Binding在Slider控件与TextBox控件之间建立关联,值可以互相绑定,但是它们的数据类型是不同的,Slider是Double类型,Text为String。原来,Binding有一种机制称为数据转换(Data Converter),当数据绑定的源与目标不同类型时,处理比较简单时,系统就自动的进行了类型转换,但是对于相对复杂的类型转换时,就需要我们手动进行了。
下面用一个例子来说明Convert的应用,程序的用途是在列表里面向玩家显示一些球的状态。
首先创建几个自定义数据类型:
public enum Category
{
Basketball,
football
}
public enum State
{
Available,
Locked,
Unknown
}
public class ball
{
public Categroy Categroy { get;set; }
public string Name { get; set; }
public State State { get; set; }
}
程序后面要用到的图片已经加载到程序中,球的State属性在UI里被映射为CheckBox。因为存在两个映射关系,我们需要提供两个Convert:一个是由Category类型单向转换为string,另一个State与bool?类型之间相互转换。代码如下:
public class CategoryToSourceConverter : IValueConverter
{
//将Category转换为Uri
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
Category c = (Category)value;
switch (c)
{
case Category.Basketball:
return @"basketball.png";
case Category.football:
return @"football.png";
default:
return null;
}
}
//不会被调用
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
public class StateToNullableBoolConvert : IValueConverter
{
//将state转换为bool?
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
State s = (State)value;
switch (s)
{
case State.Locked:
return false;
case State.Available:
return true;
case State.Unknown:
default:
return null;
}
}
//将bool?转换为State
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
bool? nb = (bool?)value;
switch (nb)
{
case true:
return State.Available;
case false:
return State.Locked;
case null:
default:
return State.Unknown;
}
}
}
下面我们看看如何在XAML里消费这些Converter。XAML代码的框架如下:
<Window x:Class="DataConverter.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:DataConverter"
Title="MainWindow" Height="350" Width="525"> <Window.Resources>
<local:CategoryToSourceConverter x:Key="cts"/>
<local:StateToNullableBoolConvert x:Key="stnb"/>
</Window.Resources>
<Grid>
<StackPanel Background="LightBlue">
<ListBox x:Name="listBoxPlan" Height="160" Margin="5,0"></ListBox>
<Button x:Name="buttonLoad" Content="Load" Height="25" Margin="5,0"></Button>
<Button x:Name="buttonSave" Content="Save" Height="25" Margin="5,5"></Button>
</StackPanel>
</Grid>
</Window>
XAML代码中已经添加了对程序集的引用并映射为名称空间local,以资源的形式创建了两个Convert的实例。名为ListBoxPlan的ListBox控件需要为它添加用于显示数据的DataTemplate。我们把焦点集中在ListBox控件的ItemTemplate属性上:
<ListBox x:Name="listBoxPlan" Height="160" Margin="5,0">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Image Width="20" Height="20"
Source="{Binding Path=Category,Converter={StaticResource cts}}"></Image>
<TextBlock Text="{Binding Path=Name}" Width="60" Margin="80,0"/>
<CheckBox IsThreeState="True"
IsChecked="{Binding Path=State,Converter={StaticResource stnb}}"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
Load按钮的Click事件处理负责把一组球的数据赋值给ListBox的ItemSource属性,Save按钮的Click事件处理器负责把用户更改过的数据写入文件:
//Load按钮Click事件处理器
private void buttonLoad_Click(object sender, RoutedEventArgs e)
{
List<Ball> ballList = new List<Ball>()
{
new Ball(){Category = Category.Basketball,Name = "NBA",State = State.Unknown},
new Ball(){Category = Category.Basketball,Name = "CBA",State = State.Unknown},
new Ball(){Category = Category.football,Name = "世界杯",State = State.Unknown},
new Ball(){Category = Category.football,Name = "欧冠",State = State.Unknown},
new Ball(){Category = Category.Basketball,Name = "WNBA",State = State.Unknown},
new Ball(){Category = Category.football,Name = "英超",State = State.Unknown}
};
this.listBoxPlan.ItemsSource = ballList;
}
//Save按钮Click事件处理器
private void buttonSave_Click(object sender, RoutedEventArgs e)
{
StringBuilder sb = new StringBuilder();
foreach (Ball b in listBoxPlan.Items)
{
sb.AppendLine(string.Format("Category={0},Name={1},State={2}", b.Category, b.Name, b.State));
}
File.WriteAllText(@"D:\BallList.txt", sb.ToString());
}
运行程序并单击CheckBox更改State,效果图如下:
单击Save按钮后打开D:\BallList.txt:
注*读《深入浅出WPF》读书笔记