XAML: x:DeferLoadStrategy, x:Null

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
        }
    }
}