让INotifyPropertyChanged的实现更优雅一些
转自：http://www.cnblogs.com/024hi/archive/2009/05/17/Let_Implementation_Of_INotifyPropertChanged_More_Graceful.html
我想大部分的WPF和SL开发者都应该对INotifyPropertyChanged这个接口再熟悉不过了。当我们向UI传递属性变化的通知并更新客户端UI时就必须应用到它。（这里插一句，当一个集合中的项改变时我们则需要使用ObservableCollection<T>泛型集合）
假设我们有一个叫做MenuButtonModel的类，其中一个属性为HasFocus，那么通常情况下我们会这样实现INotifyPropertyChanged接口
public class MenuButtonModel : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; private bool? _hasFocus; public bool? HasFocus { get { return _hasFocus; } set { _hasFocus = value; if (PropertyChanged != null) { PropertyChanged(this,new PropertyChangedEventArgs("HasFocus")) } } } }
当这个类的属性比较多的时候（如多了一个IconPath属性），我们又将NotifyPropertyChanged独立出来，如：
public class MenuButtonModel : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public void NotifyPropertyChanged(string propertyName) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } private bool? _hasFocus; public bool? HasFocus { get { return _hasFocus; } set { _hasFocus = value; NotifyPropertyChanged("HasFocus"); } } private string _iconPath; public string IconPath { get { return _iconPath; } set { _iconPath = value; NotifyPropertyChanged("IconPath"); } } }
当我们的类越来越多时，写这样的代码又显得有些疲于奔命。于是乎大家想到了为这些类建立一个共同的基类。所有该基类的派生类自然都实现了IPropertyChanged接口。如下例中的PropertyChangedBase类：
public class PropertyChangedBase : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public void NotifyPropertyChanged(string propertyName) { if (this.PropertyChanged != null) { this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } }
然后我们改动MenuButton类，使其继承PropertyChangedBase：
public class MenuButtonModel : PropertyChangedBase { private bool? _hasFocus; public bool? HasFocus { get { return _hasFocus; } set { _hasFocus = value; NotifyPropertyChanged("HasFocus"); } } private string _iconPath; public string IconPath { get { return _iconPath; } set { _iconPath = value; NotifyPropertyChanged("IconPath"); } }
现在看起来好多了。不过做为有洁癖的程序员，这样的方法显然不会满足俺们的要求，因为我们仍然需要硬编码一个Name（如:IconPath），这很不爽。即使写错了，编译器也不会发现这个潜在的错误。从成千上万个类中寻找一个硬编码问题会让人抓狂。老赵说过要尽量使用强类型，这绝对是至理名言。。。，那么表达式树正式登场！我们需要稍微的改动一下PropertyChangedBase类（这里注意需要using System.Linq.Expressions）：
public class PropertyChangedBase : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public void NotifyPropertyChanged<T>(Expression<Func<T>> propertyName) { if (this.PropertyChanged != null) { var memberExpression = propertyName.Body as MemberExpression; if (memberExpression != null) { this.PropertyChanged(this, new PropertyChangedEventArgs(memberExpression.Member.Name)); } } } }
经过如上改动，我们就可以在派生类中使用优雅的强类型啦～，如下：
public class MenuButtonModel : PropertyChangedBase { private bool? _hasFocus; public bool? HasFocus { get { return _hasFocus; } set { _hasFocus = value; NotifyPropertyChanged<bool?>(() => this.HasFocus); } } private string _iconPath; public string IconPath { get { return _iconPath; } set { _iconPath = value; NotifyPropertyChanged<string>(() => this.IconPath); } } }
看起来还不错。不过<bool?>，<string>依然不是很爽，如果可以this.NotifyPropertyChanged( p => p.HasFocus)，那多帅啊。要实现这个目的，我们需要使用扩展方法。搞一个静态类出来，如下：
public static class PropertyChangedBaseEx { public static void NotifyPropertyChanged<T, TProperty>(this T propertyChangedBase, Expression<Func<T, TProperty>> expression) where T : PropertyChangedBase { var memberExpression = expression.Body as MemberExpression; if (memberExpression != null) { string propertyName = memberExpression.Member.Name; propertyChangedBase.NotifyPropertyChanged(propertyName); } else throw new NotImplementedException(); } }
然后我们就可以在类中用优雅的语法实现PropertyChanged了。
全部代码如下：
MenuButton.cs
public class MenuButtonModel : PropertyChangedBase { private bool? _hasFocus; public bool? HasFocus { get { return _hasFocus; } set { _hasFocus = value; this.NotifyPropertyChanged(p => p.HasFocus); } } private string _iconPath; public string IconPath { get { return _iconPath; } set { _iconPath = value; this.NotifyPropertyChanged(p => p.IconPath); } } }
PropertyChangedBase.cs以及PropertyChangedBaseEx.cs
public class PropertyChangedBase : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public void NotifyPropertyChanged(string propertyName) { if (this.PropertyChanged != null) { this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } } public static class PropertyChangedBaseEx { public static void NotifyPropertyChanged<T, TProperty>(this T propertyChangedBase, Expression<Func<T, TProperty>> expression) where T : PropertyChangedBase { var memberExpression = expression.Body as MemberExpression; if (memberExpression != null) { string propertyName = memberExpression.Member.Name; propertyChangedBase.NotifyPropertyChanged(propertyName); } else throw new NotImplementedException(); } }
需要注意的是，因为是扩展方法，所有this关键字不能省略。