I'm trying to achieve the following but am running into issues :-)
create a protocol that UIViewController and UIView subclass can adopt
which contain one static method to be called on this class (call it
configuration
I then want to use the objectiveC runtime to find the classes that adopt this protocol
On each of those class I want to call the configuration method
The configuration method is to return a dictionary (key: a description string, value: a selector to be called on the class)
So far I was able to create the protocol, find the class implementing the protocol but i'm running into compiling issues.
Here is the protocol
#objc public protocol MazeProtocol: NSObjectProtocol{
#objc static func configurations() -> NSDictionary
}
Here is the extension to adopt the protocol on one of my class:
extension MapCoordinatorViewController: MazeProtocol {
static func configurations() -> NSDictionary {
let returnValue = NSMutableDictionary()
returnValue.setObject(#selector(test), forKey: "test" as NSString)
return returnValue
}
#objc static func test() {
print("test")
}}
and here is the code i'm using to try to call the selector returned from the configuration method:
let selectorKey = controllerClass.configurations().allKeys[indexPath.row]
let selector = controllerClass.configurations().object(forKey: selectorKey)
controllerClass.performSelector(selector) <================ error here
ControllerClass is declared as let controllerClass: MazeProtocol.Type
I get the following compile warning:
Instance member 'performSelector' cannot be used on type 'MazeProtocol'
What am I missing?
You can technically force this to work. Please don't. This is horrible Swift. To get this to work, you have to undermine everything Swift is trying to do. But yes, with warnings, you can technically get this to compile and work. Please, please don't.
First, you need to make selector be a Selector. You're using an NSDictionary, which is terrible in Swift, and so you get Any? back. But, yes, you can as! cast it into what you want:
let selector = controllerClass.configurations().object(forKey: selectorKey) as! Selector
And then, defying all the type gods, you can just declare that classes are actually NSObjectProtocol, because why not?
(controllerClass as! NSObjectProtocol).perform(selector)
This will throw a warning "Cast from 'MapCoordinatorViewController.Type' to unrelated type 'NSObjectProtocol' always fails", but it will in fact succeed.
After all that "don't do this," how should you do this? With closures.
public protocol MazeProtocol {
static var configurations: [String: () -> Void] { get }
}
class MapCoordinatorViewController: UIViewController {}
extension MapCoordinatorViewController: MazeProtocol {
static let configurations: [String: () -> Void] = [
"test": test
]
static func test() {
print("test")
}
}
let controllerClass = MapCoordinatorViewController.self
let method = controllerClass.configurations["test"]!
method()
I had a trick to help test UIAlertController that worked in Swift 2.x:
extension UIAlertController {
typealias AlertHandler = #convention(block) (UIAlertAction) -> Void
func tapButtonAtIndex(index: Int) {
let block = actions[index].valueForKey("handler")
let handler = unsafeBitCast(block, AlertHandler.self)
handler(actions[index])
}
}
This fails under Swift 3.x with fatal error: can't unsafeBitCast between types of different sizes, which tempts me to believe there might be a way to make the cast work. Can anyone figure it out?
Found a solution that works in Swift 3.0.1
extension UIAlertController {
typealias AlertHandler = #convention(block) (UIAlertAction) -> Void
func tapButton(atIndex index: Int) {
if let block = actions[index].value(forKey: "handler") {
let blockPtr = UnsafeRawPointer(Unmanaged<AnyObject>.passUnretained(block as AnyObject).toOpaque())
let handler = unsafeBitCast(blockPtr, to: AlertHandler.self)
handler(actions[index])
}
}
}
(Originally, the block value was the actual block, not a pointer to the block—which you obviously can't cast to a pointer to AlertHandler)
My answer is based on #Robert Atkins's, but shorter.
The problem here is that, valueForKey returns a Any typed object, and because in Swift,
MemoryLayout<Any>.size == 32
MemoryLayout<AnyObjcBlockType>.size == 8
an assertion will be triggered in unsafeBitCast when casting between types of different sizes.
One work-around is to create an intermediate wrapper and transform back to raw pointer, which satisfies MemoryLayout<UnsafeRawPointer>.size == 8.
A much simpler way is to create an indirect reference directly using protocol AnyObject, relying on the fact that MemoryLayout<AnyObject >.size == 8, we can write following valid code:
typealias AlertHandler = #convention(block) (UIAlertAction) -> Void
func tapButton(atIndex index: Int) {
if let block = actions[index].value(forKey: "handler") {
let handler = unsafeBitCast(block as AnyObject, to: AlertHandler.self)
handler(actions[index])
}
}
If your UIAlertController is an action sheet you can modify Robert's answer to dismiss the UIAlertController before you executed the handler.
dismiss(animated: true, completion: {() in handler(self.actions[index])})
I was using this extension for testing and without this modification my assertions for presented view controller were failing.
I called perform selector function of NSObjectProtocol and try to get return value, crash why? Does anyone can help me. thx.
class Convert: NSObject {
func value(_ value: CGFloat, from srcUnit: DWUnitType, to dstUnit: DWUnitType) {
let selector = Selector("_centimeterToMillimeter:")
let newValue = perform(selector, with: value).takeUnretainedValue() as? CGFloat
print(newValue)
}
func _centimeterToMillimeter(_ value: CGFloat) -> CGFloat {
return value * 10.0
}
}
Not sure whether the bridging of CGFloat to Objective-C will work (possibly this is why you are crashing). Try this
func _centimeterToMillimeter(_ value: Any) -> Any {
return (value as! CGFloat) * 10.0
}
instead for your conversion function (or something to this effect). Just as a general comment, using the runtime (e.g. perform:_:with) your are abandoning the comfort of compile time checks and stepping into a world of hard to debug crashes. Better know what you are doing.
I've been trying to use UIInterfaceOrientationMask in my Swift code. In Objective-C, I would use it as an enum, and when I needed to use, for example, the portrait mask, I would simply use UIPortraitOrientationMask in my code, like so:
NSUInteger orientations = UIInterfaceOrientationMaskPortrait;
I'm not quite sure what I'm supposed to do with the documentation, and I haven't been able to find anything about a "Raw Option Set" anywhere in the tutorial book or the documentation.
As of Xcode 7, Swift Version 2.0:
override func supportedInterfaceOrientations() -> UIInterfaceOrientationMask { return UIInterfaceOrientationMask.Portrait }
Original obsolete answer below:
As of Xcode 6.2 or 6.3, or Swift version 1.2:
override func supportedInterfaceOrientations() -> Int { return Int(UIInterfaceOrientationMask.Portrait.rawValue) }
supportedInterfaceOrientations is defined to return an Int, but UIInterfaceOrientationMask is an enumeration. So, use the rawValue property of the enumeration to find its raw value, which gives a UInt. The Int() function takes the UInt as an input and returns an Int.
Use the '.' format to set / check for a value, like so:
import UIKit
var orientation : UIInterfaceOrientationMask = .Portrait
orientation = .All
if (orientation == .Landscape) {
// do something
}
You can get the same value in Swift like this: UIInterfaceOrientationMask.Portrait
If so, are there any key differences that weren't otherwise present when using key-value observation in Objective-C?
You can use KVO in Swift, but only for dynamic properties of NSObject subclass. Consider that you wanted to observe the bar property of a Foo class. In Swift 4, specify bar as dynamic property in your NSObject subclass:
class Foo: NSObject {
#objc dynamic var bar = 0
}
You can then register to observe changes to the bar property. In Swift 4 and Swift 3.2, this has been greatly simplified, as outlined in Using Key-Value Observing in Swift:
class MyObject {
private var token: NSKeyValueObservation
var objectToObserve = Foo()
init() {
token = objectToObserve.observe(\.bar) { [weak self] object, change in // the `[weak self]` is to avoid strong reference cycle; obviously, if you don't reference `self` in the closure, then `[weak self]` is not needed
print("bar property is now \(object.bar)")
}
}
}
Note, in Swift 4, we now have strong typing of keypaths using the backslash character (the \.bar is the keypath for the bar property of the object being observed). Also, because it's using the completion closure pattern, we don't have to manually remove observers (when the token falls out of scope, the observer is removed for us) nor do we have to worry about calling the super implementation if the key doesn't match. The closure is called only when this particular observer is invoked. For more information, see WWDC 2017 video, What's New in Foundation.
In Swift 3, to observe this, it's a bit more complicated, but very similar to what one does in Objective-C. Namely, you would implement observeValue(forKeyPath keyPath:, of object:, change:, context:) which (a) makes sure we're dealing with our context (and not something that our super instance had registered to observe); and then (b) either handle it or pass it on to the super implementation, as necessary. And make sure to remove yourself as an observer when appropriate. For example, you might remove the observer when it is deallocated:
In Swift 3:
class MyObject: NSObject {
private var observerContext = 0
var objectToObserve = Foo()
override init() {
super.init()
objectToObserve.addObserver(self, forKeyPath: #keyPath(Foo.bar), options: [.new, .old], context: &observerContext)
}
deinit {
objectToObserve.removeObserver(self, forKeyPath: #keyPath(Foo.bar), context: &observerContext)
}
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
guard context == &observerContext else {
super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
return
}
// do something upon notification of the observed object
print("\(keyPath): \(change?[.newKey])")
}
}
Note, you can only observe properties that can be represented in Objective-C. Thus, you cannot observe generics, Swift struct types, Swift enum types, etc.
For a discussion of the Swift 2 implementation, see my original answer, below.
Using the dynamic keyword to achieve KVO with NSObject subclasses is described in the Key-Value Observing section of the Adopting Cocoa Design Conventions chapter of the Using Swift with Cocoa and Objective-C guide:
Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects. You can use key-value observing with a Swift class, as long as the class inherits from the NSObject class. You can use these three steps to implement key-value observing in Swift.
Add the dynamic modifier to any property you want to observe. For more information on dynamic, see Requiring Dynamic Dispatch.
class MyObjectToObserve: NSObject {
dynamic var myDate = NSDate()
func updateDate() {
myDate = NSDate()
}
}
Create a global context variable.
private var myContext = 0
Add an observer for the key-path, and override the observeValueForKeyPath:ofObject:change:context: method, and remove the observer in deinit.
class MyObserver: NSObject {
var objectToObserve = MyObjectToObserve()
override init() {
super.init()
objectToObserve.addObserver(self, forKeyPath: "myDate", options: .New, context: &myContext)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
if context == &myContext {
if let newValue = change?[NSKeyValueChangeNewKey] {
print("Date changed: \(newValue)")
}
} else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
}
}
deinit {
objectToObserve.removeObserver(self, forKeyPath: "myDate", context: &myContext)
}
}
[Note, this KVO discussion has subsequently been removed from the Using Swift with Cocoa and Objective-C guide, which has been adapted for Swift 3, but it still works as outlined at the top of this answer.]
It's worth noting that Swift has its own native property observer system, but that's for a class specifying its own code that will be performed upon observation of its own properties. KVO, on the other hand, is designed to register to observe changes to some dynamic property of some other class.
(Edited to add new info): consider whether using the Combine framework can help you accomplish what you wanted, rather than using KVO
Yes and no. KVO works on NSObject subclasses much as it always has. It does not work for classes that don't subclass NSObject. Swift does not (currently at least) have its own native observation system.
(See comments for how to expose other properties as ObjC so KVO works on them)
See the Apple Documentation for a full example.
Both yes and no:
Yes, you can use the same old KVO APIs in Swift to observe Objective-C objects.
You can also observe dynamic properties of Swift objects inheriting from NSObject.
But... No it's not strongly typed as you could expect Swift native observation system to be.
Using Swift with Cocoa and Objective-C | Key Value Observing
No, currently there is no builtin value observation system for arbitrary Swift objects.
Yes, there are builtin Property Observers, which are strongly typed.
But... No they are not KVO, since they allow only for observing of objects own properties, don't support nested observations ("key paths"), and you have to explicitly implement them.
The Swift Programming Language | Property Observers
Yes, you can implement explicit value observing, which will be strongly typed, and allow for adding multiple handlers from other objects, and even support nesting / "key paths".
But... No it will not be KVO since it will only work for properties which you implement as observable.
You can find a library for implementing such value observing here:
Observable-Swift - KVO for Swift - Value Observing and Events
Yes.
KVO requires dynamic dispatch, so you simply need to add the dynamic modifier to a method, property, subscript, or initializer:
dynamic var foo = 0
The dynamic modifier ensures that references to the declaration will be dynamically dispatched and accessed through objc_msgSend.
An example might help a little here. If I have an instance model of class Model with attributes name and state I can observe those attributes with:
let options = NSKeyValueObservingOptions([.New, .Old, .Initial, .Prior])
model.addObserver(self, forKeyPath: "name", options: options, context: nil)
model.addObserver(self, forKeyPath: "state", options: options, context: nil)
Changes to these properties will trigger a call to:
override func observeValueForKeyPath(keyPath: String!,
ofObject object: AnyObject!,
change: NSDictionary!,
context: CMutableVoidPointer) {
println("CHANGE OBSERVED: \(change)")
}
In addition to Rob's answer. That class must inherit from NSObject, and we have 3 ways to trigger property change
Use setValue(value: AnyObject?, forKey key: String) from NSKeyValueCoding
class MyObjectToObserve: NSObject {
var myDate = NSDate()
func updateDate() {
setValue(NSDate(), forKey: "myDate")
}
}
Use willChangeValueForKey and didChangeValueForKey from NSKeyValueObserving
class MyObjectToObserve: NSObject {
var myDate = NSDate()
func updateDate() {
willChangeValueForKey("myDate")
myDate = NSDate()
didChangeValueForKey("myDate")
}
}
Use dynamic. See Swift Type Compatibility
You can also use the dynamic modifier to require that access to members be dynamically dispatched through the Objective-C runtime if you’re using APIs like key–value observing that dynamically replace the implementation of a method.
class MyObjectToObserve: NSObject {
dynamic var myDate = NSDate()
func updateDate() {
myDate = NSDate()
}
}
And property getter and setter is called when used. You can verify when working with KVO. This is an example of computed property
class MyObjectToObserve: NSObject {
var backing: NSDate = NSDate()
dynamic var myDate: NSDate {
set {
print("setter is called")
backing = newValue
}
get {
print("getter is called")
return backing
}
}
}
Currently Swift does not support any built in mechanism for observing property changes of objects other than 'self', so no, it does not support KVO.
However, KVO is such a fundamental part of Objective-C and Cocoa that it seems quite likely that it will be added in the future. The current documentation seems to imply this:
Key-Value Observing
Information forthcoming.
Using Swift with Cocoa and Objective-C
One important thing to mention is that after updating your Xcode to 7 beta you might be getting the following message:
"Method does not override any method from its superclass". That's because of the arguments' optionality. Make sure that your observation handler looks exactly as follows:
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [NSObject : AnyObject]?, context: UnsafeMutablePointer<Void>)
This may be prove helpful to few people -
// MARK: - KVO
var observedPaths: [String] = []
func observeKVO(keyPath: String) {
observedPaths.append(keyPath)
addObserver(self, forKeyPath: keyPath, options: [.old, .new], context: nil)
}
func unObserveKVO(keyPath: String) {
if let index = observedPaths.index(of: keyPath) {
observedPaths.remove(at: index)
}
removeObserver(self, forKeyPath: keyPath)
}
func unObserveAllKVO() {
for keyPath in observedPaths {
removeObserver(self, forKeyPath: keyPath)
}
}
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
if let keyPath = keyPath {
switch keyPath {
case #keyPath(camera.iso):
slider.value = camera.iso
default:
break
}
}
}
I had used KVO in this way in Swift 3. You can use this code with few changes.
Overview
It is possible using Combine without using NSObject or Objective-C
Availability: iOS 13.0+, macOS 10.15+, tvOS 13.0+, watchOS 6.0+, Mac Catalyst 13.0+, Xcode 11.0+
Note: Needs to be used only with classes not with value types.
Code:
Swift Version: 5.1.2
import Combine //Combine Framework
//Needs to be a class doesn't work with struct and other value types
class Car {
#Published var price : Int = 10
}
let car = Car()
//Option 1: Automatically Subscribes to the publisher
let cancellable1 = car.$price.sink {
print("Option 1: value changed to \($0)")
}
//Option 2: Manually Subscribe to the publisher
//Using this option multiple subscribers can subscribe to the same publisher
let publisher = car.$price
let subscriber2 : Subscribers.Sink<Int, Never>
subscriber2 = Subscribers.Sink(receiveCompletion: { print("completion \($0)")}) {
print("Option 2: value changed to \($0)")
}
publisher.subscribe(subscriber2)
//Assign a new value
car.price = 20
Output:
Option 1: value changed to 10
Option 2: value changed to 10
Option 1: value changed to 20
Option 2: value changed to 20
Refer:
https://developer.apple.com/documentation/combine
https://developer.apple.com/documentation/combine/receiving_and_handling_events_with_combine
https://developer.apple.com/documentation/combine/published
Another example for anyone who runs into a problem with types such as Int? and CGFloat?. You simply set you class as a subclass of NSObject and declare your variables as follows e.g:
class Theme : NSObject{
dynamic var min_images : Int = 0
dynamic var moreTextSize : CGFloat = 0.0
func myMethod(){
self.setValue(value, forKey: "\(min_images)")
}
}