XAML语法之CententPage框架及扩展标记

编写一个ContentPage：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"  //访问Xamarin.forms类
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" //引用Xaml的标签和特性
             x:Class="XamlSamples.XamlPlusCodePage"
             Title="XAML + Code Page">
    <ContentPage.Content>
        <StackLayout>
            <StackLayout.Children>
                <Slider VerticalOptions="CenterAndExpand"
                        ValueChanged="OnSliderValueChanged" />

                <Label x:Name="valueLabel"
                       Text="A simple Label"
                       FontSize="Large"
                       HorizontalOptions="Center"
                       VerticalOptions="CenterAndExpand" />

                <Button Text="Click Me!"
                      HorizontalOptions="Center"
                      VerticalOptions="CenterAndExpand"
                      Clicked="OnButtonClicked" />
            </StackLayout.Children>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

分为三级：<ContentPage<ContentPage.Content<Children>>>

<ContentPage中的内容自动为Content，四大布局（继承于Layout<View>）中的内容自动为Children

避免平台差异的方法（如iOS设备状态栏被覆盖，需要padding）：

方法一：C#中判断
if (Device.RuntimePlatform == Device.iOS)
{
    Padding = new Thickness(0, 20, 0, 0);
}
方法二：XAML中添加内容
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="...">
    <ContentPage.Padding>
        <OnPlatform x:TypeArguments="Thickness">//Thickness为Padding的属性
            <On Platform="iOS" Value="0, 20, 0, 0" />//value为Thickness的值
            <On Platform="Android, UWP" Value="0, 0, 0, 0" />
        </OnPlatform>
    </ContentPage.Padding>
  ...
</ContentPage>

XAML扩展标记语言（implement IMarkupExtension）（通常用花括号表示）：

1.合并重复的属性代码，例如：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="XamlSamples.SharedResourcesPage"
             Title="Shared Resources Page">

    <ContentPage.Resources>
        <ResourceDictionary> //资源字典
            <LayoutOptions x:Key="horzOptions"    //key为唯一键值
                           Alignment="Center" />

            <LayoutOptions x:Key="vertOptions"
                           Alignment="Center"
                           Expands="True" />

            <x:Double x:Key="borderWidth">3</x:Double>

            <x:Double x:Key="rotationAngle">-15</x:Double>

            <OnPlatform x:Key="textColor"     //平台差异化
                        x:TypeArguments="Color">
                <On Platform="iOS" Value="Red" />
                <On Platform="Android" Value="Aqua" />
                <On Platform="UWP" Value="#80FF80" />
            </OnPlatform>

            <x:String x:Key="fontSize">Large</x:String>
        </ResourceDictionary>
    </ContentPage.Resources>

    <StackLayout>
        <Button Text="Do this!"
                HorizontalOptions="{StaticResource horzOptions}"//静态引用
                VerticalOptions="{StaticResource vertOptions}"
                BorderWidth="{StaticResource borderWidth}"
                Rotation="{StaticResource rotationAngle}"
                TextColor="{StaticResource textColor}"
                FontSize="{StaticResource fontSize}" />

        <Button Text="Do that!"
                HorizontalOptions="{StaticResource horzOptions}"
                VerticalOptions="{StaticResource vertOptions}"
                BorderWidth="{StaticResource borderWidth}"
                Rotation="{StaticResource rotationAngle}"
                TextColor="{StaticResource textColor}"
                FontSize="{StaticResource fontSize}" />

        <Button Text="Do the other thing!"
                HorizontalOptions="{StaticResource horzOptions}"
                VerticalOptions="{StaticResource vertOptions}"
                BorderWidth="{StaticResource borderWidth}"
                Rotation="{StaticResource rotationAngle}"
                TextColor="{StaticResource textColor}"
                FontSize="{StaticResource fontSize}" />

    </StackLayout>
</ContentPage>

staticresource返回一个实力，而x:static可以返回本地类或者系统类的公共静态属性、字段等（需要声明xmlns:local）

其他扩展标记：{x:null}给予属性空值；{x: Type  someClass}定义扩展类型；相对布局中的限制性扩展标记