focus-fixer/FocusFixer/WindowLocator.swift

import CoreGraphics
import os

/// A single on-screen window as reported by the window server, reduced to
/// the fields FocusFixer needs. Kept separate from the raw CGWindowList
/// dictionary so the resolution logic below can be unit tested with
/// synthetic data instead of the real window server.
struct WindowInfo: Equatable {
    let ownerPID: pid_t
    let layer: Int
    let bounds: CGRect
}

enum WindowLocator {
    private static let logger = Logger(subsystem: "com.local.focusfixer", category: "WindowLocator")

    /// Resolves the owning process ID of the frontmost on-screen window
    /// under `point`, using the live window server state.
    static func ownerPID(at point: CGPoint) -> pid_t? {
        guard let list = CGWindowListCopyWindowInfo(.optionOnScreenOnly, kCGNullWindowID) as? [[String: Any]] else {
            logger.error("CGWindowListCopyWindowInfo returned nil")
            return nil
        }
        return ownerPID(at: point, in: list.compactMap(parse))
    }

    /// Pure resolution logic: `windows` is assumed to be in front-to-back
    /// z-order, matching CGWindowListCopyWindowInfo's contract. Only
    /// layer-0 windows (ordinary app windows, excluding menu bar, Dock,
    /// Spotlight, and Control Center) are considered.
    static func ownerPID(at point: CGPoint, in windows: [WindowInfo]) -> pid_t? {
        for window in windows where window.layer == 0 && window.bounds.contains(point) {
            return window.ownerPID
        }
        return nil
    }

    private static func parse(_ dict: [String: Any]) -> WindowInfo? {
        guard
            let pid = dict[kCGWindowOwnerPID as String] as? pid_t,
            let layer = dict[kCGWindowLayer as String] as? Int,
            let boundsDict = dict[kCGWindowBounds as String] as? [String: Any],
            let bounds = CGRect(dictionaryRepresentation: boundsDict as CFDictionary)
        else {
            return nil
        }
        return WindowInfo(ownerPID: pid, layer: layer, bounds: bounds)
    }
}