Developing a FileSystemWatcher for OS X by using FSEvents with Swift 2.0

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When Swift was announced at WWDC 2014, I was very disappointed to not be able to use some of powerful C APIs such as FSEvents, to write applications in pure Swift language.

A lot of C APIs needs a C function pointer to pass a callback function when you use the API. From Swift 1.0 to Swift 1.2, it was not possible to use C function pointer in pure Swift language.

It was possible to use FSEvents by wrapping it in an Objective-C class, and by calling this class in Swift by using the bridging features offered by the language, but it was not what I attempted to accomplish.

But things change and at WWDC 2015, Apple announced the possibility to use C function pointers with Swift 2.0.

Today I’m very pleased to give you a sample code which is a good starting point to create your own FileSystemWatcher using the FSEvents API written with Swift 2.0.

import Foundation

public class FileSystemWatcher {

    // MARK: - Initialization / Deinitialization
    
    public init(pathsToWatch: [String], sinceWhen: FSEventStreamEventId) {
        self.lastEventId = sinceWhen
        self.pathsToWatch = pathsToWatch
    }

    convenience public init(pathsToWatch: [String]) {
        self.init(pathsToWatch: pathsToWatch, sinceWhen: FSEventStreamEventId(kFSEventStreamEventIdSinceNow))
    }
    
    deinit {
        stop()
    }

    // MARK: - Private Properties
    
    private let eventCallback: FSEventStreamCallback = { (stream: ConstFSEventStreamRef, contextInfo: UnsafeMutablePointer<Void>, numEvents: Int, eventPaths: UnsafeMutablePointer<Void>, eventFlags: UnsafePointer<FSEventStreamEventFlags>, eventIds: UnsafePointer<FSEventStreamEventId>) in
        print("***** FSEventCallback Fired *****", appendNewline: true)
        
        let fileSystemWatcher: FileSystemWatcher = unsafeBitCast(contextInfo, FileSystemWatcher.self)
        let paths = unsafeBitCast(eventPaths, NSArray.self) as! [String]
        
        for index in 0..<numEvents {
            fileSystemWatcher.processEvent(eventIds[index], eventPath: paths[index], eventFlags: eventFlags[index])
        }
        
        fileSystemWatcher.lastEventId = eventIds[numEvents - 1]
    }
    private let pathsToWatch: [String]
    private var started = false
    private var streamRef: FSEventStreamRef!
    
    // MARK: - Private Methods
    
    private func processEvent(eventId: FSEventStreamEventId, eventPath: String, eventFlags: FSEventStreamEventFlags) {
        print("\t\(eventId) - \(eventFlags) - \(eventPath)", appendNewline: true)
    }
    
    // MARK: - Pubic Properties
    
    public private(set) var lastEventId: FSEventStreamEventId
    
    // MARK: - Pubic Methods
    
    public func start() {
        guard started == false else { return }
        
        var context = FSEventStreamContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
        context.info = UnsafeMutablePointer<Void>(unsafeAddressOf(self))
        let flags = UInt32(kFSEventStreamCreateFlagUseCFTypes | kFSEventStreamCreateFlagFileEvents)
        streamRef = FSEventStreamCreate(kCFAllocatorDefault, eventCallback, &context, pathsToWatch, lastEventId, 0, flags)
        
        FSEventStreamScheduleWithRunLoop(streamRef, CFRunLoopGetMain(), kCFRunLoopDefaultMode)
        FSEventStreamStart(streamRef)
        
        started = true
    }
    
    public func stop() {
        guard started == true else { return }

        FSEventStreamStop(streamRef)
        FSEventStreamInvalidate(streamRef)
        FSEventStreamRelease(streamRef)
        streamRef = nil

        started = false
    }
    
}

I hope this will help you in your future applications and feel free to share this article if you found it helpful 😉

20 thoughts on “Developing a FileSystemWatcher for OS X by using FSEvents with Swift 2.0”

SutoCom June 27, 2015 at 4:11 PM

Reblogged this on SutoCom Solutions.

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Thomas Kilian July 13, 2015 at 4:32 PM

Nice work. When I run this I eventually get EXC_BAD_ACCESS in libswiftCore.dylib`_swift_release_dealloc: once I Alt-Tab from the app to the Finder. Any thought?

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Thomas Kilian July 13, 2015 at 4:41 PM

The culprit seems to be in
let fileSystemWatcher = unsafeBitCast(contextInfo, FileSystemWatcher.self)
When I comment this (and the depending part below) I don’t get the above error any more.

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1. Stéphane Cordonnier (@s_cordonnier) Post authorJuly 15, 2015 at 8:40 PM
  
  What’s your code to instantiate the FileSystemWatcher object and where your instance was declared ?
  
  Regarding the errror message, it seems that when the callback closure is called then when the code try to get the instance from the context, the object was deallocated.
  
  In my own application, my code which works looks like this :
  
  class MainController: NSViewController {
  let fileSystemWatcher = FileSystemWatcher(…)
  }
  
  The instance has the same liftetime as the main application view controller.
  
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  1. Thomas Kilian July 15, 2015 at 9:27 PM
    
    Huu. I don’t know what happened. I put this aside for the moment and worked on other stuff. Now when I started it once again I had no more issues. I guess you know what I mean 😉 I’ll dive into this more seriously the next days and will come back with good (or bad) details. Thanks for the fast response.
    
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  2. Bipin Vayalu October 30, 2015 at 7:36 PM
    
    Hi Thomas & Stephane, I am getting same EXC_BAD_ACCESS error. Any idea how can i call closure from eventCallback.
    
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  3. Thomas Kilian October 30, 2015 at 8:34 PM
    
    Sorry, no idea. As told the error went away just after some time doing other stuff.
    
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Pingback: Developing a FileSystemWatcher for OS X by using FSEvents with Swift 2.0 | Dinesh Ram Kali.
Bipin Vayalu October 29, 2015 at 7:22 PM

Hi Stephane, I am setting closer in start function and want to call it in eventCallback like fileSystemWatcher.eventHandler(eventPath:eventPath). But i am getting nil exception. Looks like object is changed in eventCallback. Any idea how can i do that.

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1. Stéphane Cordonnier (@s_cordonnier) Post authorOctober 30, 2015 at 8:54 PM
  
  Hi Bipin. Without a sample of what you try to do, it’s pretty difficult to help you and see why you get an exception.
  
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  1. Bipin Vayalu October 30, 2015 at 9:05 PM
    
    Thanks Stephane for quick response, Here is my code:
    Add some stuff to FileSystemWatcher.swift class
    
    // created eventHandler property
    public var eventHandler: ((eventPath: String)->())?
    
    // setting up eventHandler property
    func start(handler: (eventPath: String) -> ()) {
    self.eventHandler = handler
    // other code as it is
    }
    
    private let eventCallback: FSEventStreamCallback = { (stream: ConstFSEventStreamRef, contextInfo: UnsafeMutablePointer, numEvents: Int, eventPaths: UnsafeMutablePointer, eventFlags: UnsafePointer, eventIds: UnsafePointer) in
    print(“***** FSEventCallback Fired *****”)
    
    let fileSystemWatcher = unsafeBitCast(contextInfo, FileSystemWatcher.self)
    let paths = unsafeBitCast(eventPaths, NSArray.self) as! [String]
    
    for index in 0..<numEvents {
    fileSystemWatcher.processEvent(eventIds[index], eventPath: paths[index], eventFlags: eventFlags[index])
    // Calling eventHandler closure here – Getting error here
    fileSystemWatcher.eventHandler!(eventPath:paths[index])
    }
    fileSystemWatcher.lastEventId = eventIds[numEvents – 1]
    }
    
    I hope this will help you to figure out issue.
    
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  2. Stéphane Cordonnier (@s_cordonnier) Post authorOctober 30, 2015 at 9:57 PM
    
    Which version of Swift are you using ? The code of this article was written in Swift 2.0. I’ve just tested and it also crashes on my machine with Swift 2.1 (same error as yours).
    
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  3. Stéphane Cordonnier (@s_cordonnier) Post authorOctober 30, 2015 at 11:03 PM
    
    I found the problem and there was a bug in the code.
    
    I updated the article and the problem was corrected with these 2 lines :
    
    var context = FSEventStreamContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
    context.info = UnsafeMutablePointer(unsafeAddressOf(self))
    
    Now it should work as expected 😉
    
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Bipin Vayalu October 30, 2015 at 10:03 PM

I am using Swift version 2.0 and xCode 7.

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1. Bipin Vayalu October 30, 2015 at 11:21 PM
  
  Yup, it’s working fine now. Great thanks @Stephane for quick help.
  
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StyleKit (@stylekit_org) April 8, 2016 at 12:34 PM

Cant get this to work. Could you write some example code how to init this instance? I tried:

let fileSystemWatcher = FileSystemWatcher(pathsToWatch: [“~/Desktop/”.tildePath],sinceWhen: 0)
fileSystemWatcher.start()

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warlockosx June 13, 2016 at 3:14 PM

Hello, all works fine, but I have a problem to convert eventFlag in

private func processEvent(eventId: FSEventStreamEventId, eventPath: String, eventFlag: FSEventStreamEventFlags) {
print(“\t\(eventId) – \(eventFlag)) – \(eventPath)”)
}

from Int value like 107776 to something like that as it described in documentation
kFSEventStreamEventFlagItemCreated = 0x00000100

Thanks!

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ctietze November 8, 2016 at 3:27 PM

Thanks for sharing the wrapper! It demonstrates very well how to use a C API (which I never tried before) and it works like a charm. I’ve converted the code to Swift 3 here:
https://gist.github.com/DivineDominion/56e56f3db43216d9eaab300d3b9f049a

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eric October 26, 2017 at 7:39 PM

Hello, this code does not compile anymore with Swift 3.2 . and XCode 9.. any idea on how to fix it? Requested Fix make it crash… any idea on how to fix it? Thanks

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ctietze October 27, 2017 at 6:34 PM

The fixes to my code are trivial once you let Xcode 9 do the migration for you. There are some parameters that used to be optional and now aren’t. My project has changed heavily so I cannot paste a 1:1 upgrade, though. I performed the changes manually in the gist:

	//
	// Based on: https://blog.beecomedigital.com/2015/06/27/developing-a-filesystemwatcher-for-os-x-by-using-fsevents-with-swift-2/
	//

	import Foundation

	public struct Event: CustomStringConvertible {

	public let eventId: FSEventStreamEventId
	public let eventPath: String
	public let eventFlags: FSEventStreamEventFlags

	public var description: String {
	return "\(eventId) – \(eventFlags) – \(eventPath)"
	}
	}

	public class FolderContentMonitor {

	let callback: (Event) -> Void

	public init(pathsToWatch: [String], sinceWhen: FSEventStreamEventId = FSEventStreamEventId(kFSEventStreamEventIdSinceNow), callback: @escaping (Event) -> Void) {

	self.lastEventId = sinceWhen
	self.pathsToWatch = pathsToWatch
	self.callback = callback
	}

	deinit {
	stop()
	}

	// MARK: – Private Properties

	private let eventCallback: FSEventStreamCallback = {
	(stream: ConstFSEventStreamRef,
	contextInfo: UnsafeMutableRawPointer?,
	numEvents: Int,
	eventPaths: UnsafeMutableRawPointer,
	eventFlags: UnsafePointer<FSEventStreamEventFlags>,
	eventIds: UnsafePointer<FSEventStreamEventId>) in

	let fileSystemWatcher: FolderContentMonitor = unsafeBitCast(contextInfo, to: FolderContentMonitor.self)

	guard let paths = unsafeBitCast(eventPaths, to: NSArray.self) as? [String] else { return }

	for index in 0..<numEvents {
	fileSystemWatcher.processEvent(eventId: eventIds[index], eventPath: paths[index], eventFlags: eventFlags[index])
	}

	fileSystemWatcher.lastEventId = eventIds[numEvents – 1]
	}

	private let pathsToWatch: [String]
	private var started = false
	private var streamRef: FSEventStreamRef!

	private func processEvent(eventId: FSEventStreamEventId, eventPath: String, eventFlags: FSEventStreamEventFlags) {

	let event = Event(eventId: eventId, eventPath: eventPath, eventFlags: eventFlags)
	callback(event)
	}

	public private(set) var lastEventId: FSEventStreamEventId

	public func start() {
	guard started == false else { return }

	var context = FSEventStreamContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
	context.info = Unmanaged.passUnretained(self).toOpaque()
	let flags = UInt32(kFSEventStreamCreateFlagUseCFTypes \| kFSEventStreamCreateFlagFileEvents)
	streamRef = FSEventStreamCreate(kCFAllocatorDefault, eventCallback, &context, pathsToWatch as CFArray, lastEventId, 0, flags)

	FSEventStreamScheduleWithRunLoop(streamRef, CFRunLoopGetMain(), CFRunLoopMode.defaultMode.rawValue)
	FSEventStreamStart(streamRef)

	started = true
	}

	public func stop() {
	guard started == true else { return }

	FSEventStreamStop(streamRef)
	FSEventStreamInvalidate(streamRef)
	FSEventStreamRelease(streamRef)
	streamRef = nil

	started = false
	}
	}