Add a text measurement API

[cedrickcooke]

Summary

Krayon could draw text (Kanvas.drawText) but couldn't measure it, so layout code had no way to reserve space for labels ahead of time. This adds a primitive, single-line text-measurement API. Line wrapping is an explicit non-goal.

The design mirrors the existing IsPointInPath optional-capability paradigm:

• kanvas common:
  • TextMetrics(width, ascent, descent) with a height convenience. ascent/descent are baseline-relative (positive-up) font metrics; width is the string's advance.
  • fun interface MeasureText { fun measureText(text: CharSequence, paint: Paint.Text): TextMetrics }.
• Concrete backends implement MeasureText: AndroidKanvas, HtmlKanvas, CGContextKanvas, and ComposeKanvas. SvgKanvas deliberately does not (it has no font engine — same reason it doesn't implement IsPointInPath). The test CallRecordingKanvas is untouched since the capability is a separate interface.
• Standalone measurers for layout-without-a-Kanvas (chart code runs layout in UpdateElement.update(...) with no Kanvas, exactly like interaction uses standalone IsPointInPaths):
  • AndroidTextMeasurement(context), object CoreTextMeasurement (Core Text), HtmlTextMeasurement() (offscreen canvas), and Compose's public textMeasurement() factory.
  • The platform Kanvas classes delegate to these to avoid duplicated font resolution.

Each implementation reuses the exact font resolution already used for drawing, so measurements match what gets rendered.

Usage

val paint = Paint.Text(black, size = 14f, Paint.Text.Alignment.Left, Font(sansSerif))

// From a measuring Kanvas:
val metrics = (kanvas as MeasureText).measureText("Hello", paint)

// Or, during chart layout, from a standalone measurer (no Kanvas needed):
val measurer: MeasureText = AndroidTextMeasurement(context) // or HtmlTextMeasurement() / CoreTextMeasurement / compose textMeasurement()
val width = measurer.measureText("Hello", paint).width

Testing

Per-platform tests cover every measurement implementation:

Platform	Test	Where it runs
kanvas common	MeasureTextTests	every target
kanvas Android	AndroidTextMeasurementTests (Robolectric, native graphics)	JVM/CI
kanvas JS	HtmlTextMeasurementTests	ChromeHeadless (jsBrowserTest)
kanvas Apple	CoreTextMeasurementTests (appleTest)	macOS CI
compose skia	SkiaMeasureTextTests (via textMeasurement())	JVM/CI
compose Android	AndroidMeasureTextTests (Robolectric, native graphics)	JVM/CI

Each asserts real behavior: empty string → 0 width, non-empty → positive width/ascent/descent, longer strings measure wider, and vertical metrics are independent of the text.

Also verified: compiles on all targets (jvm, android, js, wasmJs, Apple macos/ios); ./gradlew apiCheck (full project) and lintKotlin pass; API dumps regenerated for kanvas and compose.

Notes / out of scope

• No line wrapping (explicit non-goal).
• ContinuousAxis is intentionally left behavior-stable; this API makes label auto-sizing possible as a follow-up.

  

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^{Reviewed by Cursor Bugbot for commit 0a03fc9. Configure here.}

[cursor]

HTML vertical metrics fallback wrong

Medium Severity

When fontBoundingBoxAscent / fontBoundingBoxDescent are unavailable, HtmlTextMeasurement falls back to actualBoundingBoxAscent / actualBoundingBoxDescent or zero. Those values are string-specific, not font metrics, so ascent/descent can vary by label text or become zero while width stays positive—contradicting TextMetrics and breaking layout that reserves height from measurement.

 

^{Reviewed by Cursor Bugbot for commit 0a03fc9. Configure here.}

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Test Results

85 tests  +6   85 ✅ +6   2s ⏱️ ±0s
17 suites +2    0 💤 ±0 
17 files   +2    0 ❌ ±0 

Results for commit 0a03fc9. ± Comparison against base commit 1589cbc.