class SuperDelegate <T: AnyObject> {
func addDelegate(delegate: T)
{
}
}
My question is about T key, does it mean the same as id in Objective-c? I mean about case of uses.
how to understand the first line class SuperDelegate <T: AnyObject> Sorry I am new in Swift.
As Objective C program for me this line means that we make class to conform a protocol that has to implement all required method. But I don't understand func addDelegate(delegate: T) is this the same like
- (void)addDelegate:(id)delegate which is a property id <T> delegate.
Yes you are correct in your assumptions that AnyObject behaves like id:
You can call any Objective-C method and access any property on an
AnyObject value without casting to a more specific class type. This
includes Objective-C compatible methods and properties marked with the
#objc attribute.
but you have used it here as a generic type rather than as a concrete type that should be cast to. The class is requiring a type that adheres to the AnyObject protocol but it isn't forcing it to be AnyObject (see header files: cmd + click on AnyObject inside Xcode).
So your instance could be instantiated SuperDelegate<AnyObject> but it could also be instantiated SuperDelegate<NSDate>. This means that the whole subset of ObjC methods and properties cannot be guaranteed as they can with a cast to AnyObject as a concrete type because at runtime T might represent NSDate or NSNumber or any other class.
To achieve what you want you would need to write:
class SuperDelegate {
func addDelegate(delegate: AnyObject)
{
}
}
But Swift is a strongly-typed language and it would normally be the case that you had a delegate protocol and that the delegate for your type adhered to the delegate protocol:
protocol DelegateProtocol {
func somethingHappened()
}
struct MyTypeDelegate:DelegateProtocol {
func somethingHappened() {
print("Thanks for telling me!")
}
}
struct MyType {
var delegate:DelegateProtocol?
func tellDelegateSomethingHappened() {
delegate?.somethingHappened()
}
}
let del = MyTypeDelegate()
var type = MyType()
type.delegate = del
type.tellDelegateSomethingHappened()
Related
How can I provide default implementations for Objective-C optional protocol methods?
Ex.
extension AVSpeechSynthesizerDelegate {
func speechSynthesizer(synthesizer: AVSpeechSynthesizer, didFinishSpeechUtterance utterance: AVSpeechUtterance) {
print(">>> did finish")
}
}
Expectation: Whatever class that conforms to AVSpeechSynthesizerDelegate should run the above function whenever a speech utterance finishes.
You do it just exactly as you've implemented it. The difference ends up being in how the method is actually called.
Let's take this very simplified example:
#objc protocol FooProtocol {
optional func bar() -> Int
}
class Omitted: NSObject, FooProtocol {}
class Implemented: NSObject, FooProtocol {
func bar() -> Int {
print("did custom bar")
return 1
}
}
By adding no other code, I'd expect to have to use this code as such:
let o: FooProtocol = Omitted()
let oN = o.bar?()
let i: FooProtocol = Implemented()
let iN = i.bar?()
Where oN and iN both end up having type Int?, oN is nil, iN is 1 and we see the text "did custom bar" print.
Importantly, not the optionally chained method call: bar?(), that question mark between the method name in the parenthesis. This is how we must call optional protocol methods from Swift.
Now let's add an extension for our protocol:
extension FooProtocol {
func bar() -> Int {
print("did bar")
return 0
}
}
If we stick to our original code, where we optionally chain the method calls, there is no change in behavior:
However, with the protocol extension, we no longer have to optionally unwrap. We can take the optional unwrapping out, and the extension is called:
The unfortunate problem here is that this isn't necessarily particularly useful, is it? Now we're just calling the method implemented in the extension every time.
So there's one slightly better option if you're in control of the class making use of the protocol and calling the methods. You can check whether or not the class responds to the selector:
let i: FooProtocol = Implemented()
if i.respondsToSelector("bar") {
i.bar?()
}
else {
i.bar()
}
This also means you have to modify your protocol declaration:
#objc protocol FooProtocol: NSObjectProtocol
Adding NSObjectProtocol allows us to call respondsToSelector, and doesn't really change our protocol at all. We'd already have to be inheriting from NSObject in order to implement a protocol marked as #objc.
Of course, with all this said, any Objective-C code isn't going to be able to perform this logic on your Swift types and presumably won't be able to actually call methods implemented in these protocol extensions it seems. So if you're trying to get something out of Apple's frameworks to call the extension method, it seems you're out of luck. It also seems that even if you're trying to call one or the other in Swift, if it's a protocol method mark as optional, there's not a very great solution.
I want to use a strategy pattern to register a set of objects that implement a protocol. When I set this up, I get a compile error when trying to set the delegate that is part of the protocol.
For discussion purposes, I have slightly reworked the DiceGame from the Swift eBook's Delegation chapter. The changes of significance are:
protocol DiceGame - declares a delegate
class SnakesAndLadders implements DiceGame (and therefore the protocol and delegate)
class Games holds 3 instances of SnakesAndLadders as
1) a concrete class of SnakesAndLadders
2) a 'let' constant of protocol DiceGame
3) a 'var' variable of protocol DiceGame
We can set the delegate fine if we use the concrete class (snakesAndLadders). However, there is a compile error if we use 'let' to hold it as a protocol (diceGameAsLet) but it compiles if we hold the variable as a 'var' (diceGameAsVar).
It is easy to work around, however, the delegate itself never changes so should be held as a 'let' constant, as it is only the internal property that changes. I must not understand something (possibly subtle but significant) about protocols and how they work and should be used.
class Dice
{
func roll() -> Int
{
return 7 // always win :)
}
}
protocol DiceGame
{
// all DiceGames must work with a DiceGameDelegate
var delegate:DiceGameDelegate? {get set}
var dice: Dice {get}
func play()
}
protocol DiceGameDelegate
{
func gameDidStart( game:DiceGame )
func gameDidEnd( game:DiceGame )
}
class SnakesAndLadders:DiceGame
{
var delegate:DiceGameDelegate?
let dice = Dice()
func play()
{
delegate?.gameDidStart(self)
playGame()
delegate?.gameDidEnd(self)
}
private func playGame()
{
print("Playing the game here...")
}
}
class Games : DiceGameDelegate
{
let snakesAndLadders = SnakesAndLadders()
// hold the protocol, not the class
let diceGameAsLet:DiceGame = SnakesAndLadders()
var diceGameAsVar:DiceGame = SnakesAndLadders()
func setupDelegateAsClass()
{
// can assign the delegate if using the class
snakesAndLadders.delegate = self
}
func setupDelegateAsVar()
{
// if we use 'var' we can assign the delegate
diceGameAsVar.delegate = self
}
func setupDelegateAsLet()
{
// DOES NOT COMPILE - Why?
//
// We are not changing the dice game so want to use 'let', but it won't compile
// we are changing the delegate, which is declared as 'var' inside the protocol
diceGameAsLet.delegate = self
}
// MARK: - DiceGameDelegate
func gameDidStart( game:DiceGame )
{
print("Game Started")
}
func gameDidEnd( game:DiceGame )
{
print("Game Ended")
}
}
DiceGame is a heterogeneous protocol that you're using as a type; Swift will treat this type as a value type, and hence (just as for a structures), changing its mutable properties will mutate also the instance of the protocol type itself.
If you, however, add the : class keyword to the DiceGame protocol, Swift will treat it as a reference type, allowing you to mutate members of instances of it, without mutating the instance itself. Note that this will constraint the protocol as conformable to only by class types.
protocol DiceGame: class { ... }
With the addition of the above, the mutation of immutable diceGameAsLet:s properties will be allowed.
In this context, it's worth mentioning that the : class keyword is usually used to constrain protocols used as delegates (e.g., DiceGameDelegate in your example) as conformable to only by class types. With this additional constraint, the delegates can be used as types to which the delegate owner (e.g. some class) only hold a weak reference, useful in contexts where a strong reference to the delegate could create a retain cycle.
See e.g. the 2nd part of this answer for details.
The issue is that when you store something as a Protocol, even if it is a class, swift considers them to be a value type, instead of the reference type you are expecting them to be. Therefore, no part of it is allowed to be changed. Take a look at this reference for more information.
I have some swift structs for which protocol compliance is generated with individual extensions with equal methods names which just differ in their return types which are struct dependent. On top of That I want to use them in a generic function which Calls a protocol conforming function for a generic type).
I tried to accomplish this like that:
//: Playground - noun: a place where people can play
import UIKit
protocol FooProt {
typealias T;
static func createMe<T>()->T;
}
struct FooStruct{
}
extension FooStruct: FooProt{
typealias T = FooStruct;
static func createMe () -> FooStruct{
return FooStruct();
}
}
class Creator{
fun createOne<T where T:FooProt>(type:T.Type){
let instance = T.createMe();
}
}
Unfortunately I get the following error :
/var/folders/sn/78_zvfd15d74dzn01mdv258h0000gq/T/./lldb/3741/playground6.swift:7 :17: note: protocol requires function 'createMe()' with type ' () -> T' (aka '<τ_1_0> () -> τ_1_0')
static func createMe()->T;
What exactly doesn't comply here and is there a workaround ?
There are several problems with your code. On the one hand you have defined a protocol with an associated type. However, you define your createMe() method as a generic which uses some other type. I don't think that was your intent. I think your intent was to have a createMe() method that returns the same type as the protocol's associated type. In this case you need to remove the from the createMe() method. Also, the name createMe() implies that you aren't just returning any type, but the type of the object on which this method is being called. In this case, you don't even need an associated type protocol. You just need a protocol with a Self constraint which allows your code to be a bit simpler. In your Creator's createOne method, your type constraint is more complex than needed.
I think you want the following code:
protocol FooProt {
static func createMe()->Self;
}
struct FooStruct{
}
extension FooStruct: FooProt {
static func createMe() -> FooStruct {
return FooStruct();
}
}
class Creator{
func createOne<T:FooProt>(type: T.Type) -> T {
return T.createMe()
}
}
let foo = Creator().createOne(FooStruct.self)
Here is an alternate solution using an initializer in the protocol instead of a static method.
protocol FooProt {
init()
}
struct FooStruct{
}
extension FooStruct: FooProt {
}
class Creator{
func createOne<T:FooProt>(type: T.Type) -> T {
return T.init()
}
}
let foo = Creator().createOne(FooStruct.self)
I couldn't find any good explanation to my questions so I'd like to ask you directly. First of all I'd like to refine my code in this post.
My problem is the protocol AnyObject and the Self type. I didn't implement AnyObject into my code because it is marked with #objc and I don't want any Objective-C stuff involved in my code (don't judge me for that). I also couldn't find any explanation about the Self type. It just worked as expected, but Xcode does not replace Self with the type the static function is called at.
Here is some example:
extension Int : Instance {}
Int.singleton { (customInstanceName) -> Self in 0 } // Self shall be replaced with Int
As you can see Xcode produces a Self instead an Int. Is there any chance I could fix this? Am I right that Self does return the dynamicType and my implementation is fine as it is in my post above? I would really appreciate any good explanation about the Self type.
As you have seen in my code. I am using a custom protocol to check whether my instance is a class or not. Is there any other shiny implementation to check my instances if they are classes or structure types, or am I forced to use AnyObject if I want to get rid of my ClassInstance protocol?
Thank you for your time.
UPDATE:
protocol Test {}
class A : Test {}
struct B : Test {}
let aClass : Test = A()
let aStruct : Test = B()
if let someClass = aClass as? AnyObject {
print(someClass) // only this will print
}
if let someStruct = aStruct as? AnyObject {
print(someStruct)
}
This will work, but AnyObject is still marked as an #objc protocol.
The Self type can be only used in protocols where it is a implicit typealias of the type which conforms to it:
protocol Testable {
func test() -> Self
}
If you want to conform to this protocol you than have to replace Self with the name of the type. For instance:
struct Product: Testable {
func test() -> Product {
return Product()
}
}
Important Edit:
As DevAndArtist pointed out in the comments there is a working class check in Swift 1.2 (without automatic bridging to Objective C) but not Swift 2 (Xcode 7 beta 3; probably a bug):
if instance.dynamicType is AnyClass {
// instance is a class
} else {
// instance is not a class
}
You can see workaround (mainly) for Swift 2 below.
End Edit
With respect to classes you should use AnyObject if you want to keep it simple but you can also use reflection which would be much more effort.
Below you can see some reflection results of string interpolations (only the first few characters):
"\(reflect(classType))" // Swift._ClassMirror
"\(reflect(0))" // Swift._LeafMirror
"\(reflect(enumType))" // Swift._EnumMirror
"\(reflect(structure))" // Swift._StructMirror
"\(reflect([0, 4]))" // Swift._ArrayTypeMirror
"\(reflect(NSDate()))" // Foundation._NSDateMirror
"\(reflect(NSURLRelationship.Contains))" // Swift._EnumMirror
"\(reflect(Int?(2)))" // Swift._OptionalMirror
As you can see enums are consistent if they are not defined in the Swift standard library (unfortunately also Optional...). So you can distinguish also structs and enums:
public enum Type {
case Enum, Class, Struct
}
public func getType<T>(anything: T) -> Type {
if anything is AnyObject {
return .Class
}
if "\(reflect(anything))".hasPrefix("Swift._EnumMirror") {
return .Enum
}
return .Struct
}
So for a better result you have to put some effort into it to differentiate between all the different cases.
But the easiest way to distinguish only between reference types and value types (aka classes and structs/enums) is still (unfortunately only works for own declared structs and not built in types because they can be bridged to Objective C; I'm working on it...):
if instance is AnyObject {}
// or: if instance is of type Any
if let classInstance = instance as? AnyObject {}
Both this declaration
protocol SomeProtocol : AnyObject {
}
and this declaration
protocol SomeProtocol : class {
}
seem to make it so that only classes can conform to this protocol (i.e. that the instances of the protocol are references to objects), and have no other effects.
Is there any difference between them? Should one be preferred over the other? If not, why is there two ways to do the same thing?
I am using the latest released Xcode 6.3.1.
This was answered by an official Swift developer (Slava_Pestov) on the Swift forums. Here is the summary:
You should use AnyObject (protocol SomeProtocol: AnyObject).
AnyObject and class are equivalent. There is no difference.
class will eventually be deprecated.
Regarding the answer https://forums.swift.org/t/class-only-protocols-class-vs-anyobject/11507/4, this answer is deprecated. These words are the same now.
DEPRECATED
Update: After consulting with the powers that be, the two definitions are supposed to be equivalent, with AnyObject being used as a stand-in while class was being finished. In the future the latter will obviate the former but, for now, they do present a few minor differences.
The difference lies in the semantics of #objc declarations. With AnyObject, the expectation is that conforming classes may or may not be proper Objective-C objects, but the language treats them as such anyway (in that you lose static dispatch sometimes). The takeaway from this is that you can treat an AnyObject et al. protocol constraint as a way to ask for #objc member functions as shown in the example in documentation for AnyObject in the STL:
import Foundation
class C {
#objc func getCValue() -> Int { return 42 }
}
// If x has a method #objc getValue()->Int, call it and
// return the result. Otherwise, return nil.
func getCValue1(x: AnyObject) -> Int? {
if let f: ()->Int = x.getCValue { // <===
return f()
}
return nil
}
// A more idiomatic implementation using "optional chaining"
func getCValue2(x: AnyObject) -> Int? {
return x.getCValue?() // <===
}
// An implementation that assumes the required method is present
func getCValue3(x: AnyObject) -> Int { // <===
return x.getCValue() // x.getCValue is implicitly unwrapped. // <===
}
The same example falls over immediately if you change that to a class-deriving protocol:
import Foundation
protocol SomeClass : class {}
class C : SomeClass {
#objc func getCValue() -> Int { return 42 }
}
// If x has a method #objc getValue()->Int, call it and
// return the result. Otherwise, return nil.
func getCValue1(x: SomeClass) -> Int? {
if let f: ()->Int = x.getCValue { // <=== SomeClass has no member 'getCValue'
return f()
}
return nil
}
// A more idiomatic implementation using "optional chaining"
func getCValue2(x: SomeClass) -> Int? {
return x.getCValue?() // <=== SomeClass has no member 'getCValue'
}
// An implementation that assumes the required method is present
func getCValue3(x: SomeClass) -> Int { // <===
return x.getCValue() // <=== SomeClass has no member 'getCValue'
}
So it seems class is a more conservative version of AnyObject that should be used when you only care about reference semantics and not about dynamic member lookups or Objective-C bridging.
In the Swift programming language guide for protocols, under the Class-Only Protocols section. It only mentioned AnyObject, but not class.
You can limit protocol adoption to class types (and not structures or enumerations) by adding the AnyObject protocol to a protocol’s inheritance list.
protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol {
// class-only protocol definition goes here
}
For that reason, I will suggest using AnyObject over class for new code or new project. Other than that, I don't see any obvious difference between them.
From 2021, Xcode 12.5, Big Sur OS:
Usage of class is deprecated by apple.
Use AnyObject instead.
Happy Coding.
AnyObject is a protocol to which all classes implicitly conform (source). So I would say there is no difference: you can use either to require class constraint.
If you open the help (alt-click) in Xcode 9 for class in a line such as protocol P: class {}, you will get typealias AnyObject.
Thus, the code compiled (in Swift 4) will be the same whether you constrain the protocol to class or AnyObject.
That said, there is also the question of style and future options — a future Swift version might want to treat class and AnyObject differently in some subtle way, even if that is not the case right now.
(Edit: This has finally happened in Swift 5.4/Xcode 12.5.)
I misspoke before. #MartinR should really answer this, since he's the one who corrected me and provided the correct information.
The real difference is that a protocol with the class qualifier can only be applied to a class, not a struct or enum.
Martin, why don't you answer and the OP can accept your answer?