我们平常用的最多的APP可能就是企鹅和微信了。有没有想过聊天窗口如何实现的？本篇我们将简单模拟一个聊天窗口。
聊天窗口大致上就是消息的一个集合列表。集合列表最常见的展现形式无非就是ListView。可能有些童鞋会觉得ListView的样式和聊天窗口相去甚远，虽然我们可以通过自定义ItemTemplate来修改元素的显示效果，但如何将ListView的元素以不同样式展现呢？这就要通过ListView的ItemTempateSelector属性来实现了。基本原理是通过条件判断，返回不同的DataTemplate给ListViewItem使用。
首先我们来定义ViewModel层的消息集合，集合共有两种不同类型的消息，分别是普通文本信息和红包消息。这两种又需要区别是接受的消息（来自对方）和发送消息（自己发出）。
public abstract class MessageBase
{
public string Name { get; set; } public DateTime Published { get; set; }
} public class Message : MessageBase
{
public string Comment { get; set; } public bool IsSelf { get; set; }
} public class Gift : Message
{
public int Amount { get; set; }
}
其次我们要定义展现消息的DataTemplate，同样分为普通文字消息和红包消息。MessageDataTemplate和SelfMessageDataTemplate就是左右对称的对话框气泡，GiftDataTemplate就是红包的样式啦。
<DataTemplate x:Key="MessageDataTemplate">
<Grid HorizontalAlignment="Left" >
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Grid Grid.Row="0" CornerRadius="4" Background="LightGray" Padding="15">
<TextBlock Text="{Binding Comment}"></TextBlock>
</Grid>
<StackPanel Grid.Row="1" Orientation="Horizontal">
<TextBlock Text="{Binding Name}" Foreground="LightGray"></TextBlock>
<TextBlock Text="{Binding Published}" Foreground="LightGray" Margin="10,0,0,0"></TextBlock>
</StackPanel>
</Grid>
</DataTemplate> <DataTemplate x:Key="SelfMessageDataTemplate">
<Grid HorizontalAlignment="Right" >
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Grid Grid.Row="0" CornerRadius="4" Background="Green" Padding="15">
<TextBlock Text="{Binding Comment}" Foreground="White"></TextBlock>
</Grid>
<StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Right">
<TextBlock Text="{Binding Published}" Foreground="LightGray" Margin="0,0,10,0"></TextBlock>
<TextBlock Text="{Binding Name}" Foreground="LightGray"></TextBlock>
</StackPanel>
</Grid>
</DataTemplate> <DataTemplate x:Key="GiftDataTemplate">
<Grid HorizontalAlignment="Left" >
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Grid Grid.Row="0" CornerRadius="4" Background="Orange" Padding="10">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Ellipse Grid.Row="0" Width="50" Height="50" Fill="Red" Stroke="Gold" StrokeThickness="4"></Ellipse>
<TextBlock Grid.Row="0" Text="红包" TextAlignment="Center" VerticalAlignment="Center"></TextBlock>
<TextBlock Grid.Row="1" Text="{Binding Amount}" TextAlignment="Center"></TextBlock>
<TextBlock Grid.Row="2" Text="{Binding Comment}" TextAlignment="Center" ></TextBlock>
</Grid>
<StackPanel Grid.Row="1" Orientation="Horizontal">
<TextBlock Text="{Binding Name}" Foreground="LightGray"></TextBlock>
<TextBlock Text="{Binding Published}" Foreground="LightGray" Margin="10,0,0,0"></TextBlock>
</StackPanel>
</Grid>
</DataTemplate>
接下来就是如何应用ItemTemplateSelector了，我们需要创建继承自DataTemplateSelector的子类，并且override SelectTemplateCore方法。该方法将我们在App.xaml中定义的DataTemplate资源文件根据集合众不同的元素返回对应的模板。
public class MessageItemDataTemplateSelector : DataTemplateSelector
{
protected override DataTemplate SelectTemplateCore(object item, DependencyObject container)
{
if (item is Gift)
{
return App.Current.Resources["GiftDataTemplate"] as DataTemplate;
}
else if (item is Message)
{
if ((item as Message).IsSelf)
{
return App.Current.Resources["SelfMessageDataTemplate"] as DataTemplate;
}
else
{
return App.Current.Resources["MessageDataTemplate"] as DataTemplate;
}
} return base.SelectTemplateCore(item);
}
}
使用之前，我们需要在XAML中声明一个MessageItemDataTemplateSelector类型的实例。
<local:MessageItemDataTemplateSelector x:Key="MessageItemDataTemplateSelector"></local:MessageItemDataTemplateSelector>
然后Binding到ListView的ItemTemplateSelector属性。
<ListView Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
ItemTemplateSelector="{StaticResource MessageItemDataTemplateSelector}"
ItemsSource="{Binding MessageList}">
<ListView.ItemContainerStyle>
<Style TargetType="ListViewItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch"></Setter>
<Setter Property="Margin" Value="10"></Setter>
</Style>
</ListView.ItemContainerStyle>
</ListView>运行看一下效果，是不是蛮简单的？
Sample的代码放在全球最大同性交友网站上，地址如下：