertyChanged with an empty string to refresh all the elements which are bound from the View. I don't see a point in automating this.

answered Apr 23 '12 at 20:01

Lev G.

286
1
6

Some of the getters in other properties are slow enough to cause noticeable lag so I wouldn't want to cause those to update unecessarily. – axanpi Apr 23 '12 at 20:09
1

Then there is a problem in your design. Getters should be as simple as possible. In most cases, the calculations should be done outside of the getters. Event handlers are great for such calculations. – Lev G. Apr 24 '12 at 07:18
I agree with this, except I don't have control over the model part of the design. – axanpi Apr 24 '12 at 12:41
Though, I suppose I could do something like cache all my get results, use a `BackgroundWorker` to do the full get, and update the results if necessary. – axanpi Apr 24 '12 at 12:58
This is an option, but try not to fall into a place where you will spend a lot of time maintaning the cache and taking care of it to be up-to-date. – Lev G. Apr 24 '12 at 14:02

In MVVM, is there a standard way to automatically NotifyPropertyChanged when the value of one property depends on another?

2 Answers2