Can anyone please explain class only protocols to me in Swift. I understand what protocols are and why we use them. I also understand that its recommended to use class only protocols when we use reference type objects in it and want to limit the protocol conformation to classes only. However, I can't find any good answer to support that recommendation. Why is it recommended? What is the drawbacks of using normal protocols in that case.
One use case:
You have a "delegate" protocol and someone wants to have a weak property of that protocol type. weak can only be used on reference types; therefore, the protocol must be class-only.
Another use case used to be to extend reference types to adopt protocols. You can't extend AnyObject itself (to inherit from another protocol, or for any other reason) but you have any reference types you want adopt a protocol inherited from AnyObject.
For example, having class equality based on unique identity is often a perfectly fine solution, so you could adopt EquatableObject:
public protocol EquatableObject: AnyObject, Equatable { }
public extension EquatableObject {
static func == (class0: Self, class1: Self) -> Bool {
class0 === class1
}
}
But now, we have protocol extension constraints (using where clauses), so instead of extending AnyObject, we can extend the protocol, and use AnyObject as a constraint. It looks backwards but operates identically, and removes the need for our own protocol. 🥳
public extension Equatable where Self: AnyObject {
static func == (class0: Self, class1: Self) -> Bool {
class0 === class1
}
}
Related
Suppose I have this:
protocol MyStuff: Hashable {
var stuff: String { get }
}
extension MyStuff {
func hash(into hasher: inout Hasher) {
hasher.combine(stuff)
}
static func == (lhs: Self, rhs: Self) -> Bool {
return lhs.stuff == rhs.stuff
}
}
struct Stuff: MyStuff {
let stuff: String
}
Where I have a protocol which conforms to Hashable, and I extend that protocol to implement the requirements of Hashable. This works great.
I'm now trying to use the api like this:
let set: Set<MyStuff> = [Stuff(stuff: "Stuff")]
However I get this error:
Protocol 'MyStuff' as a type cannot conform to 'Hashable'
This question is actually a follow up to this answer written by George, and in that answer it says that
I believe SE-0309 (Unlock existentials for all protocols) could fix this.
I was wondering if that (the above link) would actually apply into this situation (listed in the code block above), as I'm having trouble fully understanding the proposal.
There are a couple of issues here, not necessarily due to the code you wrote, but due to how Swift is currently designed:
Protocols don't conform to other protocols, which means that MyStuff is not a sub-type of Hashable, which means you cannot use it as argument for the generic Set
Protocols with associated types, or self requirements, like Equatable, from which Hashable derives, can't be used as generic arguments, can only be used as generic constraints
The compiler runs into issue #1 from above, and that one gives the (clear maybe) message that protocols as types cannot be used as generic arguments instead of other protocols.
SE-0309 might solve issue #2, however you're stuck on #1, so you'll have to change your design.
Solutions? As others have suggested in the comments:
use a type eraser (e.g. https://stackoverflow.com/a/64476569/1974224)
use classes, and replace the protocol by a base class
I have a base protocol that describes the router behavior:
protocol BaseRouterProtocol: AnyObject {
associatedtype View: MainView
func dismiss(viewController: ViewController<View>?)
}
extension BaseRouterProtocol {
func dismiss(viewController: ViewController<View>?) {
viewController?.navigationController?.popViewController(animated: true)
}
}
I want to adopt this protocol to another like this:
protocol StartRouterProtocol: BaseRouterProtocol where View == StartView {
func showTermsVC()
func showSignInVC()
}
But when I create a variable of this type:
let router: StartRouterProtocol
Compiler throws me an error:
Protocol 'StartRouterProtocol' can only be used as a generic constraint because it has Self or associated type requirements
Why does this happening if I have described the type that I expect?
Once a protocol has an associated type, that protocol can't be used as a type by itself for instance declarations-- only for generic constraints and declaring conformance.
So in this case, Swift is saying "yeah, but what is the concrete type for StartRouterProtocol's associated type?"
In this case, it's asking you to either:
Use a concrete type directly, i.e. let router: MyStartViewClass with this conformance declaration, elsewhere: class MyStartViewClass: StartRouterProtocol { ... })
OR, push the need for a concrete type up one layer, as a generic constraint i.e.
class MyRouterController<T: StartRouterProtocol> {
let router: T
}
This is probably not what you were hoping for, but unfortunately associated types add complexity to how you use protocols, especially if you're familiar with generics & interfaces from other languages. (i.e. Java/C# interfaces)
You can work around some aspects of associated types by using a concept called "type erasure" -- but that can cause other problems and complexity.
Here's some further reading that may help: https://medium.com/monstar-lab-bangladesh-engineering/swift-from-protocol-to-associatedtype-then-type-erasure-a4093f6a2d08
I am trying to create a Dictionary (actually a HashSet) keyed on a custom protocol in Swift, but it is giving me the error in the title:
Protocol 'myProtocol' can only be used as a generic constraint because it has Self or associated type requirements
and I can't make heads nor tails of it.
protocol Observing: Hashable { }
var observers = HashSet<Observing>()
Protocol Observing inherits from protocol Hashable, which in turn inherits from protocol Equatable. Protocol Equatable has the following requirement:
func ==(lhs: Self, rhs: Self) -> Bool
And a protocol that contains Self somewhere inside it cannot be used anywhere except in a type constraint.
Here is a similar question.
To solve this you could use generics. Consider this example:
class GenericClass<T: Observing> {
var observers = HashSet<T>()
}
I want a protocol that can only be followed by two classes。(ClassA or ClassB).
protocol MyProtocol where Self: ClassA || ClassB {
}
This requirement almost certainly indicates a design problem. If any piece of your system cares what is implementing the protocol, then the protocol does not capture the whole interface. For example, if at any point you use as? ClassA, then this is a dangerous use of protocols.
It is impossible in Swift to have a protocol that I can see but that I cannot implement. There are languages like Go where it's possible (though still not in exactly the way you describe), but it's impossible in Swift today. As #MartinR notes, the answer by J. Doe doesn't actually change anything. Outside parties can still just implement both protocols.
With that said, it's possible to achieve things very similar to what you're describing.
First, and most obviously: use access controls.
private protocol MyProtocol {}
public class ClassA: MyProtocol {}
public class ClassB: MyProtocol {}
This works if all the classes and protocol are in one file. If not, put them all in a framework and use internal rather than private. Now nothing outside this file/module can implement MyProtocol. The limitation is that MyProtocol is also can't be seen outside the file/module.
This can be resolved by lifting a struct:
public struct MyStruct: MyProtocol {
private let value: MyProtocol
public init(classA: ClassA) { value = classA }
public init(classB: ClassB) { value = classB }
// Methods to conform to MyProtocol by forwarding to value
}
With this, it's impossible to generate a MyStruct that is initialized with anything but a ClassA or ClassB.
Alternately, if you really literally mean "ClassA or ClassB" that's not a struct or a protocol. That's a enum:
enum MyEnum {
case classA(ClassA)
case classB(ClassB)
}
Lets say I have two protocols:
protocol TheirPcol {}
protocol MyPcol {
func extraFunc()
}
What I want to do is to create a protocol extension for 'TheirPcol' which lets extraFunc() work on anything which conforms to 'TheirPcol'. So something like this:
extension TheirPcol : MyPcol { // Error 'Extension of protocol 'TheirPcol' cannot have an inheritance clause.
func extraFunc() { /* do magic */}
}
struct TheirStruct:TheirPcol {}
let inst = TheirStruct()
inst.extraFunc()
The kicker in this is that 'TheirPcol', 'TheirStruct' are all handled by an external API which I do not control. So I'm passed the instance 'inst'.
Can this be done? Or am I going to have to do something like this:
struct TheirStruct:TheirPcol {}
let inst = TheirStruct() as! MyPcol
inst.extraFunc()
It seems there are two use-cases of why you may want to do what you are doing. In the first use-case, Swift will allow you to do what you want, but not very cleanly in the second use-case. I'm guessing you fall into the second category, but I'll go through both.
Extending the functionality of TheirPcol
One reason why you might want to do this is simply to give extra functionality to TheirPcol. Just like the compiler error says, you cannot extend Swift protocols to conform to other protocols. However, you can simply extend TheirPcol.
extension TheirPcol {
func extraFunc() { /* do magic */ }
}
Here, you are giving all objects that conform to TheirPcol the method extraFunc() and giving it a default implementation. This accomplishes the task of extending functionality for the objects conforming to TheirPcol, and if you want it to apply to your own objects as well then you could conform your objects to TheirPcol. In many situations, however, you want to keep MyPcol as your primary protocol and just treat TheirPcol as conforming to MyPcol. Unfortunately, Swift does not currently support protocol extensions declaring conformance to other protocols.
Using TheirPcol objects as if they were MyPcol
In the use case (most likely your use case) where you really do need the separate existence of MyPcol, then as far as I am aware there is no clean way to do what you want yet. Here's a few working but non-ideal solutions:
Wrapper around TheirPcol
One potentially messy approach would be to have a struct or class like the following:
struct TheirPcolWrapper<T: TheirPcol>: MyPcol {
var object: T
func extraFunc() { /* Do magic using object */ }
}
You could theoretically use this struct as an alternative to casting, as in your example, when you need to make an existing object instance conform to MyPcol. Or, if you have functions that accept MyPcol as a generic parameter, you could create equivalent functions that take in TheirPcol, then convert it to TheirPcolWrapper and send it off to the other function taking in MyPcol.
Another thing to note is if you are being passed an object of TheirPcol, then you won't be able to create a TheirPcolWrapper instance without first casting it down to an explicit type. This is due to some generics limitations of Swift. So, an object like this could be an alternative:
struct TheirPcolWrapper: MyPcol {
var object: MyPcol
func extraFunc() { /* Do magic using object */ }
}
This would mean you could create a TheirPcolWrapper instance without knowing the explicit type of the TheirPcol you are given.
For a large project, though, both of these could get messy really fast.
Extending individual objects using a child protocol
Yet another non-ideal solution is to extend each object that you know conforms to TheirPcol and that you know you wish to support. For example, suppose you know that ObjectA and ObjectB conform to TheirPcol. You could create a child protocol of MyPcol and then explicitly declare conformance for both objects, as below:
protocol BridgedToMyPcol: TheirPcol, MyPcol {}
extension BridgedToMyPcol {
func extraFunc() {
// Do magic here, given that the object is guaranteed to conform to TheirPcol
}
}
extension ObjectA: BridgedToMyPcol {}
extension ObjectB: BridgedToMyPcol {}
Unfortunately, this approach breaks down if there are a large number of objects that you wish to support, or if you cannot know ahead of time what the objects will be. It also becomes a problem when you don't know the explicit type of a TheirPcol you are given, although you can use type(of:) to get a metatype.
A note about Swift 4
You should check out Conditional conformances, a proposal accepted for inclusion in Swift 4. Specifically, this proposal outlines the ability to have the following extension:
extension Array: Equatable where Element: Equatable {
static func ==(lhs: Array<Element>, rhs: Array<Element>) -> Bool { ... }
}
While this is not quite what you are asking, at the bottom you'll find "Alternatives considered", which has a sub-section called "Extending protocols to conform to protocols", which is much more what you're trying to do. It provides the following example:
extension Collection: Equatable where Iterator.Element: Equatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
// ...
}
}
Then states the following:
This protocol extension would make any Collection of Equatable elements Equatable, which is a powerful feature that could be put to good use. Introducing conditional conformances for protocol extensions would exacerbate the problem of overlapping conformances, because it would be unreasonable to say that the existence of the above protocol extension means that no type that conforms to Collection could declare its own conformance to Equatable, conditional or otherwise.
While I realize you're not asking for the ability to have conditional conformances, this is the closest thing I could find regarding discussion of protocols being extended to conform to other protocols.