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
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 ///
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
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