Xamarin.Forms-标签FontSize OnPlatform-XAML错误 [英] Xamarin.Forms - Label FontSize OnPlatform - XAML error

问题描述

我有此代码:

  <Label x:Name="questionGroupHintInfoLabel" FontAttributes="Bold"  Text="Folgende Hinweismeldung wurde für die aktuelle Fragengruppe hinterlegt:">
      <Label.FontSize>
        <OnPlatform x:TypeArguments="NamedSize"
                    iOS="Small"
                    Android="Small" />
      </Label.FontSize>
    </Label>

...并收到此错误:

...and get this error:

No property, bindable property, or event found for FontSize

我做错了什么?

谢谢.

推荐答案

通常，当我们设置FontSize="value"，

Usually when we set FontSize="value", FontSizeConverter does the conversion to expected type (which is double) to set the value.

但是当我们使用OnPlatform时，似乎未使用此转换器.因此，我们有两个选择:

But it looks like this converter is not used when we use OnPlatform. So we have two options:

1. 使用OnPlatform和x:Double作为类型参数.

<OnPlatform x:TypeArguments="x:Double"
    iOS="20"
    Android="25" />

或者，欺骗XAML处理器为我们进行转换-我们可以使用StaticResource标记扩展名来进行转换. 注意: 这仅在未应用XAMLC的情况下有效.

Or, trick the XAML processor to do the conversion for us - we can do that by using the StaticResource markup extension. Note: This only works if XAMLC is not applied.

<!-- App.Resources or ContentPage.Resources -->
<ResourceDictionary>
    <OnPlatform x:Key="FontNamedSize" x:TypeArguments="x:String"
        iOS="Small"
        Android="Large" />
</ResourceDictionary>

<!-- now you can use static-resource extension to use above defined value -->
<Label x:Name="questionGroupHintInfoLabel" 
       FontAttributes="Bold"  
       Text="Folgende Hinweismeldung wurde für die aktuelle Fragengruppe hinterlegt:" 
       FontSize="{StaticResource FontNamedSize}" />

推荐使用NamedSize可绑定属性扩展Label并转换为FontSize(基本上是FontSizeConverter的作用).

Recommended Extend Label with NamedSize bindable property and convert to FontSize (basically what the FontSizeConverter does).

public class ExLabel : Label
{
    public static readonly BindableProperty FontNamedSizeProperty =
        BindableProperty.Create(
            "FontNamedSize", typeof(NamedSize), typeof(ExLabel),
            defaultValue: default(NamedSize), propertyChanged: OnFontNamedSizeChanged);

    public NamedSize FontNamedSize
    {
        get { return (NamedSize)GetValue(FontNamedSizeProperty); }
        set { SetValue(FontNamedSizeProperty, value); }
    }

    private static void OnFontNamedSizeChanged(BindableObject bindable, object oldValue, object newValue)
    {
        ((ExLabel)bindable).OnFontNamedSizeChangedImpl((NamedSize)oldValue, (NamedSize)newValue);
    }

    protected virtual void OnFontNamedSizeChangedImpl(NamedSize oldValue, NamedSize newValue)
    {
        FontSize = Device.GetNamedSize(FontNamedSize, typeof(Label));
    }
}

<!-- Usage -->
<local:ExLabel HorizontalOptions="Center" VerticalOptions="Center" Text="This is a custom label">
    <local:ExLabel.FontNamedSize>
        <OnPlatform x:TypeArguments="NamedSize"
            iOS="Large"
            Android="Medium" />
    </local:ExLabel.FontNamedSize>
</local:ExLabel>

选项2仅在未应用XAMLC的情况下有效.

EDIT 1: Option 2 only works if XAMLC is not applied.

添加选项3.

EDIT 2: Add option 3.

注意:之前的版本中提供了错误修复程序发布版本，也可以将其视为替代修补程序.但是我无法确认它是否在最新版本中已修复.

Note: There is a bug fix available in pre-release versions that can also be considered as an alternative fix. But I haven't been able to confirm if it is fixed in the latest release.

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