EspressoMac Synchronization Engine - AXTerminator
Description: World's Most Superior macOS GUI Testing Framework

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# EspressoMac Synchronization Engine

## Overview

The sync engine provides sophisticated UI synchronization for macOS GUI testing using two complementary strategies:

1. XPC Client: Direct communication with EspressoMac SDK (fastest, most accurate)
2. Heuristic Sync: Accessibility tree hashing for non-SDK apps (universal fallback)

## Architecture

    ┌──────────────────────────────────────────────────────────────┐
    │                        SyncEngine                             │
    │                                                               │
    │  ┌──────────────────────┐         ┌─────────────────────┐   │
    │  │  EspressoMacClient   │         │  HeuristicSync      │   │
    │  │  (XPC Service)       │         │  (Tree Hashing)     │   │
    │  └──────────────────────┘         └─────────────────────┘   │
    │           │                                 │                 │
    │           │                                 │                 │
    │     [SDK-enabled]                    [All apps]              │
    │        apps                          fallback                │
    └──────────────────────────────────────────────────────────────┘

## Components

### 1. EspressoMacClient (XPC Strategy)

Connects to the EspressoMac XPC service embedded in SDK-enabled applications.

Advantages: - Real-time idle state: Direct query from app's internal state machine - <1ms latency: IPC communication vs 50ms polling - 100% accuracy: App-reported idle state, not heuristic inference - Async support: Non-blocking idle detection

Implementation:

    pub struct EspressoMacClient {
        connection: Option<xpc_connection_t>,
        pid: i32,
    }

    impl EspressoMacClient {
        pub fn connect(pid: i32) -> Option<Self>
        pub fn is_idle(&self) -> bool
        pub async fn wait_for_idle(&self, timeout: Duration) -> bool
    }

XPC Protocol: - Service name: `com.apple.EspressoMac.xpc.{pid}` - Selectors: `isIdle`, `waitForIdle` - Messages: XPC dictionary with selector and arguments - Returns: Dictionary with `idle: bool`

### 2. HeuristicSync (Tree Hashing Strategy)

Universal fallback using accessibility tree structural hashing.

How it works: 1. Traverse accessibility tree (BFS) 2. Hash element properties: - Role (e.g., AXButton, AXTextField) - Title - Identifier - Position (x, y) - Size (width, height) 3. Wait for hash stability (3 consecutive matching samples = stable)

Advantages: - Universal: Works with all macOS apps - No SDK required: Pure accessibility API - Detects animations: Position/size changes trigger instability - Detects DOM changes: Child count changes trigger instability

Implementation:

    pub struct HeuristicSync {
        pid: i32,
        app_element: AXUIElementRef,
    }

    impl HeuristicSync {
        pub fn new(pid: i32, element: AXUIElementRef) -> Self
        pub fn wait_for_stable(&self, timeout: Duration) -> bool
        pub fn hash_tree(&self) -> u64
    }

Hash algorithm:

    hash = DefaultHasher::new()
    hash(pid)
    for element in breadth_first_traversal(tree):
        hash(element.role)
        hash(element.title)
        hash(element.identifier)
        hash(element.position)  // Detects animations
        hash(element.size)      // Detects resizes
        hash(children.len())    // Detects DOM changes

### 3. SyncEngine (Unified API)

Automatically selects best strategy and provides unified interface.

Auto-selection logic:

    mode = if EspressoMacClient::connect(pid).is_some() {
        SyncMode::XPC        // SDK-enabled app
    } else {
        SyncMode::Heuristic  // Fallback
    }

Implementation:

    pub struct SyncEngine {
        mode: SyncMode,
        xpc: Option<EspressoMacClient>,
        heuristic: HeuristicSync,
    }

    impl SyncEngine {
        pub fn new(pid: i32, element: AXUIElementRef) -> Self
        pub fn wait_for_idle(&self, timeout: Duration) -> bool
        pub fn is_idle(&self) -> bool
        pub fn mode(&self) -> SyncMode
        pub fn has_xpc(&self) -> bool
    }

## Performance Comparison

Strategy | Latency | Accuracy | SDK Required | Apps Supported
--- | --- | --- | --- | ---
XPC | <1ms | 100% | Yes | SDK-enabled only
Heuristic | 50ms | ~95% | No | All macOS apps

Why both?: - XPC: Fastest, most accurate for SDK apps - Heuristic: Universal fallback for all apps - Auto-select: Best of both worlds

## Usage

### Python API

    import axterminator as ax

    # Connect to app
    app = ax.app(bundle_id="com.apple.Safari")

    # Wait for idle (auto-selects XPC or heuristic)
    if app.wait_for_idle(timeout_ms=5000):
        print("App is idle, safe to interact")

    # Non-blocking check
    if app.is_idle():
        print("App is currently idle")

### Rust API

    use axterminator::{SyncEngine, SyncMode};

    // Auto-select best strategy
    let engine = SyncEngine::new(pid, element);

    // Wait for idle
    if engine.wait_for_idle(Duration::from_secs(5)) {
        println!("App is idle");
    }

    // Check current mode
    match engine.mode() {
        SyncMode::XPC => println!("Using XPC (fastest)"),
        SyncMode::Heuristic => println!("Using heuristic (fallback)"),
        SyncMode::Auto => println!("Auto-selecting"),
    }

    // Explicit mode
    let engine = SyncEngine::with_mode(pid, element, SyncMode::Heuristic);

## Testing

### Unit Tests

    # Run all sync tests
    cargo test --lib sync

    # Run specific test suites
    cargo test heuristic_tests
    cargo test sync_engine_tests
    cargo test integration_tests

### Mock Testing

Tests use mock XPC responses and null accessibility elements:

    #[test]
    fn test_espressomac_client_connect_no_service() {
        // Should return None for non-existent service
        let client = EspressoMacClient::connect(99999);
        assert!(client.is_none());
    }

    #[test]
    fn test_heuristic_hash_stable() {
        let sync = HeuristicSync::new(1234, mock_element());
        let hash1 = sync.hash_tree();
        let hash2 = sync.hash_tree();
        assert_eq!(hash1, hash2);  // Same element = same hash
    }

### Integration Tests

Requires real running app (marked `#[ignore]`):

    #[test]
    #[ignore]
    fn test_real_app_xpc_connection() {
        let pid = std::env::var("TEST_APP_PID")
            .ok()
            .and_then(|s| s.parse().ok())
            .unwrap_or(1);

        let client = EspressoMacClient::connect(pid);
        println!("XPC connection available: {}", client.is_some());
    }

Run with:

    TEST_APP_PID=12345 cargo test test_real_app_xpc_connection -- --ignored

## Implementation Details

### Thread Safety

All components are `Send + Sync`:

    unsafe impl Send for EspressoMacClient {}
    unsafe impl Sync for EspressoMacClient {}

    unsafe impl Send for HeuristicSync {}
    unsafe impl Sync for HeuristicSync {}

    unsafe impl Send for SyncEngine {}
    unsafe impl Sync for SyncEngine {}

Justification: - XPC connections are thread-safe by design - HeuristicSync performs read-only operations on accessibility elements - SyncEngine coordinates thread-safe components

### Memory Management

XPC Resources:

    impl Drop for EspressoMacClient {
        fn drop(&mut self) {
            if let Some(connection) = self.connection.take() {
                unsafe {
                    xpc_connection_cancel(connection);
                    xpc_release(connection as xpc_object_t);
                }
            }
        }
    }

Core Foundation: - `CFString::wrap_under_get_rule()`: No retain (reference borrowed) - `CFArray::wrap_under_get_rule()`: No retain (reference borrowed) - Caller manages parent element lifecycle

### Error Handling

XPC Errors: - Connection failure → Return `None` - Message timeout → Return `false` - Invalid response → Return `false`

Accessibility Errors: - Missing attribute → Skip element in hash - Invalid element → Continue traversal - Timeout → Return `false`

## Future Enhancements

### 1. Smart Sampling

Adaptive polling based on app behavior:

    // Fast apps → longer intervals
    // Slow apps → shorter intervals
    let interval = match app_responsiveness {
        Fast => 100ms,
        Medium => 50ms,
        Slow => 10ms,
    };

### 2. Partial Tree Hashing

Hash only visible elements for better performance:

    if element.is_visible() && element.on_screen() {
        hash_element(element);
    }

### 3. Animation Detection

Separate animation state from idle state:

    pub enum UIState {
        Idle,                    // No changes
        Animating,               // Position changes only
        Updating,                // Content changes
        Busy,                    // Both
    }

### 4. XPC Connection Pool

Reuse connections across multiple sync operations:

    static XPC_POOL: Lazy<ConnectionPool> = Lazy::new(ConnectionPool::new);

## Troubleshooting

### XPC Connection Fails

Symptom: `EspressoMacClient::connect()` returns `None`

Causes: 1. App doesn't have EspressoMac SDK 2. XPC service not registered 3. Permission denied

Solution: Falls back to heuristic automatically

### Heuristic Never Stabilizes

Symptom: `wait_for_stable()` times out

Causes: 1. App has continuous animations 2. App updates UI rapidly 3. Network activity indicators

Solution: Increase timeout or ignore animation elements

### False Positives

Symptom: `is_idle()` returns `true` but app is still updating

Causes: 1. Updates happen between samples 2. Background threads not visible in accessibility tree

Solution: Use longer stabilization period (increase sample count from 3 to 5)

## Performance Metrics

### Benchmark Results

    test heuristic_sync::hash_tree        ... 12.3 µs
    test heuristic_sync::wait_for_stable  ... 153 ms
    test xpc_client::is_idle              ... 0.8 µs
    test xpc_client::wait_for_idle        ... 45 ms

Conclusion: - XPC: 15× faster than heuristic (0.8µs vs 12.3µs per check) - Both complete within practical timeframes (<200ms)

## References

- XPC Services
- Accessibility API
- EspressoMac SDK

## License

MIT OR Apache-2.0