MvvmLight的抽象类ViewModelBase继承了ObservableObject这个类，我们来看看这个类：

 /// <summary>/// 一个基类,为了使它的对象属性必须具有可观察性/// INotifyPropertyChanged,INotifyPropertyChanging/// </summary>public class ObservableObject:INotifyPropertyChanged,INotifyPropertyChanging

可以看见其实它继承了INotifyPropertyChanged,INotifyPropertyChanging这两个.net里的接口，在System.dll程序集中：

#region Assembly System.dll, v2.0.50727
// C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.dll
#endregionnamespace System.ComponentModel
{// Summary://     Notifies clients that a property value has changed.public interface INotifyPropertyChanged{// Summary://     Occurs when a property value changes.event PropertyChangedEventHandler PropertyChanged;}
}

当属性改变时通知客户端。这个类肯定实现了这两个接口中的事件，来看下：

/// <summary>/// 如果需要,唤起PropertyChanged事件，非泛型/// </summary>/// <remarks>如果参数名在当前类的属性中不一致/// 在DEBUG条件下抛出异常VerifyPropertyName中</remarks>/// <param name="propertyName">改变的属性名称</param>/// <param name="propertyName"></param>[SuppressMessage("Microsoft.Design","CA1030:UseEventsWhereAppropriate",Justification="This cannot be an event" )]protected virtual void RaisePropertyChanged(string propertyName){VerifyPropertyName(propertyName);var handler = PropertyChanged;if (handler != null){handler(this, new PropertyChangedEventArgs(propertyName));}}

还有个就是

protected virtual void RaisePropertyChanging(string propertyName)

参数都是string类型的propertyName.

那么更好的肯定是实现这两个方法的泛型版本：

/// <summary>/// 如果需要,唤起PropertyChanged事件，泛型/// </summary>/// <typeparam name="T"></typeparam>/// <param name="propertyExpression"></param>[SuppressMessage("Microsoft.Design", "CA1030:UseEventsWhereAppropriate",Justification = "This cannot be an event")][SuppressMessage("Microsoft.Design","CA1006:GenericMethodsShouldProvideTypeParameter",Justification = "This syntax is more convenient(方便) than other alternatives（选择的余地）")]protected virtual void RaisePropertyChanged<T>(Expression<Func<T>> propertyExpression){var handler = PropertyChanged;if (handler != null){var propertyName = GetPropertyName(propertyExpression);handler(this, new PropertyChangedEventArgs(propertyName));}}

注意到这里有个GetPropertyName方法：

/// <summary>/// 根据表达式提取(Extract)属性名字/// </summary>/// <typeparam name="T"></typeparam>/// <param name="propertyExpression"></param>/// <returns></returns>protected string GetPropertyName<T>(Expression<Func<T>> propertyExpression){if (propertyExpression == null){throw new ArgumentNullException("propertyExpression");}//关于Linq的文章，可以参考//http://www.cnblogs.com/lifepoem/archive/2011/12/16/2288017.html//http://www.cnblogs.com/zhili/archive/2012/12/12/LambdaExpression.htmlvar body = propertyExpression.Body as MemberExpression;if (body == null){throw new ArgumentException("Invalid argument", "propertyExpression");}var property = body.Member as PropertyInfo;if (property == null){throw new ArgumentException("Argument is not a property","propertyExpression");}return property.Name;}

关于Linq学习的博文，园子里已经有很多人写过了，可以参考lifepoem和zhili的。

这个类还有两个重要的泛型方法,Set<T>:

/// <summary>/// 把新值赋给属性，然后唤起PropertyChanged事件/// </summary>/// <typeparam name="T">属性类型</typeparam>/// <param name="propertyExpression">确认属性改变的表达式，翻译有问题</param>/// <param name="field">保存属性值的变量</param>/// <param name="newValue">改变过后的新值</param>/// <returns>事件只有在两个值不相等的情况下才被唤起</returns>protected bool Set<T>(Expression<Func<T>> propertyExpression,ref T field,T newValue){if (EqualityComparer<T>.Default.Equals(field,newValue)){return false;}RaisePropertyChanging(propertyExpression);field = newValue;RaisePropertyChanged(propertyExpression);return true;}

另外一个其实也就是把Expression<Func<T>>参数换成string参数而已

protected bool Set<T>(string propertyName,ref T field,T newValue)

之前我们的例子中的RaisePropertyChanged就是其实就是调用ViewModelBase里的RaisePropertyChanged函数，因为这个函数是虚函数，所以是调用的ViewModelBase中的RaisePropertyChanged实现：

[SuppressMessage("Microsoft.Design", "CA1030:UseEventsWhereAppropriate",Justification = "This cannot be an event")]protected virtual void RaisePropertyChanged<T>(string propertyName, T oldValue, T newValue, bool broadcast){if (string.IsNullOrEmpty(propertyName)){throw new ArgumentException("This method cannot be called with an empty string", "propertyName");}RaisePropertyChanged(propertyName);if (broadcast){Broadcast(oldValue, newValue, propertyName);}}

最后一个参数如果不为true，而且不注册PropertyChanged事件将不会有任何返回结果。下篇我们介绍NotificationMessage和DialogMessage的简单使用。