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CallerMemberName not that great for INotifyPropertyChanged

I've blogged before about the perf improvements of using the CallerMemberName attribute over an expression tree to avoid using hard-coded property strings names when firing the PropertyChanged event on INotifyPropertyChanged.

This works great for the majority simple cases.

Setting the property & notifying:
 or just notifying:
The problem is when it's not the simple case - when setting a property on a view model and wanting to force the updating of another property (on the view model) it doesn't work obviously.
At this point I'm thinking I'll go back to the expression tree approach.

Comments

  1. A solution I'm trying out at the moment uses two parameters for the RaisePropertyChanged method:

    protected void RaisePropertyChanged(string property = null, [CallerMemberName]string caller = "")
    {
    var handler = PropertyChanged;
    if (handler != null)
    handler(this, new PropertyChangedEventArgs(property ?? caller));
    }

    With this implementation a call to "RaisePropertyChanged()" will raise the event with the caller name, while "RaisePropertyChanged("Message")" will raise it with "Message" as the property name, regardless of where it's called from.

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  2. Huh? A single parameter is enough. [CallerMemberName] only changes the default value when you don't pass anything.

    protected void RaisePropertyChanged([CallerMemberName] string propertyName = "")

    RaisePropertyChanged(); in a setter will pass the current property name.
    RaisePropertyChanged("RelatedProperty"); anywhere will always pass "RelatedProperty".

    Your code looks wrong. It isn't sufficient to simply call the PropertyChanged(...) event. There are additional checks.

    ReplyDelete
  3. Consider a different approach altogether...
    (as detailed here http://northhorizon.net/2011/the-right-way-to-do-inotifypropertychanged/)

    Instead of having your SubCategories property be a simple getter that needs notified to "refresh", have all your properties be a simple get/set using the SetProperty method, but with a slight change.

    Add two additional parameters, onChanged, and onChanging (or just onChanged if you don't need the onChanging handler)

    ie.
    SetProperty( ref T currentValue, T newValue, [CallerMemberName]string propertyName = "", Action onChanged = null, Action onChanging = null)

    ( In the SetProperty code you will need to call onChanged if it is not null and the property is changing )

    Now your SubCategories property becomes this:

    public IEnumerable SubCategories
    {
    get { return _subCategories; }
    set { SetProperty(ref _subCategories, value);
    }

    your SelectedCategory property this:

    public string SelectedCategory
    {
    get{ return _selectedCategory; }
    set { SetProperty(ref _selectedCategory, value, onChanged:OnSelectedCategoryChanged);}
    }

    And you have an OnChanged handler that does all the work.
    When SelectedCategory is set, this will be called and it will in turn update your SubCategories property.

    private void OnSelectedCategoryChanged()
    {
    if(_selectedCategory == _categories.Keys.First() || string.IsNullOrWhiteSpace(_selectedCategory))
    {
    SubCategories = _categories.SelectMany(c=>c.Value);
    return;
    }

    SubCategories = _categories[_selectedCategory];
    }

    Now you don't need to worry about manually raising the property changed notification for the SubCategories property, you set the SubCategories property from the OnChanged handler, which will in turn do the notify for you.

    If you follow this approach everywhere, you'll find that your property declarations become exactly what they are, boilerplate code. You can just create templates/snippets to create them and then wrap them in a region and never look at them again. You will never really need to manually call OnPropertyChanged again.

    ReplyDelete

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