重新想象 Windows 8.1 Store Apps (82) - 绑定: DataContextChanged, TargetNullValue, FallbackValue, UpdateSourceTrigger

原文:重新想象 Windows 8.1 Store Apps (82) - 绑定: DataContextChanged, TargetNullValue, FallbackValue, UpdateSourceTrigger[源码下载]

[align=center]重新想象 Windows 8.1 Store Apps (82) - 绑定: DataContextChanged, TargetNullValue, FallbackValue, UpdateSourceTrigger[/align]

作者：webabcd

介绍
重新想象 Windows 8.1 Store Apps 之绑定

DataContextChanged - FrameworkElement 的 DataContext 发生变化时触发的事件

TargetNullValue - 当绑定数据为 null 时所需要显示的值

FallbackValue - 当绑定失败（无法返回值）的时候所需要显示的值

UpdateSourceTrigger - UI 上数据更新的触发方式

示例
1、演示 DataContextChanged 的应用
DataContextChanged.xaml

<Page
x:Class="Windows81.Binding.DataContextChanged"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows81.Binding"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Grid Background="Transparent">

<StackPanel Margin="120 0 0 0">

<TextBlock Name="lblMsg" FontSize="14.667" />

<Button x:Name="btnChange" Content="改变数据上下文" Click="btnChange_Click" Margin="0 10 0 0" />

<ListBox x:Name="listBox" ItemsSource="{Binding}" DataContextChanged="listBox_DataContextChanged" Margin="0 10 0 0" />

</StackPanel>
</Grid>
</Page>

DataContextChanged.xaml.cs

/*
* DataContextChanged - FrameworkElement 的 DataContext 发生变化时触发的事件
*
*
* 关于绑定的基础请参见：
* http://www.cnblogs.com/webabcd/archive/2013/08/19/3267115.html * http://www.cnblogs.com/webabcd/archive/2013/08/22/3274099.html * http://www.cnblogs.com/webabcd/archive/2013/08/26/3281822.html * http://www.cnblogs.com/webabcd/archive/2013/08/29/3288304.html */

using System;
using System.Collections.Generic;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace Windows81.Binding
{
public sealed partial class DataContextChanged : Page
{
public DataContextChanged()
{
this.InitializeComponent();
this.Loaded += new RoutedEventHandler(DataContextChanged_Loaded);
}

void DataContextChanged_Loaded(object sender, RoutedEventArgs e)
{
// 指定数据上下文
listBox.DataContext = new List<string> { "a", "b", "c" };
}

private void btnChange_Click(object sender, RoutedEventArgs e)
{
// 修改数据上下文
listBox.DataContext = new List<string> { "a", "b", new Random().Next(0, 1000).ToString().PadLeft(3, '0') };
}

private void listBox_DataContextChanged(FrameworkElement sender, DataContextChangedEventArgs args)
{
/*
* FrameworkElement.DataContextChanged - 数据上下文发生改变后所触发的事件
*/

// 数据上下文发生改变后
lblMsg.Text = "数据源发生改变：" + DateTime.Now.ToString("hh:mm:ss");

}
}
}

2、演示 TargetNullValue 和 FallbackValue 的应用
TargetNullValueFallbackValue.xaml

<Page
x:Class="Windows81.Binding.TargetNullValueFallbackValue"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows81.Binding"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Grid Background="Transparent">

<StackPanel Name="stackPanel" Margin="120 0 0 0">

<!--
FallbackValue - 当绑定失败（无法返回值）的时候所需要显示的值
-->
<TextBlock FontSize="14.667" Text="{Binding Path=xxx, FallbackValue='绑定失败时的默认值'}" />

<!--
TargetNullValue - 当绑定数据为 null 时所需要显示的值
-->
<TextBlock FontSize="14.667" Text="{Binding Path=Name, TargetNullValue='绑定返回值为 null'}" Margin="0 10 0 0" />

</StackPanel>
</Grid>
</Page>

TargetNullValueFallbackValue.xaml.cs

/*
* TargetNullValue - 当绑定数据为 null 时所需要显示的值
* FallbackValue - 当绑定失败（无法返回值）的时候所需要显示的值
*
*
* 关于绑定的基础请参见：
* http://www.cnblogs.com/webabcd/archive/2013/08/19/3267115.html * http://www.cnblogs.com/webabcd/archive/2013/08/22/3274099.html * http://www.cnblogs.com/webabcd/archive/2013/08/26/3281822.html * http://www.cnblogs.com/webabcd/archive/2013/08/29/3288304.html */

using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Navigation;

namespace Windows81.Binding
{
public sealed partial class TargetNullValueFallbackValue : Page
{
public TargetNullValueFallbackValue()
{
this.InitializeComponent();
}

protected override void OnNavigatedTo(NavigationEventArgs e)
{
stackPanel.DataContext = new TargetNullValueTest { Name = null };
}
}

public sealed class TargetNullValueTest
{
public string Name { get; set; }
}
}

3、演示 UpdateSourceTrigger 的应用
UpdateSourceTrigger.xaml

<Page
x:Class="Windows81.Binding.UpdateSourceTrigger"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows81.Binding"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Grid Background="Transparent">

<StackPanel Name="stackPanel" Margin="120 0 0 0">

<TextBlock Name="lblMsg" />

<!--
UpdateSourceTrigger - UI 上数据更新的触发方式
Default - 失去焦点后触发
PropertyChanged - 属性值发生改变后触发
Explicit - 需要通过 BindingExpression.UpdateSource() 显示触发
-->

<TextBox Text="{Binding Text, Mode=TwoWay, ElementName=lblMsg, UpdateSourceTrigger=Default}" Margin="0 10 0 0" />
<TextBox Text="{Binding Text, Mode=TwoWay, ElementName=lblMsg, UpdateSourceTrigger=PropertyChanged}" Margin="0 10 0 0" />
<TextBox Name="txtExplicit" Text="{Binding Text, Mode=TwoWay, ElementName=lblMsg, UpdateSourceTrigger=Explicit}" Margin="0 10 0 0" />

<Button Name="btnBinding" Content="显示触发更新" Click="btnBinding_Click" Margin="0 10 0 0" />

</StackPanel>
</Grid>
</Page>

UpdateSourceTrigger.xaml.cs

/*
* UpdateSourceTrigger - UI 上数据更新的触发方式
*     Default - 失去焦点后触发
*     PropertyChanged - 属性值发生改变后触发
*     Explicit - 需要通过 BindingExpression.UpdateSource() 显示触发
*
*
* 关于绑定的基础请参见：
* http://www.cnblogs.com/webabcd/archive/2013/08/19/3267115.html * http://www.cnblogs.com/webabcd/archive/2013/08/22/3274099.html * http://www.cnblogs.com/webabcd/archive/2013/08/26/3281822.html * http://www.cnblogs.com/webabcd/archive/2013/08/29/3288304.html */

using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;

namespace Windows81.Binding
{
public sealed partial class UpdateSourceTrigger : Page
{
public UpdateSourceTrigger()
{
this.InitializeComponent();
}

private void btnBinding_Click(object sender, RoutedEventArgs e)
{
// 显示触发 txtExplicit 的数据更新
BindingExpression be = txtExplicit.GetBindingExpression(TextBox.TextProperty);
be.UpdateSource();
}
}
}

OK
[源码下载]