I have a BaseViewController that is defined this way:
open class BaseViewController<ViewModel: ViewModelType, View: BaseView>: UIViewController { ... }
I'm trying to write an extension for a protocol that has a default behaviour for all my BaseViewControllers. Something like this:
protocol MyProtocol {
func doStuff()
}
extension MyProtocol where Self: BaseViewController<BaseViewModel, BaseView> {
func doStuff() { ... }
}
However. If I only tells that the generic types are the BaseViewModel and BaseView it doesn't works for my subclasses, even though those are the most lower level classes used here.
How can I write my protocol extension in a way it will works for all my BaseViewController classes?
Related
I'm running into an issue where type constrained protocols defined in a separate module aren't acknowledging the constrained type.
For example if I define the following protocol:
public protocol SomeVCProtocol where Self: UIViewController {
func something()
}
And conform to it:
extension SomeViewController: SomeVCProtocol {
func something() { }
}
let someVC: SomeVCProtocol = SomeViewController()
someVC.something()
someVC.present(UIViewController(), animated: true)
There's no issue here. I can call the protocol-specific methods and UIViewController methods. However the moment I define SomeVCProtocol in a separate module, I no longer have access to the UIViewController methods. Is this just a shortcoming of Swift, or am I just missing something?
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 found an interesting behaviour which seems like a bug...
Based on the behaviour described the following articles:
https://medium.com/ios-os-x-development/swift-protocol-extension-method-dispatch-6a6bf270ba94
http://nomothetis.svbtle.com/the-ghost-of-swift-bugs-future
The output is not what I expect, when I add SomeSuperclass, rather than directly adopting the protocol.
protocol TheProtocol {
func method1()
}
extension TheProtocol {
func method1() {
print("Called method1 from protocol extension")
}
func method2NotInProtocol() {
print("Called method2NotInProtocol from protocol extension")
}
}
// This is the difference - adding a superclass
class SomeSuperclass: TheProtocol {
}
// It works as expected when it simply adopts TheProtocol, but not when it inherits from a class that adopts the protocol
class MyClass: SomeSuperclass {
func method1() {
print("Called method1 from MyClass implementation")
}
func method2NotInProtocol() {
print("Called method2NotInProtocol from MyClass implementation")
}
}
let foo: TheProtocol = MyClass()
foo.method1() // expect "Called method1 from MyClass implementation", got "Called method1 from protocol extension"
foo.method2NotInProtocol() // Called method2NotInProtocol from protocol extension
Do you know if this is a bug, or by design? A colleague suggested perhaps mixing inheritance and protocol extensions might not work as expected. I was intending to use the protocol extension to provide a default implementation... if I can't do it, then I will sadly have to mark it #objc and go back to an optional protocol.
From the post The Ghost of Swift Bugs Future, here are the rules for dispatch for protocol extensions that are mentioned at the end of the post.
IF the inferred type of a variable is the protocol:
AND the method is defined in the original protocol THEN the
runtime type’s implementation is called, irrespective of whether
there is a default implementation in the extension.
AND the method is not defined in the original protocol, THEN the
default implementation is called.
ELSE IF the inferred type of the variable is the type THEN the type’s implementation is called.
So in your condition, you're saying that method1() is defined in the protocol and it has been implemented in the subclass. But your superclass is adopting the protocol but it is not implementing the method1() and subclass just inherits from the Superclass and doesn't adopt to the protocols directly. That's why I believe that is the reason when you call foo.method1(), it doesn't invoke the the subclass implementation as stated by point 1 & 2.
But when you do,
class SomeSuperclass: TheProtocol {
func method1(){
print("super class implementation of method1()")}
}
class MyClass : SomeSuperclass {
override func method1() {
print("Called method1 from MyClass implementation")
}
override func method2NotInProtocol() {
print("Called method2NotInProtocol from MyClass implementation")
}
}
and then when you call,
let foo: TheProtocol = MyClass()
foo.method1() // Called method1 from MyClass implementation
foo.method2NotInProtocol()
So what could be the workaround for this bug (which seems to be a bug) is that, you need to implement the protocol method in the superclass and then you need to override the protocol method in the sub class. HTH
Please check the code below:
import UIKit
protocol TheProtocol {
func method1()
func method2NotInProtocol()
}
extension NSObject {
func method1() {
print("Called method1 from protocol extension")
}
func method2NotInProtocol() {
print("Called method2NotInProtocol from protocol extension")
}
}
// This is the difference - adding a superclass
class SomeSuperclass :NSObject, TheProtocol {
override func method1() {
print("Called method1 from SomeSuperclass implementation")
}
}
// It works as expected when it simply adopts TheProtocol, but not when it inherits from a class that adopts the protocol
class MyClass : SomeSuperclass {
override func method1() {
print("Called method1 from MyClass implementation")
}
override func method2NotInProtocol() {
print("Called method2NotInProtocol from MyClass implementation")
}
}
let foo: TheProtocol = MyClass()
foo.method1() // expect "Called method1 from MyClass implementation", got "Called method1 from protocol extension"
foo.method2NotInProtocol() // Called method2NotInProtocol from protocol extension
Instead of writing extension to TheProtocol, write extension to a abstract class(NSObject in above code). This works as expected.
An option that hasn't been considered is to split up your protocol into several smaller ones, such that the superclass does not need to conform to protocols containing methods it does not intend to implement. The subclass can then subscribe to these other protocols separately.
I am trying to implement some sort of delegate broadcaster (Observer Pattern) in Swift to register multiple delegates. To use the "isEqual" function I need the generic to inherit from NSObject
To avoid duplicate code I prepared a generic DelegateBroadcaster:
import UIKit
class DelegateBroadcaster<T : NSObject>: NSObject {
var delegates : [T]
override init() {
delegates = []
}
func addDelegate(newDelegate : T) {
delegates.append(newDelegate)
}
func removeDelegate(oldDelegate : T) {
for i in 0...delegates.count-1 {
if (oldDelegate.isEqual(delegates[i])) {
delegates.removeAtIndex(i)
break
}
}
}
}
and subclass this for any specific broadcaster.
import UIKit
class NavigationControllerBroadcaster : DelegateBroadcaster<UINavigationControllerDelegate> {
}
But I get a strange error: "DelegateBroadcaster requires that 'UINavigationControllerDelegate' inherit from NSObject"
This is strange because the class reference by apple (Class Reference) says that UINavigationControllerDelegate inherits from NSObject.
So why do I get an error?
You are confusing class NSObject (NSObject class) and protocol NSObject (NSObject protocol, in Swift called NSObjectProtocol).
UINavigationControllerDelegate is a protocol and cannot inherit from class NSObject, it inherits from NSObjectProtocol (switch your documentation to Swift, you will see the difference).
UINavigationControllerDelegate is not a concrete type, it is a Protocol and therefore cannot be used as the type signature for the DelegateBroadcaster
I've created a protocol in one of my ViewControllers above the class declaration like so:
#class_protocol protocol CRAAddCredentialDelegate {
func didAddCredential()
}
class CRAAddCredentialTableViewController: UITableViewController {
....
}
However, when I try to conform to this protocol:
class CRAMainViewController: UIViewController, UITableViewDataSource, UITableViewDelegate, CRAAddCredentialDelegate {
....
}
I get an error:
What am I doing wrong?
Have you added the function didAddCredential() to your CRAMainViewController class?
By adding the , CRAAddCredentialDelegate to the list of protocols, you're indicating that your class will provide all of the variables and functions that protocol includes.
So you need to actually provide them.
class CRAAddCredentialTableViewController: UITableViewController {
func didAddCredential() {
// add code here
}
....
}
You should implement the required protocol. In your case func didAddCredential() declared in the protocol CRAAddCredentialDelegate is not implemented , so its giving error.