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 = De         


        

Answer 1

Using the info in the answer provided @SushiHangover, I was able to implement the following renderer strategy for iOS:

Create a renderer for each control that you want to support dynamic text on. In my case, this included Labels, Buttons, and Entrys. The renderers assign the Control.Font to a UIFontDescriptor with the PointSize percentage adjusted based on the NamedSize assigned to the Xamarin Forms control.

Example:

[assembly: ExportRenderer(typeof(Button), typeof(CustomButtonRenderer))]
namespace iOS.Controls
{
    public class CustomButtonRenderer : ButtonRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs e)
        {
            base.OnElementChanged(e);

            var view = e.NewElement as Button;
            if(Control != null && view != null)
            {
                var descriptor = UIFontDescriptor.PreferredBody;
                var pointSize = descriptor.PointSize;

                var size = view.FontSize;
                if(size == Device.GetNamedSize(NamedSize.Large, typeof(Button)))
                {
                    pointSize *= 1.4f;
                }
                else if(size == Device.GetNamedSize(NamedSize.Small, typeof(Button)))
                {
                    pointSize *= .8f;
                }
                else if(size == Device.GetNamedSize(NamedSize.Micro, typeof(Button)))
                {
                    pointSize *= .6f;
                }

                Control.Font = UIFont.FromDescriptor(descriptor, pointSize);
            }
        }
    }
}

You could even dynamically support fixed font sizes this way by taking their percentage value based on the base NamedSize.Default size.

Example:

[assembly: ExportRenderer(typeof(Button), typeof(CustomButtonRenderer))]
namespace iOS.Controls
{
    public class CustomButtonRenderer : ButtonRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs e)
        {
            base.OnElementChanged(e);

            var view = e.NewElement as Button;
            if(Control != null && view != null)
            {
                var descriptor = UIFontDescriptor.PreferredBody;
                var percent = view.FontSize / Device.GetNamedSize(NamedSize.Default, typeof(Button));

                Control.Font = UIFont.FromDescriptor(descriptor, percent * descriptor.PointSize);
            }
        }
    }
}

For reference, full implementation examples can be found in this GitHub project.