WPF入门（3）——命令
命令是ICommand类型的属性，binding到控件上，用于代替事件，个人认为事件也很好，命令只是轻度解耦前后端。
闲话少说，上代码，示例是ScreenToGif的源代码中的一个命令：
public ICommand OpenOptions
{
get
{
return new RelayCommand
{
CanExecutePredicate = a => true, //TODO: Always let this window opens or check if there's any other recorder active?
ExecuteAction = a =>
{
var options = Application.Current.Windows.OfType<Options>().FirstOrDefault();
var tab = a as int? ?? 0; //Parameter that selects which tab to be displayed.
if (options == null)
{
options = new Options(tab);
options.Closed += (sender, args) =>
{
CloseOrNot();
};
//TODO: Open as dialog or not? Block other windows?
options.Show();
}
else
{
if (options.WindowState == WindowState.Minimized)
options.WindowState = WindowState.Normal;
options.SelectTab(tab);
options.Activate();
}
}
};
}
}
这样看有些麻烦，我们省略一些暂时对理解不重要的东西：
public ICommand OpenOptions
{
get
{
return new RelayCommand
{
CanExecutePredicate = a => true,
ExecuteAction = a =>
{
//这是个lambda表达式
}
};
}
}
实际上就是返回了一个new RelayCommand，这个RelayCommand是作者自定义的一个路由命令的类，代码如下：
/// <summary>
/// 路由命令
/// </summary>
internal class RelayCommand : ICommand
{
/// <summary>
/// 作者自定义的字段
/// </summary>
public Predicate<object> CanExecutePredicate { get; set; }
/// <summary>
/// 作者自定义的字段
/// </summary>
public Action<object> ExecuteAction { get; set; }
/// <summary>
/// 构造函数
/// </summary>
/// <param name="canExecute"></param>
/// <param name="execute"></param>
public RelayCommand(Predicate<object> canExecute, Action<object> execute)
{
CanExecutePredicate = canExecute;
ExecuteAction = execute;
}
/// <summary>
/// ICommand字段
/// </summary>
public RelayCommand()
{ }
/// <summary>
/// ICommand字段
/// </summary>
public event EventHandler CanExecuteChanged
{
add => CommandManager.RequerySuggested += value;
remove => CommandManager.RequerySuggested -= value;
}
/// <summary>
/// ICommad字段
/// </summary>
/// <param name="parameter"></param>
/// <returns></returns>
public bool CanExecute(object parameter)
{
return CanExecutePredicate == null || CanExecutePredicate(parameter);
}
/// <summary>
/// ICommand字段
/// </summary>
/// <param name="parameter"></param>
public void Execute(object parameter)
{
ExecuteAction(parameter);
}
}
使用路由的好处就是不管啥命令返回的都是RelayCommand类的实例，只要给该实例绑定上相应的方法就好了，方法可以随意写，上面作者就是为路由实例赋值了一个lambda表达式。
下面使用实例操作一下，下面的例子按下按钮之后修改button的context，为了方便我就直接使用上面ScreenToGif的RelayCommand。为了不与之前的代码冲突，我新加一个button：
新加的button的context同样是binding到Name属性，Command则是binding到了名为“ChangeName”命令上，该命令如下：
此时，MainViewModel.cs的所有代码如下：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
namespace WpfApp.ViewModel
{
class MainViewModel : INotifyPropertyChanged
{
#region INPC
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
#endregion
private string name;
public string Name
{
get { return name; }
set
{
name = value;
OnPropertyChanged("Name");
}
}
public ICommand ChangeName
{
get
{
return new RelayCommand() {
CanExecutePredicate = a => true,
ExecuteAction = a =>
{
Name = "ChangeName Command";
}
};
}
}
}
/// <summary>
/// 路由命令
/// </summary>
internal class RelayCommand : ICommand
{
public Predicate<object> CanExecutePredicate { get; set; }
public Action<object> ExecuteAction { get; set; }
/// <summary>
/// 构造函数
/// </summary>
/// <param name="canExecute"></param>
/// <param name="execute"></param>
public RelayCommand(Predicate<object> canExecute, Action<object> execute)
{
CanExecutePredicate = canExecute;
ExecuteAction = execute;
}
/// <summary>
/// ICommand字段
/// </summary>
public RelayCommand()
{ }
/// <summary>
/// ICommand字段
/// </summary>
public event EventHandler CanExecuteChanged
{
add => CommandManager.RequerySuggested += value;
remove => CommandManager.RequerySuggested -= value;
}
/// <summary>
/// ICommad字段
/// </summary>
/// <param name="parameter"></param>
/// <returns></returns>
public bool CanExecute(object parameter)
{
return CanExecutePredicate == null || CanExecutePredicate(parameter);
}
/// <summary>
/// ICommand字段
/// </summary>
/// <param name="parameter"></param>
public void Execute(object parameter)
{
ExecuteAction(parameter);
}
}
}
按照我的设定，按下第一个按钮会触发Button_Click方法（该方法在view的后台代码中，也就是我的工程中的MainWindow.xaml.cs文件）这是事件的方法，按下后会更改Name属性的内容，所有binding到该属性的控件内容都会跟随更改。
按下第二个按钮，则是触发ChangeName命令，该命令则是view的DataContext（也就是MainViewModel类）的ChangeName属性。按下后同样会修改Name属性的内容，所有所有binding到该属性的控件内容都会跟随更改。
下面debug一下：
喜大普奔，没有啥bug，哈哈哈哈哈哈
工程源代码上传在GitHub上了：https://github.com/feipeng8848/WPF-Demo
参考：https://www.codeproject.com/Articles/1052346/ICommand-Interface-in-WPF
