just_font_scan

Dart package to scan system font families and their supported weights using platform-native APIs.

• Windows: DirectWrite COM API (dwrite.dll) via dart:ffi
• macOS: CoreText framework via dart:ffi

Features

• Retrieves all system font families grouped by the platform's native family grouping (e.g. "Source Code Pro" is one family with weights 200--900, not separate entries per variant)
• Reports supported font weights (100--950 on Windows, 100--900 on macOS) per family
• Results cached after first scan; repeated clearCache() → scan() cycles are memory-safe
• No native build step -- pure dart:ffi with system libraries

Installation

Add to your pubspec.yaml:

dependencies:
  just_font_scan:
    git:
      url: https://github.com/kihyun1998/just_font_scan.git

API Reference

FontFamily class

Represents a single system font family.

Property	Type	Description
name	String	Font family name (e.g. 'Arial', 'Source Code Pro').
weights	List<int>	Supported font weights in ascending order. Values follow the CSS/OpenType convention (see weight table below).

FontFamily supports equality comparison (==) and can be used as a map key.

JustFontScan class

All methods are static. No instantiation needed.

JustFontScan.scan()

static List<FontFamily> scan()

Scans all system font families. Returns a list sorted alphabetically by family name.

• Returns: List<FontFamily> -- all font families found on the system.
• Caching: Results are cached after the first call. Subsequent calls return the cached list instantly.
• Error handling: Returns an empty list [] if the platform is unsupported or if a native API error occurs. Never throws.
• Thread safety: The cache is isolate-local. Calling scan() from different isolates triggers separate scans.

JustFontScan.clearCache()

static void clearCache()

Clears the cached scan result. The next scan() call will rescan the system. Use this if fonts have been installed or removed since the last scan.

JustFontScan.weightsFor()

static List<int> weightsFor(String familyName)

Returns the supported weights for a specific font family.

• Parameter familyName (String): The font family name to look up. Case-insensitive (e.g. 'arial' matches 'Arial').
• Returns: List<int> -- weights in ascending order.
• Not found: Returns [400] as a default when the family does not exist in the system. To distinguish "family exists with only weight 400" from "family not found", use scan() directly and search the result.

Font weight values

Standard DWRITE_FONT_WEIGHT / CSS font-weight values:

Value	Name
100	Thin
200	ExtraLight
300	Light
350	SemiLight
400	Regular
500	Medium
600	SemiBold
700	Bold
800	ExtraBold
900	Black
950	ExtraBlack

Not all fonts support every weight. A font may have any subset of these values.

macOS weight caveat

macOS CoreText reports font weight as a normalized float in the range −1.0 to 1.0. just_font_scan snaps each value to the nearest bucket in Apple's NSFontWeight table and reports the corresponding CSS weight. This is an approximation — a font whose native weight is e.g. 0.36 (between Semibold 0.30 and Bold 0.40) will be reported as 700 because it is closer to Bold. On Windows, DWRITE_FONT_WEIGHT values map 1:1, so Windows results are exact.

The CSS weight 950 (ExtraBlack) is never produced on macOS because no public NSFontWeight constant corresponds to it.

Usage

Basic scan

import 'package:just_font_scan/just_font_scan.dart';

final families = JustFontScan.scan();
// families is List<FontFamily>, sorted by name.

for (final family in families) {
  print('${family.name}: ${family.weights}');
}
// Arial: [400, 700, 900]
// Calibri: [300, 400, 700]
// Source Code Pro: [200, 300, 400, 500, 600, 700, 800, 900]
// ...

Query a specific family

final weights = JustFontScan.weightsFor('Source Code Pro');
print(weights); // [200, 300, 400, 500, 600, 700, 800, 900]

final missing = JustFontScan.weightsFor('NonExistentFont');
print(missing); // [400]  (default fallback)

Check if a family supports a specific weight

final weights = JustFontScan.weightsFor('Arial');
if (weights.contains(700)) {
  print('Arial Bold is available');
}

Check if a family exists

final families = JustFontScan.scan();
final exists = families.any(
  (f) => f.name.toLowerCase() == 'arial',
);

Rescan after font installation

JustFontScan.clearCache();
final updated = JustFontScan.scan();

Platform support

Platform	Status	API
Windows	Supported	DirectWrite (IDWriteFactory)
macOS	Supported	CoreText (CTFontCollection)
Linux	Not yet	--

On unsupported platforms, scan() returns an empty list and weightsFor() always returns [400].

Requirements

• Dart SDK >=3.9.2
• Windows 7+ (DirectWrite is preinstalled) or macOS 10.13+ (CoreText is preinstalled)