本文的代码都是基于WPF的,对于Silverlight,这些技术也同样适用。
本文详细讨论了INotifyPropertyChanged和ObservableCollection一些使用技巧。
本文的代码都是基于WPF的,对于Silverlight,这些技术也同样适用。
(一)INotifyPropertyChanged的使用场合
先写一个最简单的数据绑定,每次点击Button后,TextBlock的值都会自增1。
效果图如下所示:
![clip_image002 clip_image002](https://images.cnblogs.com/cnblogs_com/Jax/WindowsLiveWriter/INotifyPropertyChangedObservableCollecti_FFB8/clip_image002_thumb.jpg)
这里使用了MVVM模式,并把Click事件抽象为了Command。
代码下载:WpfApplication4_1.zip
观察上面的代码,注意到几个细节:
1. UserName和Age属性作为ViewModel的两个属性,因为Age递增是基于绑定实现的,所以ViewModel要实现INotifyPropertyChanged接口。
2. 我们只在Age上添加了OnPropertyChanged方法,它会根据Age属性的变化而自动更新XAML中绑定的值。
而对于UserName属性,由于它是始终不变的,所以没有添加OnPropertyChanged方法。
由此可见,OnPropertyChanged方法决定了后台数据的变化是否能影响到前台绑定的XAML。
此外,对于一次性绑定(以后不会再改变)的属性,不要添加OnPropertyChanged方法,因为该方法会增加额外的性能开销。
但是,在MVVM模式中,我们常常将UserName和Age属性抽象出来,作为一个实体类:
public class UserInfo
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public string UserName
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public int Age
{ get; set; }
}
与此同时,在ViewModel中添加一个UserInfo类型的属性:
public class RegisterUserViewModel : INotifyPropertyChanged
{
public UserInfo User { get; set; }
注意:此时XAML中的绑定相应要修改为:
<TextBlock Name="tbUserName" Text="{Binding User.UserName}" …. />
<TextBlock Name="tbAge" Text="{Binding User. Age}" …. />
这时候问题就来了。到底是UserInfo实现INotifyPropertyChanged接口呢,还是ViewModel实现INotifyPropertyChanged接口呢?
1.如果UserInfo实现INotifyPropertyChanged接口,,那么UserInfo实体类相应地修改为:
public class UserInfo : INotifyPropertyChanged
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public string UserName
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private int age;
public int Age
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
get
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
return this.age;
}
set
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
if (this.age != value)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.age = value;
OnPropertyChanged("Age");
}
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif)
INotifyPropertyChanged Members#region INotifyPropertyChanged Members
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public event PropertyChangedEventHandler PropertyChanged;
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private void OnPropertyChanged(string propertyName)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
#endregion
}
而ViewModel相应地就简化了:
public class RegisterUserViewModel
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
public RegisterUserViewModel()
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
this.User = new UserInfo
{ UserName = "Baobao", Age = 27 };
ClickCommand = new DelegateCommand<object>(OnClick, arg => true);
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
void OnClick(object obj)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.User.Age += 1;
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public UserInfo User
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public RegisterUserView View
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ICommand ClickCommand
{ get; set; }
}
注意到,此时ViewModel不需要实现INotifyPropertyChanged接口。因为INotifyPropertyChanged监视的是UserInfo实体里面的Age属性,所以在OnClick方法中,只要修改User实体的Age属性就能通知XAML做出改变。
示例下载:WpfApplication4_2.zip
2.如果UserInfo不实现INotifyPropertyChanged接口,那么这个实体类就非常简单了:
public class UserInfo
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public string UserName
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public int Age
{ get; set; }
}
这时,ViewModel就必须要实现INotifyPropertyChanged接口,而且要为User属性添加OnPropertyChanged方法,以下是部分代码(省略了INotifyPropertyChanged接口的实现):
public class RegisterUserViewModel : INotifyPropertyChanged
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
public RegisterUserViewModel()
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
this.User = new UserInfo
{ UserName = "Baobao", Age = 27 };
ClickCommand = new DelegateCommand<object>(OnClick, arg => true);
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
void OnClick(object obj)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.User.Age += 1;
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private UserInfo user;
public UserInfo User
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
get
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
return this.user;
}
set
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
if (this.user != value)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.user = value;
OnPropertyChanged("User");
}
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public RegisterUserView View
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ICommand ClickCommand
{ get; set; }
}
代码下载:WpfApplication4_3.zip
如果运行上述代码,你会发现XAML中的Age并不会自增1。问题应该出在OnClick方法上,我在下面一行添加了断点:
this.User.Age += 1;
我发现User的Age属性确实由27变成了28,但是并没有把改变结果通知XAML。
这是因为,INotifyPropertyChanged接口只监视UserInfo这个实体的地址是否发生了改变,而目前这个地址并没有变化,只是存储在UserInfo实体中的成员Age发生了改变,而Age的地址并不在INotifyPropertyChanged接口的监视之下,所以XAML中没有任何改变。
看来只有修改UserInfo这个实体的地址了,最好的方法就是重新实例化一个UserInfo实体:
![](https://www.cnblogs.com/Images/OutliningIndicators/ContractedBlock.gif)
Code
void OnClick(object obj)
{
//this syntax below cannot run, so it is marked
//this.User.Age += 1;
this.User = new UserInfo() { UserName = this.User.UserName, Age = this.User.Age + 1 };
}
示例下载:WpfApplication4_4.zip
比较两种实现方式,我个人倾向于第一种,因为它更自然,而第二种,由于要new一个新的UserInfo对象,所以要多牺牲一些性能。但是不管哪种实现方式,都不要ViewModel和UserInfo同时实现INotifyPropertyChanged接口,这是非常消耗性能的。
(二)ObservableCollection和List的使用场合
始终对ObservableCollection怀有恐惧,因为它太笼统了。
首先它是一个集合,但它是一个既实现了INotifyPropertyChanged接口又实现了INotifyCollectionChanged接口的集合,这是它与List<T>的不同之处。
使用ObservableCollection<T>注定要比List<T>消耗性能,所以我一直在探寻它们各自不同的使用场合。而且,当泛型参数T也实现了INotifyPropertyChanged接口时,就更加复杂了。
于是分为以下四种情况:
1.最简单是List<T>,其中T没有实现INotifyPropertyChanged接口,就是一个简单的实体。
做一个简单的示例,统计一个班级的计算机成绩。
![clip_image004 clip_image004](https://images.cnblogs.com/cnblogs_com/Jax/WindowsLiveWriter/INotifyPropertyChangedObservableCollecti_FFB8/clip_image004_thumb.jpg)
由于List<T>中的T没有实现INotifyPropertyChanged接口,所以增加、删除、修改这3个Button都不会起作用。
着眼于“及格”这个CheckBox,勾选后就只显示分数不小于60的学生列表,取消勾选择显示全部学生。为此,在XAML中要把CheckBox的绑定方式设置为TwoWay(虽然CheckBox默认就是TwoWay,但还是建议显式指定):
<CheckBox Name="chkPass" IsChecked="{Binding Path=IsPassed, Mode=TwoWay}">及格</CheckBox>
在ViewModel中,关键是ModelPropertyChanged方法,在IsPassed属性变化的时候,会执行这个方法:
void ModelPropertyChanged(object sender, PropertyChangedEventArgs e)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
switch (e.PropertyName)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
case "IsPassed":
this.StudentList = this.IsPassed ? StudentService.RetrievePassedStudentList() : StudentService.RetrieveStudentList();
break;
default:
break;
}
}
看得出,根据IsPassed属性的不同,分别为StudentList分配不同的地址,所以在XAML中的DataGrid会跟着发生改变。
随着这里的ViewModel也实现了INotifyPropertyChanged接口,但这是为了IsPassed属性和StudentList属性而实现的,与Student实体类无关。
![clip_image006 clip_image006](https://images.cnblogs.com/cnblogs_com/Jax/WindowsLiveWriter/INotifyPropertyChangedObservableCollecti_FFB8/clip_image006_thumb.jpg)
代码下载:WpfApplication6_1.zip
如果大家读过我之前介绍MVP模式的一系列文章,这个小例子应该很简单。
结论:List<T>(T未实现INotifyPropertyChanged接口),适用于一次性绑定,适用于重新实例化整个List集合的绑定(例如上文的StudentList)。
2.稍微复杂点,List<T>,其中T实现了INotifyPropertyChanged接口。
也就是说,让Student实体类实现INotifyPropertyChanged接口。
public class Student : INotifyPropertyChanged
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public int UserId
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public string UserName
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private int score;
public int Score
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
get
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
return this.score;
}
set
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
if (this.score != value)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.score = value;
OnPropertyChanged("Score");
}
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif)
INotifyPropertyChanged Members#region INotifyPropertyChanged Members
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public event PropertyChangedEventHandler PropertyChanged;
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private void OnPropertyChanged(string propertyName)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
#endregion
}
其它代码保持不变。
我们看到,Modify按钮可以使用了。修改后的数据立刻展现在XAML中。
![clip_image008 clip_image008](https://images.cnblogs.com/cnblogs_com/Jax/WindowsLiveWriter/INotifyPropertyChangedObservableCollecti_FFB8/clip_image008_thumb.jpg)
但是,Add和Remove按钮仍然不能使用。因为INotifyPropertyChanged接口只是监视Student,而没有监视List<Student>集合。所以修改单笔Student数据,可以发送通知给XAML;但是修改List<Student>集合,除非像上一种方式那样重新实例化List<Student>集合(勾选CheckBox仅显示及格的学生),否则必须另辟蹊径。
代码下载:WpfApplication6_2.zip
结论:List<T>(T实现INotifyPropertyChanged接口),适用于修改集合中单笔数据的成员(比如说Student的Score属性)。
3.换个思路,用ObservableCollection<T>怎么样?其中T没有实现INotifyPropertyChanged接口。
也就是说,Student类仍然是一个简单的实体类:
public class Student
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public int UserId
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public string UserName
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public int Score
{ get; set; }
}
相应地,把ViewModel中的List<Student>全都修改为ObservableCollection<Student>,并在构造函数中把原始的List集合转换为ObservableCollection集合:
public class ScoreListViewModel : INotifyPropertyChanged
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
public ScoreListViewModel()
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.StudentList = new ObservableCollection<Student>(StudentService.RetrieveStudentList());
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
AddCommand = new DelegateCommand<object>(OnAdd, arg => true);
ModifyCommand = new DelegateCommand<object>(OnModify, arg => true);
RemoveCommand = new DelegateCommand<object>(OnRemove, arg => true);
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
this.PropertyChanged += ModelPropertyChanged;
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
void ModelPropertyChanged(object sender, PropertyChangedEventArgs e)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
switch (e.PropertyName)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
case "IsPassed":
this.StudentList = new ObservableCollection<Student>(
this.IsPassed ? StudentService.RetrievePassedStudentList() : StudentService.RetrieveStudentList());
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
break;
default:
break;
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ScoreListView View
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private bool isPassed;
public bool IsPassed
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
get
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
return this.isPassed;
}
set
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
if (this.isPassed != value)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.isPassed = value;
OnPropertyChanged("IsPassed");
}
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ObservableCollection<Student> StudentList
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ICommand AddCommand
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ICommand ModifyCommand
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
public ICommand RemoveCommand
{ get; set; }
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public void OnAdd(object obj)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
this.StudentList.Add(new Student()
{ UserId = 7, UserName = "李永京", Score = 75 });
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public void OnModify(object obj)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
var item = this.StudentList.SingleOrDefault(x => x.UserId == 1);
if (item != null)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
item.Score = 0;
//item = new Student() { UserId = item.UserId, UserName = item.UserName, Score = 0 };
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public void OnRemove(object obj)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
var item = this.StudentList.SingleOrDefault(x => x.UserId == 2);
if (item != null)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
this.StudentList.Remove(item);
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif)
INotifyPropertyChanged Members#region INotifyPropertyChanged Members
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
public event PropertyChangedEventHandler PropertyChanged;
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
private void OnPropertyChanged(string propertyName)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
#endregion
}
看一下点击Add和Remove按钮后的效果图:
![clip_image010 clip_image010](https://images.cnblogs.com/cnblogs_com/Jax/WindowsLiveWriter/INotifyPropertyChangedObservableCollecti_FFB8/clip_image010_thumb.jpg)
而由于ObservableCollection<T>中的T没有实现INotifyPropertyChanged接口,所以修改功能又不能使用了。
这时,我尝试着将
item.Score = 0;
修改为:
item = new Student() { UserId = item.UserId, UserName = item.UserName, Score = 0 };
但仍然无济于事。别灰心,还有最后一种解决方案没试呢~
代码下载:WpfApplication6_3.zip
结论:ObservableCollection<T>(T未实现INotifyPropertyChanged接口),适用于增删集合中的数据(上面的Add和Remove按钮)。但是对于单笔数据的更改,是无能为力的。
4.最复杂的情况,就是使用ObservableCollection<T>,并且T实现了INotifyPropertyChanged接口。
代码结合了上述第2、第3种编程的方式:
代码下载:WpfApplication6_4.zip
结论:所有的问题在次全都迎刃而解。这也许是最完美的解决方案了。但是,也是最消耗性能的。
(三)数据绑定中的OneWay和TwoWay
OneWay和TwoWay是最常见的两种绑定方式,描述了源数据(控件)和目标数据(控件)之间的关系。现在,让我们基于MVP模式的思想,重新认识数据绑定机制。
在下面的概念中,源数据(控件),在MVP里变成了数据(Model);目标(数据)控件,则相应地变成了XAML中绑定的控件。
在WPF和Silverlight的数据绑定中,因控件的不同,其默认的绑定方向也不一样。
在Silverlight 3.0中,BindingMode枚举有3个值,默认是OneWay:
public enum BindingMode
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
{
OneWay = 1,
OneTime = 2,
TwoWay = 3
}
OneWay和OneTime是数据(Model)发生变化时通知XAML中的控件。而TwoWay则是数据和XAML之间任何一方发生变化都会通知对方。
我们在前面的例子中已经展示过,勾选“及格”这个CheckBox后,GridView中的数据会只显示Score>=60的学生,这个就是TwoWay的最好例证。
TwoWay,它适用于那些可以和用户互动的控件,比如说CheckBox和RadioButton的选中与否,比如在TextBox中输入了那些值,比如说在ListBox、ComboBox和GridView中选中了哪一行。我会在《Prism研究 之 包氏波动理念》来详细介绍其实现技术。
对于OneWay和OneTime,需要仔细说清楚。
A) OneWay是我们使用最频繁的了,无需多说,数据上的任何变化都会通知XAML中绑定的控件。
B) OneTime仅仅是一次性绑定。之后即使数据(Model)再发生变化,XAML中的控件也不会跟着改变。
还记得我们在本文的第一节《INotifyPropertyChanged的使用场合》举出的那个例子么?其中的UserName字段,就设置为OneTime的绑定方式,因为我们在初始化将其设置为”Baobao”后,就再也没改动过它。
将不变化的字段尽可能地设置为OneTime,而不是OneWay,这不就多少又节约了一些性能嘛。
而在WPF中,BindingMode枚举有5个值,默认是Default:
public enum BindingMode
{
TwoWay = 0,
OneWay = 1,
OneTime = 2,
OneWayToSource = 3,
Default = 4
}
OneWayToSource,从字面上很容易理解,它表示当XAML中的控件发生变化时,通知数据(Model),但是在反方向上,数据发生变化却不会通知XAML中的控件。它和OneWay是正好相反的。这样,OneWayToSource在MVP模式中就没有半点用武之处(它的最大用处其实在于把非依赖属性绑定到依赖属性上,参加http://book.51cto.com/art/200908/145469.htm)。
在WPF中,绑定的默认值是Default,这不同于Silverlight 3的默认绑定OneWay。Default表示它会根据控件的不同而未控件自动选择其绑定行为。比如对于TextBox、CheckBox、RadioBtton这些用户可编辑的控件,Default值就是TwoWay;而对于只是用来显示数据的控件(比如ListBox、Label),Default值就是OneWay。