I have a protocol that declares property of type Int. I also have few classes that conforms that Protocol, and now I need to overload operator + for all of them. Since operator + will work based on the property declared, I don't want to implement that operator in each class separately.
So I have
protocol MyProtocol {
var property: Int { get }
}
And I would want to have something like
extension MyProtocol {
static func +(left: MyProtocol, right: MyProtocol) -> MyProtocol {
// create and apply operations and return result
}
}
And actually I successfully did that, but trying to work with it I get an error ambiguous reference to member '+'.
When I move operator overload func to the each class separately, issue disappeared, but I'm still looking for a solution to make it works with Protocols.
Solved, by moving func +... outside of Extension, so it is just a method in the file where MyProtocol is declared
protocol MyProtocol {
var property: Int { get }
}
func +(left: MyProtocol, right: MyProtocol) -> MyProtocol {
// create and apply operations and return result
}
Related
Consider following code
// GENERATED PROTOCOL, CANNOT BE MODIFIED
protocol SomeProtocol {
}
// GENERATED CLASS, CANNOT BE MODIFIED
class A {
}
// GENERATED CLASS, CANNOT BE MODIFIED
class B: A, SomeProtocol {
}
// I CAN CHANGE ONLY FROM HERE BELOW
extension SomeProtocol {
func someMethod() {
print("protocol implementation")
}
}
extension B {
func someMethod() {
print("class implementation")
}
}
let some: SomeProtocol = B()
some.someMethod() //this prints "protocol implementation"
I want some.someMethod() to print "class implementation". I know there are ways to achieve this, one would be to add in SomeProtocol someMethod, but, unfortunately, I cannot change none of SomeProtocol, A or B, these are generated. I can only play with extensions. Is there a way to achieve this without touching the 3 mentioned before?
If you declare the variable as the protocol type, it will always take the default implementation of the protocol method, since the method is declared in an extension of the protocol.
Without adding the method to the protocol declaration itself (which you've stated to be not possible for you), the only way to access the specific implementation of your conforming type is to downcast some to B or store it as B in the first place.
let some: SomeProtocol = B()
some.someMethod() //this prints "protocol implementation"
(some as? B)?.someMethod() // this prints "class implementation"
let someB = B()
someB.someMethod() // this prints "class implementation"
I want to extend protocol with a default implementation that satisfies OR ( || ) constraint.
class A { }
class B { }
protocol SomeProtocol { }
/// It will throw error for ||
extension SomeProtocol where Self: A || Self: B {
}
You can't extend a protocol with OR as you can't do it in a if let, because with this the compiler infers the type of self or of the var, so if it conforms 2 types, the compiler doesn't know of what type is self.
(When you type self. or any var. the compiler always knows what type of var is in compiler type, in that case it would be in runtime). So the easiest way is to make that the 2 types conforms a protocol and do a extension of that protocol. So the compiler knows that self conforms a protocol and he doesn't care of the exact type of Self (But you will be able to use only the properties declared in the protocol).
protocol ABType {
// Properties that you want to use in your extension.
}
class A: ABType, SomeProtocol { }
class B: ABType, SomeProtocol { }
protocol SomeProtocol { }
extension SomeProtocol where Self: ABType {
}
Also if you want to apply the extension to both types you have to do it one by one.
extension A: SomeProtocol { }
extension B: SomeProtocol { }
// Silly example:
(In this case is not really useful, but it is just to show how to make to make 2 classes conforms a protocol and to make a extension of it using a method declared in that protocol and creating a default implementation.)
protocol ABType {
func getName()
}
class AClass: ABType {
func getName() {
print ("A Class")
}
}
class BClass: ABType, someProtocol {
func getName() {
print ("B Class")
}
}
protocol someProtocol {
func anotherFunc()
}
extension someProtocol where Self: ABType {
func anotherFunc() {
self.getName()
}
}
let a = AClass()
// a.anotherFunc() <- Error, A cant call anotherFunc
let b = BClass()
b.anotherFunc()
I am developing an app to increase a little more my knowledge about swift. One of my questions if is it possible to delegate a optional function with a structure as an argument.
What yes Im able to do:
#objc protocol someProtocol {
optional func optionalFunc(someClass: someClass)
}
class someClass: NSObject {
}
But, what I want to do (problems representing a structure in objc):
#objc protocol someProtocol {
optional func optionalFunc(someStructure: someStructure)
}
struct someStructure {
}
And Im not able to find the way to solve this problem.
And the other thing I want, is similar to this but with enums instead of structs:
#objc protocol someProtocol {
optional func optionalFunc(someEnum: someEnum)
}
enum someEnum {
case example
}
If somebody can help me, I will be very grateful!
Lot of thanks! Luciano!
Swift 2.0 lets you do default implementations of protocols.
protocol someProtocol {
func optionalFunc(someStructure: SomeStructure)
}
extension someProtocol {
func optionalFunc(someStructure: SomeStructure){
// optional, leave empty
}
}
struct SomeStructure {
}
This way you can get around using the optional-decoration and do what you wanted.
You cannot pass the parameters as struct or enum, because it's only valid on Swift language, so it cannot be represented in Objective-C.
Another approach, you can declare a function as variable instead of func:
protocol someProtocol {
var optionalFunc: (someStructure) -> ()? { get set}
}
Implementation:
class someClass : someProtocol {
var optionalFunc: (someStructure) -> ()? = { yourStruct in
// Do anything with yourStruct
return
}
}
Using:
var someVar:someClass = someClass()
var result = someVar.optionalFunc(someStructure())
The result is a ()?. If you do not implement the variable, result will nil
EDIT:
This question was written before swift added the some keyword, making it obsolete
In objective-c I could declare a method with a return type:
-(UIView<MyProtocol> *)someMethod;
In this example the method returns a UIView that conforms to a protocol MyProtocol.
I want to do something like that in swift:
protocol MyProtocol {
var someProperty : Int {get set}
}
protocol MyDelegate {
func someMethod() -> UIView : MyProtocol // the view should conform to the protocol - I don't care what kind of view it is - I don't want to define a specific type of view
}
In general - The delegate should return a UIView with the var "someProperty"
I don't want to define a concrete UIView class.
I want the user to be able to return any type of UIView (As long as it conforms to the protocol)
The syntax I wrote is invalid - How should I be writing it?
You could just use the protocol as type:
protocol MyDelegate {
func someMethod() -> MyProtocol
}
And use it like this:
protocol MyProtocol {
var someProperty : Int {get set}
}
class CustomView: UIView, MyProtocol {
var someProperty = 2
}
protocol MyDelegate {
func someMethod() -> MyProtocol
}
struct Delegate: MyDelegate {
func someMethod() -> MyProtocol {
return CustomView()
}
}
let delegate = Delegate()
let view = delegate.someMethod()
let property = view.someProperty // property = 2
This is not possible in Swift. Not everything possible in Obj-C has to be possible in Swift. When creating a type requirement you can only combine protocols using the protocol<..., ...> syntax but you can't combine a class and a protocol.
Technically, this should be good for your architecture. You can probably find a workaround but I would advice against it. There is a reason to avoid combining classes with protocols because the interfaces are much more difficult to handle. Most OOP languages don't have that syntax. Many commonly used languages don't even have a syntax to combine protocols.
protocol MyProtocol {
var someProperty : Int {get set}
}
protocol MyDelegate {
func someMethod<T: UIView & MyProtocol>() -> T // the view should conform to the protocol - I don't care what kind of view it is - I don't want to define a specific type of view
}
class MyDelegateTestView : UIView, MyProtocol {
var someProperty: Int = 10
}
class MyDelegateTestClass : MyDelegate {
func someMethod<T>() -> T where T : UIView, T : MyProtocol {
return MyDelegateTestView() as! T
}
}
The question was written before the days of swift-ui
The "some" keyword has solved it by allowing opaque types to be returned from functions
protocol MyDelegate {
func someMethod() -> some MyProtocol
}
Here below is a way.
func myMethod(string: String) -> MyClass:MyProtocol? {
}
You can use without optional type as MyClass: MyProtocol.
I'm trying to create a delegate protocol that implements a function which passes an array of a generic type. I've tried several combinations but none of them seem to do the trick.
This is the most approximate thing i've reached to. This is the protocol:
protocol APIControllerProtocol {
typealias T
func didReceiveAPIResults(results: [T])
}
And this is the the delegator object:
class APIController<U:APIControllerProtocol> {
typealias ElementType = U
var delegate: ElementType?
init(delegate: ElementType){
self.delegate = delegate
}
func getAPIResults(){
// Perform some action before delegation
// "results" is an Array of dictionaries got from NSJSONSerialization
self.delegate?.didReceiveAPIResults(results.map{dict in Album(json:dict)})
}
}
However, the last line get this error: "Album is not convertible to U.T"
"Album" is the model object used to return the results.
What am i doing wrong?
EDIT:
Following Mike S advice, i've made the protocol method didReceiveAPIResults a generic function, and specified what T is in the delegate. However, when receiving and assigning the argument of type T to a property in the delegate, i get the error: "T is not identical to T"
class TestDelegate: APIControllerProtocol {
typealias T = Album
var albums:[T] = [T]()
func didReceiveAPIResults<T>(results: [T]) {
// ...
self.albums = results //ERROR: "T is not identical to T"
}
}
Your didReceiveAPIResults declaration in APIControllerProtocol needs to be a generic function so that the generic type T is passed along to it correctly.
protocol APIControllerProtocol {
typealias T
func didReceiveAPIResults<T>(results: [T])
}
Note: This means your delegate definition will need to define what T is:
class TestDelegate: APIControllerProtocol {
typealias T = Album
func didReceiveAPIResults<T>(results: [T]) {
// ...
}
}
Update: While the code above does get rid of the original error, it turns out that it acts more like a workaround and doesn't really address the root of the problem.
The real issue seems to be that the compiler is having trouble reconciling what U.T is with no ambiguity. That's actually easy enough to fix though, we just need to give it a more precise definition (note the where clause in the APIController definition):
protocol APIControllerProtocol {
typealias T
func didReceiveAPIResults(results: [T])
}
class APIController<U:APIControllerProtocol where U.T == Album> {
typealias ElementType = U
// ...
}
Note: I removed the <T> that I added to the function in the protocol previously; that's not needed anymore and will end up causing problems later.
With that, the TestDelegate class works as expected (you don't even need the typealias anymore):
class TestDelegate: APIControllerProtocol {
var albums: [Album]? = nil
func didReceiveAPIResults(results: [Album]) {
albums = results
}
}