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@correia
Last active April 16, 2023 02:38
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A quick example for how to use Foundation style KVO from Swift. (Official documentation from Apple is forthcoming.)
//
// Swift-KVO
//
// Created by Jim Correia on 6/5/14.
// Copyright (c) 2014-2015 Jim Correia. All rights reserved.
//
// Update: 6/17/2014
//
// KVOContext has gone away; use the same idiom you'd use from Objective-C for the context
//
// Update 6/10/2015
//
// Updated for Swift 2
// Use dynamic on observable properties
import Foundation
typealias KVOContext = UInt8
var MyObservationContext = KVOContext()
class Observer: NSObject {
func startObservingPerson(person: Person) {
let options = NSKeyValueObservingOptions([.New, .Old])
person.addObserver(self, forKeyPath: "firstName", options: options, context: &MyObservationContext)
person.addObserver(self, forKeyPath: "lastName", options: options, context: &MyObservationContext)
}
func stopObservingPerson(person: Person) {
person.removeObserver(self, forKeyPath: "firstName", context: &MyObservationContext)
person.removeObserver(self, forKeyPath: "lastName", context: &MyObservationContext)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [NSObject : AnyObject]?, context: UnsafeMutablePointer<Void>) {
guard keyPath != nil else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
return
}
switch (keyPath!, context) {
case("firstName", &MyObservationContext):
print("First name changed: \(change)")
case("lastName", &MyObservationContext):
print("Last name changed: \(change)")
case(_, &MyObservationContext):
assert(false, "unknown key path")
default:
super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
}
}
}
class Person: NSObject {
dynamic var firstName: String?
dynamic var lastName: String?
}
let person = Person()
let observer = Observer()
observer.startObservingPerson(person)
person.firstName = "Jim"
person.lastName = "Correia"
observer.stopObservingPerson(person)
@jameswomack
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An example project that demonstrates KVO being used in a UIKit interface via Swift:
https://github.com/jameswomack/kvo-in-swift

@jshah
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jshah commented Apr 22, 2015

Do the variables in the Person class, firstName and lastName need the "dynamic" property on it?

@neilpa
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neilpa commented May 29, 2015

@jshah111 - Yea, dynamic is needed if you actually want to get KVO notifications.

@correia
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Author

correia commented Jun 10, 2015

Updated for Swift 2 on 2015/06/10

@Gouldsc
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Gouldsc commented Jun 22, 2015

EDIT: This is working in Xcode 7 Beta 2. Disregard previous statement. 7/30/15

@andrewschreiber
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On line 33

override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [NSObject :        AnyObject]?, context: UnsafeMutablePointer<Void>) {
        guard keyPath != nil else {
            super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
            return
        }

I needed to replace

[NSObject: AnyObject]?

with

[String:AnyObject]?

@colinta
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colinta commented Feb 8, 2016

guard let keyPath = keyPath else { ... }

switch keyPath { ... }
--- yours
+++ patched
@@ -31,12 +31,12 @@
     }

     override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [NSObject : AnyObject]?, context: UnsafeMutablePointer<Void>) {
-        guard let keyPath = keyPath else {
+        guard keyPath != nil else {
             super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
             return
         }

-        switch (keyPath, context) {
+        switch (keyPath!, context) {
         case("firstName", &MyObservationContext):
             print("First name changed: \(change)")

@ltfschoen
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This is brilliant. Thank you!

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