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r/rust Post

Title: AXTerminator: Rust-powered macOS GUI testing with PyO3 bindings (379 us element access)

Body:

I built a macOS GUI automation library in Rust with Python bindings via PyO3. The key feature: background testing without focus stealing.

Architecture

Python API -> PyO3 FFI -> Rust Core -> AXUIElement API
                |
            ~379 us per element (Criterion benchmark, M1 MacBook Pro)

Direct Accessibility API access eliminates the HTTP/WebDriver overhead that makes Appium ~500 ms per operation.

Why Rust?

  • Performance: 379 us element access via direct FFI to Core Foundation
  • Safety: macOS Accessibility API requires careful CFTypeRef memory management (get-rule vs create-rule)
  • PyO3: Seamless Python bindings with abi3-py39 for a single wheel across Python 3.9-3.14+

Self-Healing Locators

The core implements 7 fallback strategies in Rust: 1. data_testid (AXIdentifier exact match) 2. aria_label (AXLabel/AXDescription) 3. identifier (AXIdentifier direct) 4. title (fuzzy matching via Levenshtein distance, 80% threshold) 5. xpath (structural path parser with predicate support) 6. position (spatial heuristic, 50px threshold) 7. visual_vlm (delegates to Python VLM backends)

All pure Rust except strategy 7, which calls back into Python.

Background Testing

macOS AXUIElementPerformAction works on unfocused windows. This is undocumented by Apple but verified on macOS 12-15. We use kAXPressAction and friends to click elements without bringing the target window to the foreground.

// Simplified
unsafe {
    AXUIElementPerformAction(element, kAXPressAction as CFStringRef);
}

Crate Dependencies

  • core-foundation / core-foundation-sys / core-graphics for macOS APIs
  • pyo3 for Python bindings
  • ahash for fast tree hashing (sync engine)
  • lru for element caching
  • criterion for benchmarks

GitHub: https://github.com/MikkoParkkola/axterminator Docs: https://mikkoparkkola.github.io/axterminator/

Feedback welcome, especially on the PyO3 patterns and Core Foundation memory management!