[转]5步实现Silverlight中的Command

在这里我打算分享一个十分轻量级的技巧实现Silverlight4中的Command。

Step 1 – 实现ICommand接口

第一步是新建一个类来管理Command相关的逻辑，它需要实现ICommand接口。当然，还有很多其他的方式，我在这里只介绍这种简单有效的实现。

DelegatedCommand类实现了ICommand接口定义的CanExecute、Execute方法和CanExecuteChanged事件。

public class DelegateCommand : ICommand {
    Func<object, bool> canExecute;
    Action<object> executeAction;
    bool canExecuteCache;
    public DelegateCommand(Action<object> executeAction, Func<object, bool> canExecute) {
        this.executeAction = executeAction;
        this.canExecute = canExecute;
    }

    #region ICommand 成员
    public bool CanExecute(object parameter) {
        bool temp = canExecute(parameter);
        if (canExecuteCache != temp) {
            canExecuteCache = temp;
            if (CanExecuteChanged != null) {
                CanExecuteChanged(this, new EventArgs());
            }
        }
        return canExecuteCache;
    }

    public event EventHandler CanExecuteChanged;
    public void Execute(object parameter) {
        executeAction(parameter);
    }
    #endregion
}

Step 2 – 定义Command

在你的ViewModel中增加一个公共属性对外提供ICommand。一般来说，这个属性会绑定到界面的按钮上。

public ICommand LoadProductsCommand { get; set; }

Step 3 – 创建Command

在你的ViewModel的构造函数中通过第一步中创建的DelegateCommand类实例化LoadProductsCommand。

LoadProductsCommand = new DelegateCommand(LoadProducts, CanLoadProducts);

Step 4 – 创建ViewModel

接下来你需要确认你的View可以和你的ViewModel通信。很多方式都可以实现这一点，这里我是将ViewModel在XAML中定义为一个静态资源。

<UserControl.Resources>
    <local:ProductViewModel x:Key="vm"/>
</UserControl.Resources>

Step 5 – 绑定到Command

向UI中添加一个按钮然后将你ViewModel中创建的Command属性绑定到这个按钮上。接下来你可能需要通过设置绑定中的CommandParameter向Command传递参数，这里可以是一个UI元素，如TextBox。

<Button Content="Load" Width="120"    
        Command="{Binding LoadProductsCommand}"    
        CommandParameter="{Binding ElementName=FilterTextBox, Path=Text}" />

就此5步，你的应用程序已经实现Command了。

附录 …

ProductViewModel的全部代码:

public class ProductViewModel : ViewModelBase {
    public ProductViewModel() {
        this.Products = new ObservableCollection<Product>();
        // 注意: 这里只是代码示例   
        // ViewModel中不应为Model定义数据:-)   
        // 我们可以通过调用服务得到Model所需数据. 
        this.AllProducts = new ObservableCollection<Product>();
        this.AllProducts.Add(new Product { ProductId = 1, ProductName = "Apple" });
        this.AllProducts.Add(new Product { ProductId = 2, ProductName = "Orange" });
        this.AllProducts.Add(new Product { ProductId = 3, ProductName = "Banana" });
        this.AllProducts.Add(new Product { ProductId = 4, ProductName = "Pear" });
        this.AllProducts.Add(new Product { ProductId = 5, ProductName = "Grape" });
        this.AllProducts.Add(new Product { ProductId = 6, ProductName = "Grapefruit" });
        this.AllProducts.Add(new Product { ProductId = 7, ProductName = "Strawberry" });
        this.AllProducts.Add(new Product { ProductId = 8, ProductName = "Melon" });
        this.AllProducts.Add(new Product { ProductId = 9, ProductName = "Guava" });
        this.AllProducts.Add(new Product { ProductId = 10, ProductName = "Kiwi" });
        this.AllProducts.Add(new Product { ProductId = 11, ProductName = "Pineapple" });
        this.AllProducts.Add(new Product { ProductId = 12, ProductName = "Mango" });
        LoadProductsCommand = new DelegateCommand(LoadProducts, CanLoadProducts);
    }
    private void LoadProducts(object param) {
        string filter = param as string ?? string.Empty;
        this.Products.Clear();
        var query = from p in this.AllProducts
                    where p.ProductName.ToLower().StartsWith(filter.ToLower())
                    select p;
        foreach (var item in query) {
            this.Products.Add(item);
        }
    }
    private bool CanLoadProducts(object param) {
        return true;
    }
    public ICommand LoadProductsCommand { get; set; }
    public ObservableCollection<Product> AllProducts { get; set; }
    private ObservableCollection<Product> products;
    public ObservableCollection<Product> Products {
        get {
            return products;
        }
        set {
            products = value;
            this.FirePropertyChanged("Product");
        }
    }
}

 

public abstract class ViewModelBase : INotifyPropertyChanged {
    public ViewModelBase() {
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected void FirePropertyChanged(string propertyname) {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyname));
    }
}