xamarin.forms - How Do I Support Accessibility Font Sizes in Xamarin Forms?

Question

For example, I have a label on my page:

var label = new Label
{
    Text = "Some text here.",
    LineBreakMode = LineBreakMode.WordWrap,
    FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label))
};

How do I make this label's font size increase (or decrease) depending on the user's accessibility settings for font sizes? For example, in iOS you can set the Font Size for your device under Settings > General > Accessibility > Larger Text. I believe that Apple calls this "Dynamic Text" and is almost a requirement for your app to support.

The same applies for other controls in my app (buttons, entrys, etc).

I have tried this setting on my iPhone and it does not appear to be changing all things in my app. There are a few things like TableView section headers and list view cells that are changing, but things like my standard Labels and Entrys are not.

Answer 1

You would need to supply the UIFont returned from preferredFontWithTextStyle (C# = UIFont.PreferredFontForTextStyle) as your usage context of a label, button, entry, etc... would not be known to Xamarin.Forms.

So what I did for one client was subclass the base renderers and view elements and add an iOS-only property to those elements so they could define the context of how that control is begin used in the UI and thus when rendered by iOS these controls will be subject to Dynamic Text sizing.

There are six Dynamic font types defined in iOS 9:

• UICTFontTextStyleBody
• UICTFontTextStyleCaption1
• UICTFontTextStyleCaption2
• UICTFontTextStyleFootnote
• UICTFontTextStyleHeadline
• UICTFontTextStyleSubhead

Note: Xamarin.iOS does not have constants/enum defined for these like Swift does (ObjC does not define these either), so they are passed as a NSString, see example below.

Example Renderer:

Sets UICTFontTextStyleBody for a label subclass called BodyLabel:

[assembly: ExportRenderer(typeof(BodyLabel), typeof(iOSLabelBodyRenderer))]
namespace Foobar.iOS
{
    public class iOSLabelBodyRenderer : LabelRenderer
    {
        public iOSLabelBodyRenderer() { }

        protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
        {
            base.OnElementChanged(e);
            if (Control != null)
                Control.Font = UIFont.GetPreferredFontForTextStyle(new NSString("UICTFontTextStyleBody"));
        }
    }
}

Results in:

Note: Technically you should also implement UIContentSizeCategoryDidChangeNotification notifications so you resize/invalidate your controls when the user changes the dynamic font size.