focus-fixer/FocusFixer/EventTapController.swift

import CoreGraphics
import Foundation
import os

/// Suppresses events that arrive within `interval` of the previous accepted
/// event. Clock is injectable so debounce timing can be unit tested without
/// sleeping.
struct Debouncer {
    private let interval: TimeInterval
    private let now: () -> TimeInterval
    private var lastAcceptedTime: TimeInterval?

    init(interval: TimeInterval, clock: @escaping () -> TimeInterval = { ProcessInfo.processInfo.systemUptime }) {
        self.interval = interval
        self.now = clock
    }

    mutating func shouldProcess() -> Bool {
        let current = now()
        if let last = lastAcceptedTime, current - last < interval {
            return false
        }
        lastAcceptedTime = current
        return true
    }
}

/// Owns the low-level CGEvent tap: creation, run-loop wiring, and recovery
/// when macOS disables the tap for being too slow. The tap is listen-only
/// and never mutates or swallows events. All work beyond capturing the
/// event location happens off the callback thread.
final class EventTapController {
    private let logger = Logger(subsystem: "com.local.focusfixer", category: "EventTapController")
    private let handoffQueue = DispatchQueue(label: "com.local.focusfixer.eventtap.handoff")
    private let preferences: Preferences

    private var eventTap: CFMachPort?
    private var runLoopSource: CFRunLoopSource?
    private var debouncer: Debouncer
    private var onMouseDown: ((CGPoint) -> Void)?

    init(preferences: Preferences = .shared) {
        self.preferences = preferences
        self.debouncer = Debouncer(interval: preferences.debounceInterval)
    }

    /// Starts listening for left mouse-down events. `onMouseDown` is
    /// invoked on `handoffQueue`, after debouncing, with the click location
    /// in global screen coordinates.
    func start(onMouseDown: @escaping (CGPoint) -> Void) {
        self.onMouseDown = onMouseDown

        let eventMask: CGEventMask = 1 << CGEventType.leftMouseDown.rawValue
        let selfPtr = Unmanaged.passUnretained(self).toOpaque()

        guard let tap = CGEvent.tapCreate(
            tap: .cgSessionEventTap,
            place: .headInsertEventTap,
            options: .listenOnly,
            eventsOfInterest: eventMask,
            callback: eventTapCallback,
            userInfo: selfPtr
        ) else {
            logger.error("Failed to create event tap — Accessibility permission likely missing")
            return
        }

        eventTap = tap
        let source = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, tap, 0)
        runLoopSource = source
        CFRunLoopAddSource(CFRunLoopGetMain(), source, .commonModes)
        CGEvent.tapEnable(tap: tap, enable: true)
        logger.info("Event tap started")
    }

    func stop() {
        if let tap = eventTap {
            CGEvent.tapEnable(tap: tap, enable: false)
        }
        if let source = runLoopSource {
            CFRunLoopRemoveSource(CFRunLoopGetMain(), source, .commonModes)
        }
        eventTap = nil
        runLoopSource = nil
        logger.info("Event tap stopped")
    }

    fileprivate func handle(type: CGEventType, event: CGEvent) {
        if type == .tapDisabledByTimeout || type == .tapDisabledByUserInput {
            logger.error("Event tap disabled — re-enabling")
            if let tap = eventTap {
                CGEvent.tapEnable(tap: tap, enable: true)
            }
            return
        }

        guard type == .leftMouseDown else { return }
        let location = event.location

        handoffQueue.async { [weak self] in
            guard let self else { return }
            guard self.debouncer.shouldProcess() else { return }
            self.onMouseDown?(location)
        }
    }
}

/// Non-capturing C callback required by CGEvent.tapCreate. Recovers the
/// controller instance from `refcon` rather than capturing `self`.
private func eventTapCallback(
    proxy: CGEventTapProxy,
    type: CGEventType,
    event: CGEvent,
    refcon: UnsafeMutableRawPointer?
) -> Unmanaged<CGEvent>? {
    if let refcon {
        let controller = Unmanaged<EventTapController>.fromOpaque(refcon).takeUnretainedValue()
        controller.handle(type: type, event: event)
    }
    return Unmanaged.passUnretained(event)
}