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Implementing a Custom Backend

Pretext.Contracts makes the measurement layer pluggable. A custom backend only needs to measure text widths for a parsed font descriptor and advertise a factory to the core engine.

When a custom backend makes sense

  • you want to bind Pretext to another renderer or platform text API
  • you need application-specific measurement behavior in tests
  • you want to ship a backend package independently from the main repository

Minimal shape

Implement:

  • IPretextTextMeasurer
  • IPretextTextMeasurerFactory

Advertise the factory at assembly level:

[assembly: PretextTextMeasurerFactory(typeof(MyTextMeasurerFactory))]

Typical implementation flow

  1. Parse the incoming font string with PretextFontParser.Parse(font).
  2. Map PretextFontDescriptor to the host font API.
  3. Return a measurer that can report width for arbitrary segment text.
  4. Set IsSupported according to the current runtime.
  5. Choose a Priority that makes sense beside the first-party backends.

Example skeleton

using Pretext;

public sealed class MyTextMeasurerFactory : IPretextTextMeasurerFactory
{
    public string Name => "MyBackend";

    public bool IsSupported => true;

    public int Priority => 50;

    public IPretextTextMeasurer Create(string font)
    {
        var descriptor = PretextFontParser.Parse(font);
        return new MyTextMeasurer(descriptor);
    }
}

public sealed class MyTextMeasurer : IPretextTextMeasurer
{
    public MyTextMeasurer(PretextFontDescriptor descriptor)
    {
        Descriptor = descriptor;
    }

    public PretextFontDescriptor Descriptor { get; }

    public double MeasureText(string text)
    {
        // Call into the host text API here.
        throw new NotImplementedException();
    }

    public void Dispose()
    {
    }
}

Priority guidance

  • use a priority above 0 if your backend should beat the SkiaSharp fallback
  • use the same top priority as another supported backend only if your app will always configure the backend explicitly
  • keep test-only backends explicit rather than relying on discovery