I tend to only put the necessities (stored properties, initializers) into my class definitions and move everything else into their own extension, kind of like an extension per logical block that I would group with // MARK: as well.
For a UIView subclass for example, I would end up with an extension for layout-related stuff, one for subscribing and handling events and so forth. In these extensions, I inevitably have to override some UIKit methods, e.g. layoutSubviews. I never noticed any issues with this approach -- until today.
Take this class hierarchy for example:
public class C: NSObject {
public func method() { print("C") }
}
public class B: C {
}
extension B {
override public func method() { print("B") }
}
public class A: B {
}
extension A {
override public func method() { print("A") }
}
(A() as A).method()
(A() as B).method()
(A() as C).method()
The output is A B C. That makes little sense to me. I read about Protocol Extensions being statically dispatched, but this ain't a protocol. This is a regular class, and I expect method calls to be dynamically dispatched at runtime. Clearly the call on C should at least be dynamically dispatched and produce C?
If I remove the inheritance from NSObject and make C a root class, the compiler complains saying declarations in extensions cannot override yet, which I read about already. But how does having NSObject as a root class change things?
Moving both overrides into their class declaration produces A A A as expected, moving only B's produces A B B, moving only A's produces C B C, the last of which makes absolutely no sense to me: not even the one statically typed to A produces the A-output any more!
Adding the dynamic keyword to the definition or an override does seem to give me the desired behavior 'from that point in the class hierarchy downwards'...
Let's change our example to something a little less constructed, what actually made me post this question:
public class B: UIView {
}
extension B {
override public func layoutSubviews() { print("B") }
}
public class A: B {
}
extension A {
override public func layoutSubviews() { print("A") }
}
(A() as A).layoutSubviews()
(A() as B).layoutSubviews()
(A() as UIView).layoutSubviews()
We now get A B A. Here I cannot make UIView's layoutSubviews dynamic by any means.
Moving both overrides into their class declaration gets us A A A again, only A's or only B's still gets us A B A. dynamic again solves my problems.
In theory I could add dynamic to all overrides I ever do but I feel like I'm doing something else wrong here.
Is it really wrong to use extensions for grouping code like I do?
Extensions cannot/should not override.
It is not possible to override functionality (like properties or methods) in extensions as documented in Apple's Swift Guide.
Extensions can add new functionality to a type, but they cannot override existing functionality.
Swift Developer Guide
The compiler is allowing you to override in the extension for compatibility with Objective-C. But it's actually violating the language directive.
😊That just reminded me of Isaac Asimov's "Three Laws of Robotics" 🤖
Extensions (syntactic sugar) define independent methods that receive their own arguments. The function that is called for i.e. layoutSubviews depends on the context the compiler knows about when the code is compiled. UIView inherits from UIResponder which inherits from NSObject so the override in the extension is permitted but should not be.
So there's nothing wrong with grouping but you should override in the class not in the extension.
Directive Notes
You can only override a superclass method i.e. load() initialize()in an extension of a subclass if the method is Objective-C compatible.
Therefore we can take a look at why it is allowing you to compile using layoutSubviews.
All Swift apps execute inside the Objective-C runtime except for when using pure Swift-only frameworks which allow for a Swift-only runtime.
As we found out the Objective-C runtime generally calls two class main methods load() and initialize() automatically when initializing classes in your app’s processes.
Regarding the dynamic modifier
From the Apple Developer Library (archive.org)
You can use the dynamic modifier to require that access to members be dynamically dispatched through the Objective-C runtime.
When Swift APIs are imported by the Objective-C runtime, there are no guarantees of dynamic dispatch for properties, methods, subscripts, or initializers. The Swift compiler may still devirtualize or inline member access to optimize the performance of your code, bypassing the Objective-C runtime. 😳
So dynamic can be applied to your layoutSubviews -> UIView Class since it’s represented by Objective-C and access to that member is always used using the Objective-C runtime.
That's why the compiler allowing you to use override and dynamic.
One of the goals of Swift is static dispatching, or rather the reduction of dynamic dispatching. Obj-C however is a very dynamic language. The situation you're seeing is borne out of the link between the 2 languages and the way they work together. It shouldn't really compile.
One of the main points about extensions is that they are for extending, not for replacing / overriding. It's clear from both the name and the documentation that this is the intention. Indeed if you take out the link to Obj-C from your code (remove NSObject as the superclass) it won't compile.
So, the compiler is trying to decide what it can statically dispatch and what it has to dynamically dispatch, and it's falling through a gap because of the Obj-C link in your code. The reason dynamic 'works' is because it's forcing Obj-C linking on everything so it's all always dynamic.
So, it isn't wrong to use extensions for grouping, that's great, but it is wrong to override in extensions. Any overrides should be in the main class itself, and call out to extension points.
There is a way to achieve a clean separation of class signature and implementation (in extensions) while maintaining the ability to have overrides in subclasses. The trick is to use variables in place of the functions
If you make sure to define each subclass in a separate swift source file, you can use computed variables for the overrides while keeping the corresponding implementation cleanly organized in extensions. This will circumvent Swift's "rules" and will make your class's API/signature neatly organized in one place:
// ---------- BaseClass.swift -------------
public class BaseClass
{
public var method1:(Int) -> String { return doMethod1 }
public init() {}
}
// the extension could also be in a separate file
extension BaseClass
{
private func doMethod1(param:Int) -> String { return "BaseClass \(param)" }
}
...
// ---------- ClassA.swift ----------
public class A:BaseClass
{
override public var method1:(Int) -> String { return doMethod1 }
}
// this extension can be in a separate file but not in the same
// file as the BaseClass extension that defines its doMethod1 implementation
extension A
{
private func doMethod1(param:Int) -> String
{
return "A \(param) added to \(super.method1(param))"
}
}
...
// ---------- ClassB.swift ----------
public class B:A
{
override public var method1:(Int) -> String { return doMethod1 }
}
extension B
{
private func doMethod1(param:Int) -> String
{
return "B \(param) added to \(super.method1(param))"
}
}
Each class's extension are able to use the same method names for the implementation because they are private and not visible to each other (as long as they are in separate files).
As you can see inheritance (using the variable name) works properly using super.variablename
BaseClass().method1(123) --> "BaseClass 123"
A().method1(123) --> "A 123 added to BaseClass 123"
B().method1(123) --> "B 123 added to A 123 added to BaseClass 123"
(B() as A).method1(123) --> "B 123 added to A 123 added to BaseClass 123"
(B() as BaseClass).method1(123) --> "B 123 added to A 123 added to BaseClass 123"
This answer it not aimed at the OP, other than the fact that I felt inspired to respond by his statement, "I tend to only put the necessities (stored properties, initializers) into my class definitions and move everything else into their own extension ...". I'm primarily a C# programmer, and in C# one can use partial classes for this purpose. For example, Visual Studio places the UI-related stuff in a separate source file using a partial class, and leaves your main source file uncluttered so you don't have that distraction.
If you search for "swift partial class" you'll find various links where Swift adherents say that Swift doesn't need partial classes because you can use extensions. Interestingly, if you type "swift extension" into the Google search field, its first search suggestion is "swift extension override", and at the moment this Stack Overflow question is the first hit. I take that to mean that problems with (lack of) override capabilities are the most searched-for topic related to Swift extensions, and highlights the fact that Swift extensions can't possibly replace partial classes, at least if you use derived classes in your programming.
Anyway, to cut a long-winded introduction short, I ran into this problem in a situation where I wanted to move some boilerplate / baggage methods out of the main source files for Swift classes that my C#-to-Swift program was generating. After running into the problem of no override allowed for these methods after moving them to extensions, I ended up implementing the following simple-minded workaround. The main Swift source files still contain some tiny stub methods that call the real methods in the extension files, and these extension methods are given unique names to avoid the override problem.
public protocol PCopierSerializable {
static func getFieldTable(mCopier : MCopier) -> FieldTable
static func createObject(initTable : [Int : Any?]) -> Any
func doSerialization(mCopier : MCopier)
}
.
public class SimpleClass : PCopierSerializable {
public var aMember : Int32
public init(
aMember : Int32
) {
self.aMember = aMember
}
public class func getFieldTable(mCopier : MCopier) -> FieldTable {
return getFieldTable_SimpleClass(mCopier: mCopier)
}
public class func createObject(initTable : [Int : Any?]) -> Any {
return createObject_SimpleClass(initTable: initTable)
}
public func doSerialization(mCopier : MCopier) {
doSerialization_SimpleClass(mCopier: mCopier)
}
}
.
extension SimpleClass {
class func getFieldTable_SimpleClass(mCopier : MCopier) -> FieldTable {
var fieldTable : FieldTable = [ : ]
fieldTable[376442881] = { () in try mCopier.getInt32A() } // aMember
return fieldTable
}
class func createObject_SimpleClass(initTable : [Int : Any?]) -> Any {
return SimpleClass(
aMember: initTable[376442881] as! Int32
)
}
func doSerialization_SimpleClass(mCopier : MCopier) {
mCopier.writeBinaryObjectHeader(367620, 1)
mCopier.serializeProperty(376442881, .eInt32, { () in mCopier.putInt32(aMember) } )
}
}
.
public class DerivedClass : SimpleClass {
public var aNewMember : Int32
public init(
aNewMember : Int32,
aMember : Int32
) {
self.aNewMember = aNewMember
super.init(
aMember: aMember
)
}
public class override func getFieldTable(mCopier : MCopier) -> FieldTable {
return getFieldTable_DerivedClass(mCopier: mCopier)
}
public class override func createObject(initTable : [Int : Any?]) -> Any {
return createObject_DerivedClass(initTable: initTable)
}
public override func doSerialization(mCopier : MCopier) {
doSerialization_DerivedClass(mCopier: mCopier)
}
}
.
extension DerivedClass {
class func getFieldTable_DerivedClass(mCopier : MCopier) -> FieldTable {
var fieldTable : FieldTable = [ : ]
fieldTable[376443905] = { () in try mCopier.getInt32A() } // aNewMember
fieldTable[376442881] = { () in try mCopier.getInt32A() } // aMember
return fieldTable
}
class func createObject_DerivedClass(initTable : [Int : Any?]) -> Any {
return DerivedClass(
aNewMember: initTable[376443905] as! Int32,
aMember: initTable[376442881] as! Int32
)
}
func doSerialization_DerivedClass(mCopier : MCopier) {
mCopier.writeBinaryObjectHeader(367621, 2)
mCopier.serializeProperty(376443905, .eInt32, { () in mCopier.putInt32(aNewMember) } )
mCopier.serializeProperty(376442881, .eInt32, { () in mCopier.putInt32(aMember) } )
}
}
Like I said in my introduction, this doesn't really answer the OP's question, but I'm hoping this simple-minded workaround might be helpful to others who wish to move methods from the main source files to extension files and run into the no-override problem.
Use POP (Protocol-Oriented Programming) to override functions in extensions.
protocol AProtocol {
func aFunction()
}
extension AProtocol {
func aFunction() {
print("empty")
}
}
class AClass: AProtocol {
}
extension AClass {
func aFunction() {
print("not empty")
}
}
let cls = AClass()
cls.aFunction()
Just wanted to add that for Objective-C classes, two separate categories can end up overwriting the same method, and it this case... well... unexpected things can happen.
The Objective-C runtime doesn't make any guarantees about which extension will be used, as described by Apple here:
If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime. This is less likely to be an issue if you’re using categories with your own classes, but can cause problems when using categories to add methods to standard Cocoa or Cocoa Touch classes.
It's a good thing Swift prohibits this for pure Swift classes, since this kind of overly-dynamic behaviour is a potential source of hard to detect and investigate bugs.
i am trying to do swift protocol style programming in dart because i think its clean. So the question is:
lets say i have a protocol where i must implement the methods in it, so in the class that i use the delegate, i will always have extension outside of the main class and put the delegate method inside, then next time i can even put the delegate methods in a different file, also it has good readability, for example:
// delegate method here
extension mainClass{
void delegateMethod(){}
}
then i try to do this in dart with extension, but there is an error at the home page class because it cannot find the mixin method :
class HomePage with delegateOne{
libraryexample.delegate = this;
}
extension delegateMethod on HomePage {
String getDescriptionForIndex(int index) {
// TODO: implement getDescriptionURLForIndex
return "description";
}
}
mixin delegateOne {
String getDescriptionForIndex(int index);
}
Extension methods are syntactic sugar; they don't actually add a method to instances of the class, and they therefore won't help if you need your class to conform to some interface.
I'm not familiar with "Swift protocol style programming", but typically in Dart, classes use implements to satisfy interfaces. For example:
abstract class delegateOne {
String getDescriptionForIndex(int index);
}
class HomePage implements delegateOne {
#override
String getDescriptionForIndex(int index) {
// TODO: implement getDescriptionURLForIndex
return "description";
}
}
I need to know if Swift language has some way to achieve this feature of marking some elements of an enum. For instance, I have a set of properties in Swift enum and some of the properties are animatable while others are not. I want something like:
public enum Property: String {
case prop1,
case prop2 #animatable,
case prop3 #animatable,
case prop4,
case prop5 #animatable,
...
...
...
case prop100
}
And then in a function I can pass an argument such as:
func animate(_ prop: Property #animatable, startTime:CFTimeInterval, duration:CFTimeInterval) {
}
So I pass a property which are elements of enum that are only animatable, not others. Is there a clean way to achieve something like this in Swift language (Swift 5)?
I watched "Protocol-Oriented Programming in Swift" and read the related docs, but I still think there is a conflict in the following sample code (try it in a Playground).
protocol X {
// The important part is "static" keyword
static var x: String { get }
}
extension X {
// Here "static" again
static var x: String {
get {
return "xxx"
}
}
}
// Now I'm going to use the protocol in a class, BUT
// in classes "static" is like "final class",
// i.e. CAN'T BE OVERRIDDEN, right?
// But I'd prefer to have the ability to override that property,
// so I'll try to use "class" keyword.
// Will it break the program? (spoiler: no!)
class Y: X {
// Here we are allowed to use "class" keyword (but why?).
class var x: String {
get {
return "yyy"
}
}
}
class Z: Y {
override class var x: String {
get {
return "zzz"
}
}
}
class Test<T: X> {
func test() -> String {
return T.x
}
}
// And finally the property is successfully overridden (but why?).
print(Test<Z>().test()) // "zzz\n"
Does this actually mean that static keyword from protocol (and possible default implementation) can be legitimately replaced with class keyword when the protocol used in classes? Do you know any references confirming that?
From Language Reference / Declarations we know the following.
Function Declaration
...
Special Kinds of Methods
...
Methods associated with a type rather than an instance of a type must be marked with the static declaration modifier for enumerations and structures or the class declaration modifier for classes.
I.e. the static keyword is (mainly) for enumerations and structures and the class keyword is for classes.
There is also such a note:
Type Variable Properties
...
NOTE
In a class declaration, the keyword static has the same effect as marking the declaration with both the class and final declaration modifiers.
I.e. the static keyword actually can be used in class declaration and will mean final class.
So what about protocols?
Protocol Method Declaration
...
To declare a class or static method requirement in a protocol declaration, mark the method declaration with the static declaration modifier. Classes that implement this method declare the method with the class modifier. Structures that implement it must declare the method with the static declaration modifier instead. If you’re implementing the method in an extension, use the class modifier if you’re extending a class and the static modifier if you’re extending a structure.
Here the docs state that we should replace the static keyword from protocol declaration with the class keyword when implementing the protocol in a class or a class extension (and this is the exact answer to the original question).
Bonus
There are two cases in which protocol adoption will be restricted to classes only. The first (and the least explicit) is when a protocol includes optional members:
Protocol Declaration
...
By default, types that conform to a protocol must implement all properties, methods, and subscripts declared in the protocol. That said, you can mark these protocol member declarations with the optional declaration modifier to specify that their implementation by a conforming type is optional. The optional modifier can be applied only to protocols that are marked with the objc attribute. As a result, only class types can adopt and conform to a protocol that contains optional member requirements. ...
And the second (explicit; the next paragraph):
To restrict the adoption of a protocol to class types only, mark the protocol with the class requirement by writing the class keyword as the first item in the inherited protocols list after the colon. ...
But neither of them changes the rules considering the static and the class keywords applicability.
Is it possible to overload a protocol function and have the correct definition be called when dealing directly with the protocol type?
Here's some code to illustrate the issue
protocol SomeProtocol {
func doSomething<T>(obj: T)
}
class SomeClass : SomeProtocol {
func doSomething<T>(obj: T) {
print("Generic Method")
}
func doSomething(obj: String) {
print(obj)
}
}
let testClass = SomeClass()
testClass.doSomething("I will use the string specific method")
(testClass as SomeProtocol).doSomething("But I will use the generic method")
Edit: To clarify, the code works. I want to know why both calls do not use the string specific method.
Double Edit: Removed the intermediary dispatch class for a simpler example
Is this a bug, current limitation, or intended functionality? If this is intended, can someone please explain why?
Swift 2.0, Xcode 7.0
Answer
You can't overload a protocol function and expect the correct definition to be called. This is because the definition to call is picked at compile time. Since the compiler doesn't know the concrete type, it chooses the only definition known at compile time, doSomething<T>.
I tested your code here http://swiftstub.com/ and it worked fine.
First it prints "I will use the specific method" and then "Generic Method":
I will use the specific methodGeneric Method