當T更新ObservableCollection <T>，刷新其他依賴屬性的建議

Question

在MVVM中，有什麼打算技術刷新屬性NumberOfAdults和NumberOfChildren要在UI上更新？更一般地說，People view模型如何捕獲更新並刷新依賴於OberservableCollection<Person>上的內容的屬性？

該解決方案需要更高級地使用XAML綁定語法。它是什麼？

該人士看來模式

public class Person : INotifyPropertyChanged 
{ 
    private int _age; 
    public int Age 
    { 
     get { return _age; } 
     set { _age = value; NotifyPropertyChanged("Age"); } 
    } 

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

    public event PropertyChangedEventHandler PropertyChanged; 
    protected void NotifyPropertyChanged(String info) 
    { 
     if (PropertyChanged != null) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs(info)); 
     } 
    } 
} 

人民視圖模型

using System; 
using System.ComponentModel; 
using System.Collections.ObjectModel; 

public class PeopleViewModel : INotifyPropertyChanged 
{ 
    public PeopleViewModel() 
    { 
     People.Add(new Person { Name = "Happy", Age = 12 }); 
     People.Add(new Person { Name = "Sleepy", Age = 15 }); 
     People.Add(new Person { Name = "Sneezy", Age = 17 }); 
     People.Add(new Person { Name = "Grumpy", Age = 45 }); 
     People.Add(new Person { Name = "Dopey", Age = 50 }); 
     People.Add(new Person { Name = "Bashful", Age = 60 }); 
     People.Add(new Person { Name = "Doc", Age = 75 }); 
    } 

    private ObservableCollection<Person> _people = new ObservableCollection<Person>(); 
    public ObservableCollection<Person> People 
    { 
     get { return _people;} 
     set { _people = value;} 
    } 

    public int NumberOfAdults 
    { 
     get 
     { 
      int count = 0; 
      foreach (Person p in People) 
      { 
       if (p.Age >= 18) 
       { 
        count += 1; 
       } 
      } 
      return count; 
     } 
    } 

    public int NumberOfChildren 
    { 
     get 
     { 
      int count = 0; 
      foreach (Person p in People) 
      { 
       if (p.Age < 18) 
       { 
        count += 1; 
       } 
      } 
      return count; 
     } 
    } 


    public event PropertyChangedEventHandler PropertyChanged; 
    protected void NotifyPropertyChanged(String info) 
    { 
     if (PropertyChanged != null) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs(info)); 
     } 

    } 
} 

主XAML

<Window x:Class="WpfApplication1.MainWindow" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     Title="MainWindow" Height="350" Width="525"> 
    <StackPanel> 
    <Label Content="{Binding Path=NumberOfAdults}"></Label> 
    <Label Content="{Binding Path=NumberOfChildren}"></Label> 
    <DataGrid ItemsSource="{Binding Path=People}"></DataGrid> 
    </StackPanel> 
</Window> 

Answer 1

這是我的解決方案。這不是很好，但會起作用。我在CollectionChanged事件上針對ObservableCollection實現了一個回調。在回調中，我註冊了PropertyChanged事件。

using System; 
using System.ComponentModel; 
using System.Collections.ObjectModel; 
using System.Diagnostics; 

public class PeopleViewModel : INotifyPropertyChanged 
{ 
    public PeopleViewModel() 
    { 
     People.CollectionChanged += People_CollectionChanged; 

     // Same as before.... 
    } 

    void p_PropertyChanged(object sender, PropertyChangedEventArgs e) 
    { 
     Person p = (Person)sender; 
     Debug.WriteLine(String.Format("Update occured on Name: {0} Age: {1}",p.Name,p.Age)); 

     NotifyPropertyChanged("NumberOfAdults"); 
     NotifyPropertyChanged("NumberOfChildren"); 
    } 

    void People_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) 
    { 
     Debug.WriteLine("People_CollectionChanged called."); 
     if (e.NewItems != null) 
     { 
      foreach (Person p in e.NewItems) 
      { 
       p.PropertyChanged += p_PropertyChanged; 
      } 
     } 

     if (e.OldItems != null) 
     { 
      foreach (Person p in e.OldItems) 
      { 
       p.PropertyChanged -= p_PropertyChanged; 
      } 
     } 

    } 

    // Same as before. 
}