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Backend Discovery and Overrides

The branch split the text-measurement layer out of Pretext itself. The engine is now backend-agnostic and resolves measurement through Pretext.Contracts.

Default behavior

If you do not set a factory explicitly, Pretext:

  1. scans loaded and probeable Pretext*.dll assemblies
  2. finds assembly-level PretextTextMeasurerFactoryAttribute declarations
  3. instantiates the advertised factories
  4. keeps only factories whose IsSupported is true
  5. selects the supported factory with the highest Priority

First-party priorities

Backend OS support Priority
Pretext.DirectWrite Windows 100
Pretext.FreeType Linux 100
Pretext.CoreText macOS 100
Pretext.SkiaSharp Any OS 0

That means a host-native backend wins on its matching OS, while Pretext.SkiaSharp stays available as the portable fallback.

Explicit override

Use PretextLayout.SetTextMeasurerFactory(...) when:

  • you want deterministic startup behavior regardless of discovery
  • tests should force a specific backend
  • your app ships multiple supported custom backends
  • you want to opt into a non-default backend
using Pretext;
using Pretext.CoreText;

PretextLayout.SetTextMeasurerFactory(new CoreTextTextMeasurerFactory());

Calling SetTextMeasurerFactory(...) also clears the cached font state so the new backend takes effect immediately.

Failure modes

If no supported backend is found, Pretext throws and tells you to either:

  • reference a backend package such as Pretext.SkiaSharp, or
  • call PretextLayout.SetTextMeasurerFactory(...)

If multiple supported backends exist at the same highest priority, Pretext throws and requires an explicit selection.

Host type Recommended packages
Windows-native host Pretext + Pretext.DirectWrite
Linux-native host Pretext + Pretext.FreeType
macOS-native host Pretext + Pretext.CoreText
Generic SkiaSharp host Pretext + Pretext.SkiaSharp
Uno app Pretext.Uno