List和ObservableCollection和ListBinding在MVVM模式下的对比

List

当对List进行增删操作后，并不会对View进行通知。

//Employee
public class Employee : INotifyPropertyChanged
{public event PropertyChangedEventHandler? PropertyChanged;public string Name { get; set; }private int salary;public int Salary{get { return salary; }set{salary = value;PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Salary)));}}
}

<!--View-->
<Grid><Grid.RowDefinitions><RowDefinition /><RowDefinition Height="40" /></Grid.RowDefinitions><DataGrid ItemsSource="{Binding Employees}" /><StackPanelGrid.Row="1"VerticalAlignment="Center"Orientation="Horizontal"><Button Command="{Binding addCommand}" Content="Add" /><TextBlock Text="工资总额" /><TextBox Text="{Binding TotalSalary, Mode=OneWay}" /></StackPanel>
</Grid>

//MainViewModel
public partial class MainViewModel:INotifyPropertyChanged
{public List<Employee> Employees { get; }public int TotalSalary=>Employees.Sum(x => x.Salary);public MainViewModel(){Employees = new List<Employee>() { new Employee(){Name="Tom",Salary=200},new Employee(){Name="Tim",Salary=450},new Employee(){Name="Jerry",Salary=780},new Employee(){Name="Jake",Salary=540},new Employee(){Name="Kit",Salary=670},};}[RelayCommand]private void add(){Employees.Add(new Employee() {  Name = "Lit", Salary = 260 });//通知TotalSalayPropertyChanged?.Invoke(this,new PropertyChangedEventArgs(nameof(TotalSalary)));}public event PropertyChangedEventHandler? PropertyChanged;
}

334

可以看到，点击Add后，上面列表中并没有更新，但是工资总额已经更新，这说明新的数据已经加到了List中，但是List并没有通知View。

ObservableCollection

ObservableCollection可以在增加和删除时，对View进行通知

修改MainViewModel

public partial class MainViewModel:INotifyPropertyChanged
{public ObservableCollection<Employee> Employees { get; }public int TotalSalary=>Employees.Sum(x => x.Salary);public MainViewModel(){Employees = new ObservableCollection<Employee>() { new Employee(){Name="Tom",Salary=200},new Employee(){Name="Tim",Salary=450},new Employee(){Name="Jerry",Salary=780},new Employee(){Name="Jake",Salary=540},new Employee(){Name="Kit",Salary=670},};}[RelayCommand]private void add(){Employees.Add(new Employee() {  Name = "Lit", Salary = 260 });PropertyChanged?.Invoke(this,new PropertyChangedEventArgs(nameof(TotalSalary)));}//增加一个Modify方法[RelayCommand]private void Modify(){Employees[2].Salary = 999;PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(TotalSalary)));}public event PropertyChangedEventHandler? PropertyChanged;
}

335

可以发现，点击Add，View中也相应地进行了更新，而且工资总额也相应改变。同时，无论是add还是Modify，都使用了PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(TotalSalary)));它的改进做法是使用CollectionChanged事件。

//改进做法,增加CollectionChanged，可以删除add()和Modify()的PropertyChanged...了
Employees.CollectionChanged += (_, _) => { PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(TotalSalary))); };