focus-fixer/FocusFixer/FocusEnforcer.swift

import AppKit
import ApplicationServices
import os

/// Activates the app that owns a window and reasserts keyboard focus on
/// that specific window via the Accessibility API.
final class FocusEnforcer {
    private let logger = Logger(subsystem: "com.local.focusfixer", category: "FocusEnforcer")
    private let preferences: Preferences

    init(preferences: Preferences = .shared) {
        self.preferences = preferences
    }

    /// Activates the owning app and raises/focuses its window, unless it is
    /// this app, already frontmost, or user-excluded.
    func enforceFocus(forWindowOwnedBy pid: pid_t) {
        guard pid != ProcessInfo.processInfo.processIdentifier else {
            return
        }
        guard let app = NSRunningApplication(processIdentifier: pid) else {
            logger.debug("No running application for pid \(pid, privacy: .public)")
            return
        }
        if let bundleID = app.bundleIdentifier, preferences.isExcluded(bundleID: bundleID) {
            return
        }
        guard !app.isActive else {
            return
        }

        if !app.activate(options: []) {
            logger.debug("Activation returned false for pid \(pid, privacy: .public)")
        }
        raiseFocusedWindow(pid: pid)
    }

    private func raiseFocusedWindow(pid: pid_t) {
        let axApp = AXUIElementCreateApplication(pid)
        var windowRef: CFTypeRef?
        let copyResult = AXUIElementCopyAttributeValue(axApp, kAXFocusedWindowAttribute as CFString, &windowRef)
        guard copyResult == .success, let windowRef else {
            logger.debug("Could not resolve focused window via AX for pid \(pid, privacy: .public): \(copyResult.rawValue, privacy: .public)")
            return
        }
        // AXUIElementCopyAttributeValue guarantees an AXUIElement on success; the
        // Swift compiler can't express that constraint, hence the forced cast.
        let axWindow = windowRef as! AXUIElement

        let raiseResult = AXUIElementPerformAction(axWindow, kAXRaiseAction as CFString)
        if raiseResult != .success {
            logger.debug("Raise action failed for pid \(pid, privacy: .public): \(raiseResult.rawValue, privacy: .public)")
        }

        let focusResult = AXUIElementSetAttributeValue(axApp, kAXFocusedWindowAttribute as CFString, axWindow)
        if focusResult != .success {
            logger.debug("Setting focused window failed for pid \(pid, privacy: .public): \(focusResult.rawValue, privacy: .public)")
        }
    }
}