MVVM入门

1.简介

MVVM 就是 Model – View – ViewModel 三组功能（类）分割的设计模式。WPF中使用MVVM可以降低UI显示与后端逻辑代码的耦合度，即更换界面时，只需要修改很少的逻辑代码就可以实现，甚至不用修改。在WPF中使用数据绑定机制，当数据变化后，数据会通知界面变更的发生，而不需要通过访问界面元素来修改值，这样在后端逻辑代码中也就不必操作或者很少操作界面的元素了。
使用MVVM，可以很好的配合WPF的数据绑定机制来实现UI与逻辑代码的分离，MVVM中的View表示界面，负责页面显示，ViewModel负责逻辑处理，包括准备绑定的数据和命令，ViewModel通过View的DataContext属性绑定至View，Model为业务模型，供ViewModel使用。

2.例子

废话不多说，直接看例子：
1. 需求：显示学生的学号、姓名和年龄
2. 建立Model：

public class StudentModel : INotifyPropertyChanged {
		
		private string studentId;
		public string StudentId {
			get {
				return studentId;
			}
			set {
				studentId = value;
				NotifyPropertyChanged("StudentId");
			}
		}

		
		private string studentName;
		public string StudentName {
			get {
				return studentName;
			}
			set {
				studentName = value;
				NotifyPropertyChanged("StudentName");
			}
		}

		
		private int studentAge;
		public int StudentAge {
			get {
				return studentAge;
			}
			set {
				studentAge = value;
				NotifyPropertyChanged("StudentAge");
			}
		}

		public event PropertyChangedEventHandler PropertyChanged;
		public void NotifyPropertyChanged(string propertyName) {
			if (PropertyChanged != null) {
				PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
			}
		}
}  

3.建立ViewModel：

class StudentViewModel {
		public DelegateCommand ShowCommand { get; set; }
		public StudentModel Student { get; set; }
		public StudentViewModel() {
			Student = new StudentModel();
			ShowCommand = new DelegateCommand();
			ShowCommand.ExecuteCommand = new Action<object>(ShowStudentData);
		}
		private void ShowStudentData(object obj) {
			Student.StudentId = "13818783181";
			Student.StudentName = "xiao";
			Student.StudentAge = 27;
		} 
	}

public class DelegateCommand : ICommand {
		public Action<object> ExecuteCommand = null;
		public Func<object, bool> CanExecuteCommand = null;
		public event EventHandler CanExecuteChanged;

		public bool CanExecute(object parameter) {
			if (CanExecuteCommand != null) {
				return this.CanExecuteCommand(parameter);
			} else {
				return true;
			}
		}

		public void Execute(object parameter) {
			if (this.ExecuteCommand != null) {
				this.ExecuteCommand(parameter);
			}
		}

		public void RaiseCanExecuteChanged() {
			if (CanExecuteChanged != null) {
				CanExecuteChanged(this, EventArgs.Empty);
			}
		}
}

4.绑定数据到View：

<Grid>
        <Label Content="学号" Height="28" HorizontalAlignment="Left" Margin="54,23,0,0" Name="labelStudentId" VerticalAlignment="Top" />
        <TextBox Text="{Binding Student.StudentId}" IsReadOnly="True" Height="23" HorizontalAlignment="Right" Margin="0,27,289,0" Name="textBoxStudentId" VerticalAlignment="Top" Width="120" />
        <Label Content="姓名" Height="28" HorizontalAlignment="Left" Margin="54,61,0,0" Name="labelStudentName" VerticalAlignment="Top" />
        <TextBox Text="{Binding Student.StudentName}" IsReadOnly="True" Height="23" HorizontalAlignment="Left" Margin="94,65,0,0" Name="textBoxStudentName" VerticalAlignment="Top" Width="120" />
       <Label Content="年龄" Height="28" HorizontalAlignment="Left" Margin="54,94,0,0" Name="labelStudentAge" VerticalAlignment="Top" />
        <TextBox Text="{Binding Student.StudentAge}" IsReadOnly="True" Height="23" HorizontalAlignment="Left" Margin="94,99,0,0" Name="textBoxStudentAge" VerticalAlignment="Top" Width="120" />
        <Button Command="{Binding ShowCommand}" Content="显示" Height="23" HorizontalAlignment="Left" Margin="345,27,0,0" Name="buttonShow" VerticalAlignment="Top" Width="75" />
</Grid>

5.将ViewModel设置到DataContext：

public MainWindow() {
	InitializeComponent();
	this.DataContext = new StudentViewModel();  
}

6.说明：

ICommand接口中的Execute（）方法用于命令的执行，CanExecute（）方法用于指示当前命令在目标元素上是否可用，当这种可用性发生改变时便会触发接口中的CanExecuteChanged事件。
我们可以将实现了ICommand接口的命令DelegateCommand赋值给Button（命令源）的Command属性（只有实现了ICommandSource接口的元素才拥有该属性），这样Button便与命令进行了绑定。