Summary

A marker with orient="auto-start-reverse" crashes the app on Android, and draws at the wrong angle on Apple platforms.

Orient has listed 'auto-start-reverse' as a valid value since markers were added (src/elements/Marker.tsx), but neither native renderer implements it. Android passes every non-"auto" value to Double.parseDouble:

java.lang.NumberFormatException: For input string: "auto-start-reverse"
  at com.horcrux.svg.MarkerView.renderMarker(MarkerView.java:125)
  at com.horcrux.svg.SvgView.onDraw(SvgView.java:135)

That runs on the UI thread inside onDraw, where no JS error boundary can catch it, so a single such SVG takes the whole app down. Apple does not throw — [_orient doubleValue] answers 0 for a non-numeric string — so the marker is silently drawn at the wrong angle instead.

The angle maths for this value already exists in the library: CurrentAngle adds the half turn for a start marker when auto_start_reverse_ is set (RNSVGMarkerPosition.java, RNSVGMarkerPosition.mm). Nothing ever set that flag; on Android it was even marked // TODO. So this finishes the wiring rather than adding a parallel path:

• renderMarkers already resolves the start marker before computing positions, so it passes whether that marker is auto-start-reverse into fromPath / fromCGPath:, which sets the existing flag.
• MarkerView / RNSVGMarker treat the value like "auto", and an orient that is neither "auto" nor a number falls back to the property's initial value of 0 instead of throwing.

Test Plan

What's required for testing (prerequisites)?

Two SVGs that differ in exactly one attribute — orient="auto" versus orient="auto-start-reverse" — with the same marker on both ends, so the start marker is the only thing that may change:

<svg xmlns="http://www.w3.org/2000/svg" width="400" height="120" viewBox="0 0 400 120">
  <rect width="400" height="120" fill="#ffffff"/>
  <defs>
    <marker id="a" viewBox="0 0 10 8" refX="5" refY="4" markerWidth="14" markerHeight="11" orient="auto-start-reverse">
      <path d="M0 0 L10 4 L0 8 z" fill="#000000"/>
    </marker>
  </defs>
  <path d="M80 60 L320 60" stroke="#000000" stroke-width="6" marker-start="url(#a)" marker-end="url(#a)"/>
</svg>

What are the steps to reproduce (after prerequisites)?

Render both with SvgUri on Android.

Before this change, the auto-start-reverse one kills the process: the app disappears from the launcher and logcat carries the NumberFormatException above.

After, measured on an arm64 Android 30 emulator (react-native-svg built from this branch; react-native-svg:compileDebugJavaWithJavac ran clean):

• No NumberFormatException and no MarkerView.renderMarker frame in logcat; the app stays up.

• Comparing the two renders pixel by pixel, the end marker is byte-identical (difference 0) and only the start marker changes — which is what the spec asks for.

• The start marker is not merely different, it is reversed. Measuring the triangle's thickness per column, its base and apex swap sides between the two values, and a browser rendering the same two files — Chromium implements SVG 2 here — swaps them the same way:

  	left half	right half
  Android auto	6840	2340
  Android auto-start-reverse	2340	6840
  Chromium auto	3707	1243
  Chromium auto-start-reverse	1243	3707

I have not run the Apple change on a device or simulator. It mirrors the Android one and follows from doubleValue returning 0 for a non-numeric string, but I have not verified it visually — happy to do that before merging if you'd like.

Compatibility

Web already handled this — the markup reaches the browser's own SVG engine — so nothing changed there.

Checklist

• I have tested this on a device and a simulator
• I added documentation in README.md
• I updated the typed files (typescript)
• I added a test for the API in the __tests__ folder

_{Checklist left unticked deliberately: Android was verified on an emulator only, the TypeScript already accepts this value, and I did not add a test — let me know if you'd like an e2e/cases SVG for it.}