[Silverlight入门系列]使用MVVM模式(7)：ViewModel的INotifyPropertyChanged接口实现
上一篇说到《Model的INotifyPropertyChanged接口实现》和在Prism中有《NotificationObject自动实现INotifyPropertyChanged接口》，很好，本文说说ViewModel的这个INotifyPropertyChanged接口可以用来做啥？
举例1：我有个TabControl，里面放了很多View，每个由ViewModel控制，我想是想TabSelectionChanged就打开相应的ViewModel，怎么做？
解答：用ViewModel的INotifyPropertyChanged接口实现，因为TabItem作为一个选择器就有 IsSelected属性，把这个属性绑定到ViewModel的IsSelected字段，然后这个字段改变的时候用INotifyPropertyChanged接口实现通知即可。整个流程用MVVM实现非常整洁。
Xaml
1 <TabControl ...>
2 <TabControl.ItemContainerStyle>
3 <Style TargetType="{x:Type TabItem}">
4 <Setter Property="IsSelected"
5 Value="{Binding Path=IsSelected,Mode=TwoWay}"/>
6 </Style>
7 </TabControl.ItemContainerStyle>
8 </TabControl>
ViewModel
1 public class
MyViewModel : INotifyPropertyChanged 2
{ 3 private bool
_isLoaded; 4
5 private void
Load() 6
{ 7 // code
8
} 9
10 private bool
_isSelected; 11
12 public bool
IsSelected 13
{ 14 get
15
{ 16 return this
._isSelected; 17
} 18 set
19
{ 20 if (this._isSelected !=
value) 21
{ 22 this._isSelected =
value; 23
24 if (this._isSelected && !this
._isLoaded) 25
{ 26 this
.Load(); 27 this._isLoaded = true
; 28
} 29
30 var propertyChanged = this
.PropertyChanged; 31 if (propertyChanged != null
) 32
{ 33 propertyChanged(this, new PropertyChangedEventArgs("IsSelected"
)); 34
} 35
} 36
} 37
} 38
39 public event
PropertyChangedEventHandler PropertyChanged; 40 }
举例2：我有个TreeView，里面的项非常复杂，还需要惰性加载（点击了才取数据并展开），怎么用MVVM实现？彻底晕了
解答：还是用ViewModel的INotifyPropertyChanged接口实现，要使得你的视图没有代码，就要你不再把TreeView当成一个存储数据的地方，而是看做一个展现数据的地方，那么一切都将水到渠成。这就是ViewModel这个想法的由来。而用INotifyPropertyChanged接口可以神奇的实现它们之间的解耦。
Xaml
1 <TreeView ItemsSource="{Binding FirstGeneration}">
2 <TreeView.ItemContainerStyle>
3 <!--
4
This Style binds a TreeViewItem to a PersonViewModel. 5 -->
6 <Style TargetType="{x:Type TreeViewItem}">
7 <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
8 <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
9 <Setter Property="FontWeight" Value="Normal" />
10 <Style.Triggers>
11 <Trigger Property="IsSelected" Value="True">
12 <Setter Property="FontWeight" Value="Bold" />
13 </Trigger>
14 </Style.Triggers>
15 </Style>
16 </TreeView.ItemContainerStyle>
17
18 <TreeView.ItemTemplate>
19 <HierarchicalDataTemplate ItemsSource="{Binding Children}">
20 <TextBlock Text="{Binding Name}" />
21 </HierarchicalDataTemplate>
22 </TreeView.ItemTemplate>
23 </TreeView>
ViewModel
1 public class
PersonViewModel 2
{ 3 public
PersonViewModel(Person person) 4 : this(person, null
) 5
{ 6
} 7
8 private
PersonViewModel(Person person, PersonViewModel parent) 9
{ 10 _person =
person; 11 _parent =
parent; 12
13 _children = new ReadOnlyCollection<PersonViewModel>
( 14 (from child in
_person.Children 15 select new PersonViewModel(child, this
)) 16 .ToList<PersonViewModel>
()); 17
} 18
19 private bool
_isSelected; 20 public bool
IsSelected 21
{ 22 get { return
_isSelected; } 23 set
24
{ 25 if (value !=
_isSelected) 26
{ 27 _isSelected =
value; 28 this.OnPropertyChanged("IsSelected"
); 29
} 30
} 31
} 32
33 private bool
_isExpanded; 34 public bool
IsExpanded 35
{ 36 get { return
_isExpanded; } 37 set
38
{ 39 if (value !=
_isExpanded) 40
{ 41 _isExpanded =
value; 42 this.OnPropertyChanged("IsExpanded"
); 43
} 44
45 // Expand all the way up to the root.
46 if (_isExpanded && _parent != null
) 47 _parent.IsExpanded = true
; 48
} 49
} 50 public string
Name 51
{ 52 get { return
_person.Name; } 53
} 54
} 55 public class
Person 56
{ 57 readonly List<Person> _children = new List<Person>
(); 58 public IList<Person>
Children 59
{ 60 get { return
_children; } 61
} 62
63 public string Name { get; set
; } 64 }
按需加载：
View Code
1 interface
ITreeViewItemViewModel : INotifyPropertyChanged 2
{ 3 ObservableCollection<TreeViewItemViewModel> Children { get
; } 4 bool HasDummyChild { get
; } 5 bool IsExpanded { get; set
; } 6 bool IsSelected { get; set
; } 7 TreeViewItemViewModel Parent { get
; } 8
} 9 public
TreeViewItemViewModel : ITreeViewItemViewModel 10
{ 11 protected TreeViewItemViewModel(TreeViewItemViewModel parent, bool
lazyLoadChildren) 12
{ 13 _parent =
parent; 14
15 _children = new ObservableCollection<TreeViewItemViewModel>
(); 16
17 if
(lazyLoadChildren) 18
_children.Add(DummyChild); 19
} 20 public bool
IsExpanded 21
{ 22 get { return
_isExpanded; } 23 set
24
{ 25 if (value !=
_isExpanded) 26
{ 27 _isExpanded =
value; 28 this.OnPropertyChanged("IsExpanded"
); 29
} 30
31 // Expand all the way up to the root.
32 if (_isExpanded && _parent != null
) 33 _parent.IsExpanded = true
; 34
35 // Lazy load the child items, if necessary.
36 if (this
.HasDummyChild) 37
{ 38 this
.Children.Remove(DummyChild); 39 this
.LoadChildren(); 40
} 41
} 42
} 43
44 /// <summary>
45 ///
Returns true if this object's Children have not yet been populated. 46 /// </summary>
47 public bool
HasDummyChild 48
{ 49 get { return this.Children.Count == 1 && this.Children[0] ==
DummyChild; } 50
} 51
52 /// <summary>
53 ///
Invoked when the child items need to be loaded on demand. 54 ///
Subclasses can override this to populate the Children collection. 55 /// </summary>
56 protected virtual void
LoadChildren() 57
{ 58
} 59 }
真正加载子项的工作留给子类去实现。它们重载LoadChildren方法来提供一个跟类型相关的加载子项的实现。比如下面的RegionViewModel类，它重载了该方法来加载State对象并且创建StateViewModel对象。
View Code
1 public class
RegionViewModel : TreeViewItemViewModel 2
{ 3 readonly
Region _region; 4
5 public
RegionViewModel(Region region) 6 : base(null, true
) 7
{ 8 _region =
region; 9
} 10
11 public string
RegionName 12
{ 13 get { return
_region.RegionName; } 14
} 15
16 protected override void
LoadChildren() 17
{ 18 foreach (State state in
Database.GetStates(_region)) 19 base.Children.Add(new StateViewModel(state, this
)); 20
} 21 }
Xaml
1 <TreeView ItemsSource="{Binding Regions}">
2 <TreeView.ItemContainerStyle>
3 <!--
4
This Style binds a TreeViewItem to a TreeViewItemViewModel. 5 -->
6 <Style TargetType="{x:Type TreeViewItem}">
7 <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
8 <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
9 <Setter Property="FontWeight" Value="Normal" />
10 <Style.Triggers>
11 <Trigger Property="IsSelected" Value="True">
12 <Setter Property="FontWeight" Value="Bold" />
13 </Trigger>
14 </Style.Triggers>
15 </Style>
16 </TreeView.ItemContainerStyle>
17
18 <TreeView.Resources>
19 <
HierarchicalDataTemplate 20 DataType="{x:Type local:RegionViewModel}color: #0000f
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