当 ObservableCollection 中的项目更新时更新 ItemsControl

Question

The Problem:

• You declare an ItemsControl ( or a control derived from ItemsControl ) in the view.
• You bind the ItemsControl.ItemsSource property to an ObservableCollection in your ViewModel.
• Your view updates as expected when an item is added to /removed from the ObservableCollection .
• BUT, the view does not update when you change a property of an item in the ObservableCollection .

Background:

It seems that this is a common problem many WPF developers have encountered. It has been asked a few times:

Notify ObservableCollection when Item changes

ObservableCollection not noticing when Item in it changes (even with INotifyPropertyChanged)

ObservableCollection and Item PropertyChanged

My Implementation:

I tried to implement the accepted solution in Notify ObservableCollection when Item changes . The basic idea is to hook up a PropertyChanged handler in your MainWindowViewModel for each item in the ObservableCollection . When an item's property is changed, the event handler will be invoked and somehow the View is updated.

I could not get the implementation to work. Here is my implementation.

ViewModels:

class ViewModelBase : INotifyPropertyChanged 
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName = "")
    {
        var handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

Item ViewModel:

class EmployeeViewModel : ViewModelBase
{
    private int _age;
    private string _name;

    public int Age 
    {
        get { return _age; }
        set
        {
            _age = value;
            RaisePropertyChanged("Age");
        }
    }

    public string Name  
    {
        get { return _name; }
        set
        {
            _name = value;
            RaisePropertyChanged("Name");
        }
    }

    public override string ToString()
    {
        return string.Format("{0} is {1} years old", Name, Age);
    }
}

Main Window ViewModel:

class MainWindowViewModel : ViewModelBase
{
    private ObservableCollection<EmployeeViewModel> _collection;

    public MainWindowViewModel()
    {
        _collection = new ObservableCollection<EmployeeViewModel>();
        _collection.CollectionChanged += MyItemsSource_CollectionChanged;

        AddEmployeeCommand = new DelegateCommand(() => AddEmployee());
        IncrementEmployeeAgeCommand = new DelegateCommand(() => IncrementEmployeeAge());
    }

    public ObservableCollection<EmployeeViewModel> Employees 
    {
        get { return _collection; }
    }

    public ICommand AddEmployeeCommand { get; set; }
    public ICommand IncrementEmployeeAgeCommand { get; set; }

    public void AddEmployee()
    {
        _collection.Add(new EmployeeViewModel()
            {
                Age = 1,
                Name = "Random Joe",
            });
    }

    public void IncrementEmployeeAge()
    {
        foreach (var item in _collection)
        {
            item.Age++;
        }
    }

    private void MyItemsSource_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.NewItems != null)
            foreach (EmployeeViewModel item in e.NewItems)
                item.PropertyChanged += ItemPropertyChanged;

        if (e.OldItems != null)
            foreach (EmployeeViewModel item in e.OldItems)
                item.PropertyChanged -= ItemPropertyChanged;
    }

    private void ItemPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        RaisePropertyChanged("Employees");
    }
}

View:

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero"
    xmlns:d="clr-namespace:Iress.IosPlus.DynamicOE.Controls"
    Title="MainWindow" Height="350" Width="350">

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="0.3*"></ColumnDefinition>
        <ColumnDefinition Width="0.7*"></ColumnDefinition>
    </Grid.ColumnDefinitions>

    <StackPanel Grid.Column="0">
        <Button Command="{Binding AddEmployeeCommand}">Add Employee</Button>
        <Button Command="{Binding IncrementEmployeeAgeCommand}">Increment Employee Age</Button>
    </StackPanel>

    <Grid Grid.Column="1">
        <Grid.RowDefinitions>
            <RowDefinition Height="0.1*"></RowDefinition>
            <RowDefinition></RowDefinition>
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0" Text="{Binding Path=Employees[0]}"></TextBlock>
        <ItemsControl Grid.Row="1" ItemsSource="{Binding Path=Employees}" BorderBrush="Red" BorderThickness="1"></ItemsControl>
    </Grid>
</Grid>

My Results:

To verify my implementation, I create a view like so. The TextBlock.Text is bound to the first item in the collection. The ItemsControl is bound to the collection itself.

• Pressing the "Add Employee" button adds an EmployeeViewModel object in the collection and both the TextBlock and ItemsControl are updated as expected.
• Pressing the "Add Employee" again, the ItemsControl is updated with another entry. Great!
• Pressing the "Increment Employee Age" button. The Age property of each item is incremented by 1. The PropertyChanged event is raised. The ItemPropertyChanged event handler is invoked. The Textblock is updated as expected. However, the ItemsControl is not updated.

I am under the impression that the ItemsControl should be updated too when the Employee.Age is changed according to the answer in Notify ObservableCollection when Item changes .

Answer 1

I found the answer using Snoop to debug XAML.

The issue is that you are trying to bind to the ToString() method and that does not raise the PropertyChanged event. If you look at the XAML bindings you will notice that the ObservableCollection is actually changing.

Now look at each item control and it's texts binding in the "Text" property. There are none, it's just text.

To fix this simply add an ItemsControl ItemTemplate with a DataTemplate that contains the elements you'd like to be displayed.

<ItemsControl Grid.Row="1" ItemsSource="{Binding Path=Employees, UpdateSourceTrigger=PropertyChanged}" BorderBrush="Red" BorderThickness="1" >
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <TextBlock>
                <TextBlock.Text>
                    <MultiBinding StringFormat=" {0} is {1} years old">
                        <Binding Path="Name"/>
                        <Binding Path="Age"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

We now have a green light on binding. RaisePropertyChanged is being called.

Ta-da!