[WPF/Silverlight]让INotifyPropertyChanged的实现更优雅一些

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。

ok，就到这里,have fun~