背水一战 Windows 10 (26)

wyyy721

　　[源码下载]




背水一战 Windows 10 (26) - XAML: x:DeferLoadStrategy, x:Null　　
作者：webabcd

介绍
背水一战 Windows 10 之 XAML

• x:DeferLoadStrategy="Lazy" - 用于指定一个 UIElement 为一个延迟加载元素
  
• x:Null - null
  

　　
示例
1、x:DeferLoadStrategy 通过 FindName 加载
Xaml/DeferLoadStrategy/Demo1.xaml



<Page
x:Class="Windows10.Xaml.DeferLoadStrategy.Demo1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml.DeferLoadStrategy"
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="10 0 10 10">
<TextBlock Name="lblMsg" TextWrapping="Wrap" Margin="5" />
<!--
x:DeferLoadStrategy="Lazy" - 其仅作用于 UIElement，可以指定一个 UIElement 为一个延迟加载元素
1、标记为延迟加载的元素必须要指定其 x:Name
2、UIElement 不是在任何情况下都能标记为 x:DeferLoadStrategy="Lazy" 的，不是只有 FindName 才能加载延迟加载元素，具体参见文档：https://msdn.microsoft.com/en-us/windows/uwp/xaml-platform/x-deferloadstrategy-attribute
3、将 UIElement 标记为 x:DeferLoadStrategy="Lazy" 的好处是可以减少页面的启动时间，带来的问题是会增加内存的使用量，每个延迟加载元素大约多耗费 600 字节左右的内存
4、延迟加载元素在加载后，会触发其 Loading 事件
-->
<TextBlock Name="textBlock" Margin="5" x:DeferLoadStrategy="Lazy" />
</StackPanel>
</Grid>
</Page>
　　Xaml/DeferLoadStrategy/Demo1.xaml.cs



/*
* 演示 x:DeferLoadStrategy 的相关知识点
*
* 本例演示通过“FindName”加载延迟加载元素
*/
using System;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
namespace Windows10.Xaml.DeferLoadStrategy
{
public sealed partial class Demo1 : Page
{
public Demo1()
{
this.InitializeComponent();
this.Loaded += DeferLoadStrategyDemo_Loaded;
}
private void DeferLoadStrategyDemo_Loaded(object sender, RoutedEventArgs e)
{
try
{
// 抛出异常
textBlock.Text = "我是 TextBlock";
}
catch (Exception ex)
{
lblMsg.Text += ex.ToString();
lblMsg.Text += Environment.NewLine;
}
// 可以通过 FindName() 来加载 x:DeferLoadStrategy="Lazy" 元素
this.FindName(nameof(textBlock));
textBlock.Text = "我是 TextBlock";
}
}
}
　　
2、x:DeferLoadStrategy 通过绑定加载
Xaml/DeferLoadStrategy/Demo2.xaml



<Page
x:Class="Windows10.Xaml.DeferLoadStrategy.Demo2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml.DeferLoadStrategy"
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="10 0 10 10">
<TextBox Name="textBox1" Margin="5" />
<!--延迟加载元素-->
<TextBox Name="textBox2" x:DeferLoadStrategy="Lazy" Text="我是 TextBox" Margin="5" />
<!--将 textBox2 与 textBox1 绑定后，textBox2 就会被加载-->
<Button Name="button" Content="将 textBox1 的 Text 绑定到 textBox2 的 Text" Click="button_Click" Margin="5" />
</StackPanel>
</Grid>
</Page>
　　Xaml/DeferLoadStrategy/Demo2.xaml.cs



/*
* 演示 x:DeferLoadStrategy 的相关知识点
*
* 本例演示通过“绑定”加载延迟加载元素
*/
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;
namespace Windows10.Xaml.DeferLoadStrategy
{
public sealed partial class Demo2 : Page
{
public Demo2()
{
this.InitializeComponent();
}
private void button_Click(object sender, RoutedEventArgs e)
{
// 实例化 Binding 对象
Binding binding = new Binding()
{
ElementName = nameof(textBox2), // textBox2 是延迟加载元素，将其与 textBox1 绑定后，textBox2 就会被加载
Path = new PropertyPath(nameof(TextBox.Text)),
Mode = BindingMode.TwoWay,
UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged
};
// 将目标对象的目标属性与指定的 Binding 对象关联
            BindingOperations.SetBinding(textBox1, TextBox.TextProperty, binding);
}
}
}
　　
3、x:DeferLoadStrategy 通过 Storyboard 加载
Xaml/DeferLoadStrategy/Demo3.xaml



<Page
x:Class="Windows10.Xaml.DeferLoadStrategy.Demo3"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml.DeferLoadStrategy"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Page.Resources>
<Storyboard x:Name="sb">
<ColorAnimation Storyboard.TargetName="ellipse"
Storyboard.TargetProperty="(Ellipse.Fill).(SolidColorBrush.Color)"
BeginTime="00:00:00"
From="Orange"
To="Blue"
Duration="0:0:3"
AutoReverse="true"
RepeatBehavior="Forever ">
</ColorAnimation>
</Storyboard>
</Page.Resources>
<Grid Background="Transparent">
<StackPanel Margin="10 0 10 10">
<!--延迟加载元素-->
<Ellipse x:Name="ellipse" Fill="Orange" Width="200" Height="100" x:DeferLoadStrategy="Lazy" HorizontalAlignment="Left" Margin="5" />
<!--启动一个引用了延迟加载元素的动画后，该延迟加载元素就会被加载-->
<Button Name="button" Content="开始动画" Click="button_Click" Margin="5" />
</StackPanel>
</Grid>
</Page>
　　Xaml/DeferLoadStrategy/Demo3.xaml.cs



/*
* 演示 x:DeferLoadStrategy 的相关知识点
*
* 本例演示通过“Storyboard”加载延迟加载元素
*/
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
namespace Windows10.Xaml.DeferLoadStrategy
{
public sealed partial class Demo3 : Page
{
public Demo3()
{
this.InitializeComponent();
}
private void button_Click(object sender, RoutedEventArgs e)
{
// 启动一个引用了延迟加载元素的动画后，该延迟加载元素就会被加载
            sb.Begin();
}
}
}
　　
4、x:DeferLoadStrategy 通过 Setter 加载
Xaml/DeferLoadStrategy/Demo4.xaml



<Page
x:Class="Windows10.Xaml.DeferLoadStrategy.Demo4"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml.DeferLoadStrategy"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="Transparent">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="WindowSizeStates">
<VisualState>
<VisualState.StateTriggers>
<AdaptiveTrigger MinWindowWidth="360" />
</VisualState.StateTriggers>
</VisualState>
<VisualState>
<VisualState.StateTriggers>
<AdaptiveTrigger MinWindowWidth="0" />
</VisualState.StateTriggers>
<VisualState.Setters>
<!--窗口宽度小于 360 时调用，此处 setter 引用了延迟加载元素，该延迟加载元素将会被加载-->
<Setter Target="textBox.Foreground" Value="Red" />
</VisualState.Setters>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<StackPanel Margin="10 0 10 10">
<!--延迟加载元素-->
<TextBox Name="textBox" Text="我是 TextBox" x:DeferLoadStrategy="Lazy" />
</StackPanel>
</Grid>
</Page>
　　Xaml/DeferLoadStrategy/Demo4.xaml.cs



/*
* 演示 x:DeferLoadStrategy 的相关知识点
*
* 本例演示通过“Setter”加载延迟加载元素
*/
using Windows.UI.Xaml.Controls;
namespace Windows10.Xaml.DeferLoadStrategy
{
public sealed partial class Demo4 : Page
{
public Demo4()
{
this.InitializeComponent();
}
}
}
　　
5、x:DeferLoadStrategy 通过 GetTemplateChild 加载
Xaml/DeferLoadStrategy/Demo5.xaml



<Page
x:Class="Windows10.Xaml.DeferLoadStrategy.Demo5"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml.DeferLoadStrategy"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Page.Resources>
<Style TargetType="local:TitledImage">
<Setter Property="HorizontalAlignment" Value="Left" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:TitledImage">
<Grid>
<Image Source="{TemplateBinding Source}" Width="200" Height="100" Stretch="Uniform" HorizontalAlignment="Center" VerticalAlignment="Center" />
<!--延迟加载元素，当调用 GetTemplateChild 后，它将被加载-->
<ContentPresenter x:Name="TitlePresenter" x:DeferLoadStrategy="Lazy" Content="{TemplateBinding Title}" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" Foreground="White" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Page.Resources>
<Grid Background="Transparent">
<StackPanel Margin="10 0 10 10">
<!--对应的 ControlTemplate 中的延迟加载元素 TitlePresenter 不会被加载-->
<local:TitledImage Source="/Assets/SplashScreen.png" />
<!--对应的 ControlTemplate 中的延迟加载元素 TitlePresenter 会被加载（因为调用了 GetTemplateChild）-->
<local:TitledImage Source="/Assets/SplashScreen.png" Title="image title" />
</StackPanel>
</Grid>
</Page>
　　Xaml/DeferLoadStrategy/Demo5.xaml.cs



/*
* 演示 x:DeferLoadStrategy 的相关知识点
*
* 本例演示通过“GetTemplateChild”加载延迟加载元素
*/
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;
namespace Windows10.Xaml.DeferLoadStrategy
{
public sealed partial class Demo5 : Page
{
public Demo5()
{
this.InitializeComponent();
}
}

// 自定义控件（一个可显示 Title 的 Image）
public class TitledImage : Control
{
// 此自定义控件用于显示 Title 的 ContentPresenter
private ContentPresenter _titlePresenter;
public TitledImage()
{
this.DefaultStyleKey = typeof(TitledImage);
// 注册一个回调，当 Title 发生变化时触发
this.RegisterPropertyChangedCallback(TitleProperty, TitleChanged);
}
// 定义一个依赖属性 Title（用于显示标题）
public static readonly DependencyProperty TitleProperty = DependencyProperty.Register(nameof(Title), typeof(string), typeof(TitledImage), new PropertyMetadata(null));
public string Title
{
get { return (string)GetValue(TitleProperty); }
set { SetValue(TitleProperty, value); }
}
// 定义一个依赖属性 Source（用于显示图片）
public static readonly DependencyProperty SourceProperty = DependencyProperty.Register(nameof(Source), typeof(ImageSource), typeof(TitledImage), new PropertyMetadata(null));
public ImageSource Source
{
get { return (BitmapSource)GetValue(SourceProperty); }
set { SetValue(SourceProperty, value); }
}
void TitleChanged(DependencyObject sender, DependencyProperty prop)
{
string title = (string)sender.GetValue(prop);
if (!string.IsNullOrEmpty(Title) && _titlePresenter == null)
{
// TitlePresenter 是控件模板中的一个延迟加载元素，通过 GetTemplateChild 调用后，它将被加载
_titlePresenter = (ContentPresenter)GetTemplateChild("TitlePresenter");
}
}
protected override void OnApplyTemplate()
{
base.OnApplyTemplate();
if (!string.IsNullOrEmpty(Title))
{
// TitlePresenter 是控件模板中的一个延迟加载元素，通过 GetTemplateChild 调用后，它将被加载
_titlePresenter = (ContentPresenter)GetTemplateChild("TitlePresenter");
}
}
}
}
　　
6、x:Null - null
Xaml/NullDemo.xaml



<Page
x:Class="Windows10.Xaml.NullDemo"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Windows10.Xaml"
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="10 0 10 10">
<TextBlock Name="lblMsg" TextWrapping="Wrap" Margin="5" />
<!--不指定 Tag 则其默认值为 null-->
<TextBlock Name="textBlock1" Margin="5" />
<!--指定 Tag 的值为 ""-->
<TextBlock Name="textBlock2" Margin="5" Tag="" />
<!--指定 Tag 的值为 null-->
<TextBlock Name="textBlock3" Margin="5" Tag="{x:Null}" />
</StackPanel>
</Grid>
</Page>
　　Xaml/NullDemo.xaml.cs



/*
* 演示 x:Null 的相关知识点、
*
* 在这里插一句：
* 在 xaml 使用的 {x:Null}, {Binding}, {x:Bind}, {StaticResource} 之类的这种带大括号的语法被称为标记扩展（Markup Extension），在 uwp 中无法开发自定义标记扩展（但是在 wpf 中是可以的）
*/
using System;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
namespace Windows10.Xaml
{
public sealed partial class NullDemo : Page
{
public NullDemo()
{
this.InitializeComponent();
this.Loaded += NullDemo_Loaded;
}
private void NullDemo_Loaded(object sender, RoutedEventArgs e)
{
lblMsg.Text = $"textBlock1.Tag: {textBlock1.Tag ?? "null"}"; // null
lblMsg.Text += Environment.NewLine;
lblMsg.Text += $"textBlock2.Tag: {textBlock2.Tag ?? "null"}"; // ""
lblMsg.Text += Environment.NewLine;
lblMsg.Text += $"textBlock3.Tag: {textBlock3.Tag ?? "null"}"; // null
        }
}
}
　　
OK
[源码下载]