连接CollectionChanged和PropertyChanged(或:为什么某些WPF绑定不刷新?) [英] Wiring up CollectionChanged and PropertyChanged (Or : Why do some WPF Bindings not refresh?)
问题描述
WPF DataBindings曾经让我高兴.我现在偶然发现的一件事是，在某些时候它们只是没有按计划刷新.请查看以下(非常简单的)代码:
WPF DataBindings used to make me happy. One thing I've stumbled over just now is that at some point they just don't refresh as intented. Please take a look at the following (fairly simple) code:
<Window x:Class="CVFix.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="300">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition></RowDefinition>
<RowDefinition Height="40"></RowDefinition>
</Grid.RowDefinitions>
<ListBox Grid.Row="0" Grid.Column="0"
ItemsSource="{Binding Path=Persons}"
SelectedItem="{Binding Path=SelectedPerson}"
x:Name="lbPersons"></ListBox>
<TextBox Grid.Row="1" Grid.Column="0" Text="{Binding Path=SelectedPerson.Name, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>
</Window>
XAML背后的代码:
The Code behind for the XAML:
using System.Windows;
namespace CVFix
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public ViewModel Model { get; set; }
public MainWindow()
{
InitializeComponent();
this.Model = new ViewModel();
this.DataContext = this.Model;
}
}
}
最后，这是ViewModel类:
Finally, here's the ViewModel classes:
using System.Collections.ObjectModel;
using System.ComponentModel;
namespace CVFix
{
public class ViewModel : INotifyPropertyChanged
{
private PersonViewModel selectedPerson;
public PersonViewModel SelectedPerson
{
get { return this.selectedPerson; }
set
{
this.selectedPerson = value;
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs("SelectedPerson"));
}
}
public ObservableCollection<PersonViewModel> Persons { get; set; }
public ViewModel()
{
this.Persons = new ObservableCollection<PersonViewModel>();
this.Persons.Add(new PersonViewModel() { Name = "Adam" });
this.Persons.Add(new PersonViewModel() { Name = "Bobby" });
this.Persons.Add(new PersonViewModel() { Name = "Charles" });
}
public event PropertyChangedEventHandler PropertyChanged;
}
}
public class PersonViewModel : INotifyPropertyChanged
{
private string name;
public string Name
{
get { return this.name; }
set
{
this.name = value;
if(this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs("Name"));
}
}
public override string ToString()
{
return this.Name;
}
public event PropertyChangedEventHandler PropertyChanged;
}
我想发生的事情:当我从列表框中选择一个条目并在文本框中修改其名称时，该列表就会更新以显示新值.
What I'd want to happen: As I select an entry from the ListBox and modify its Name in the TextBox, the list gets updated to display the new value.
发生了什么:什么都没有.如果我是法官，那就是正确的行为. 我确保已触发SelectedItem的PropertyChanged，但这(当然)不会导致CollectionChanged被触发.
What happens : nothing. And that is the correct behaviour, If I'm any judge. I made sure that the SelectedItem's PropertyChanged is fired, but that does (of course) not cause CollectionChanged to be fired.
要解决此问题，我创建了一个ObservableCollection派生的类，该类具有公共的OnCollectionChanged方法，请参见此处:
To fix this, I created an ObservableCollection-derived class that has a public OnCollectionChanged method, see here :
public class PersonList : ObservableCollection<PersonViewModel>
{
public void OnCollectionChanged()
{
this.OnCollectionChanged(new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Reset ));
}
}
我从ViewModel的构造函数访问它，如下所述:
I access this from the ViewModel's constructor, as described below:
public ViewModel()
{
PersonViewModel vm1 = new PersonViewModel()
{
Name = "Adam"
};
PersonViewModel vm2 = new PersonViewModel()
{
Name = "Bobby"
};
PersonViewModel vm3 = new PersonViewModel()
{
Name = "Charles"
};
vm1.PropertyChanged += this.PersonChanged;
this.Persons = new PersonList();
this.Persons.Add(vm1);
this.Persons.Add(vm2);
this.Persons.Add(vm3);
}
void PersonChanged(object sender, PropertyChangedEventArgs e)
{
this.Persons.OnCollectionChanged();
}
它可以工作，但这不是一个干净的解决方案.我的下一个想法是创建一个ObservableCollection的派生模型，该模型在CollectionChanged-handler中自动进行接线.
It works, but it's not a clean solution. My next idea would be creating a derivate of ObservableCollection that does the wiring up automatically in a CollectionChanged-handler.
public class SynchronizedObservableCollection<T> : ObservableCollection<T> where T : INotifyPropertyChanged
{
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
{
foreach (INotifyPropertyChanged item in e.NewItems)
{
item.PropertyChanged += this.ItemChanged;
}
break;
}
case NotifyCollectionChangedAction.Remove:
{
foreach (INotifyPropertyChanged item in e.OldItems)
{
item.PropertyChanged -= this.ItemChanged;
}
break;
}
}
base.OnCollectionChanged(e);
}
void ItemChanged(object sender, PropertyChangedEventArgs e)
{
this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}
}
问题是:还有更好的方法吗?这真的有必要吗?
The question is : is there a better way to do this? Is this really necessary?
非常感谢您的输入！
推荐答案
否，完全没有必要.您的样本失败的原因很微妙，但很简单.
No, it's not necessary at all. The reason your sample is failing is subtle, but quite simple.
如果不为WPF提供数据项的模板(例如列表中的Person对象)，则默认使用ToString()方法显示.那是一个成员，而不是属性，因此当值更改时，您不会收到任何事件通知.
If you don't provide WPF with a template for a data item (such as the Person objects in your list), it'll default to using the ToString() method to display. That's a member, not a property, and so you get no event notification when the value changes.
如果将DisplayMemberPath="Name"添加到列表框中，它将生成一个模板，该模板可以正确地绑定到您的个人的Name，然后模板将按照您的期望自动更新.
If you add DisplayMemberPath="Name" to your listbox, it'll generate a template that binds properly to the Name of your person - which will then update automatically as you'd expect.
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