I have the following class:
class MyClass<T: BaseClass> {
let aThing = T()
func someMethod() {
configure()
}
}
whereas
class SubTypeAOfBaseClass: BaseClass { ... }
class SubTypeBOfBaseClass: BaseClass { ... }
In configure I want to configure aThing depending on its type. Therefore I have created a protocol Configurable and an extension:
protocol Configurable {
func configure()
}
extension MyClass: Configurable where T == SubTypeAOfBaseClass {
func configure() {
print("Configuring SubTypeAOfBaseClass")
aThing.doSomethingA()
}
}
The error that I get is in MyClass.someMethod where I call configure(): "Referencing instance method configure() requires types T and SubTypeAOfBaseClass be equivalent.
Another error when I add the following extension:
extension MyClass: Configurable where T == SubTypeBOfBaseClass {
func configure() {
print("Configuring SubTypeBOfBaseClass")
aThing.doSomethingB()
}
}
The error changes to "No exact matches in call to instance method configure" and in the line
extension MyClass: Configurable where T == SubTypeBOfBaseClass {
I get the error "Conflicting conformance of MyClass to protocol configurable, there cannot more than one conformance, even with different conditional bounds."
It should work, but apparently I am missing something or my understanding of how to achieve what I want is wrong.
Consider the case when T is BaseClass, or when T is AnotherSubclass that I defined as
class AnotherSubclass : BaseClass {
}
What would happen? You haven't declared a conformance to Configure when T is AnotherSubclass!
There's really only two (not bad) choices here.
You want configure to do nothing when T is neither SubTypeAOfBaseClass nor SubTypeBOfBaseClass
you only want MyClass<SubTypeAOfBaseClass> and MyClass<SubTypeBOfBaseClass> to be valid types - MyClass<BaseClass> and MyClass<AnotherSubclass> would give compiler errors.
Choice 2 is not possible in Swift. That would require something similar to the sealed types in Java or Kotlin.
Choice 1 can be done like this:
class BaseClass {
...
func configure() {
}
}
class SubTypeAOfBaseClass: BaseClass {
...
override func configure() {
print("Configuring SubTypeAOfBaseClass")
doSomethingA()
}
}
class SubTypeBOfBaseClass: BaseClass {
...
override func configure() {
print("Configuring SubTypeAOfBaseClass")
doSomethingB()
}
}
class MyClass<T: BaseClass> {
let aThing = T()
func someMethod() {
aThing.configure()
}
}
You might notice that the each implementation of configure has been moved to the base classes. If you want to implement them all in MyClass, you must check the type by hand:
class MyClass<T: BaseClass> {
let aThing = T()
func someMethod() {
if let selfA = self as? MyClass<SubTypeAOfBaseClass> {
selfA.configure()
} else if let selfB = self as? MyClass<SubTypeBOfBaseClass> {
selfB.configure()
}
}
}
extension MyClass where T == SubTypeAOfBaseClass {
func configure() {
print("Configuring SubTypeAOfBaseClass")
aThing.doSomethingA()
}
}
extension MyClass where T == SubTypeBOfBaseClass {
func configure() {
print("Configuring SubTypeBOfBaseClass")
aThing.doSomethingB()
}
}
This is because of the second problem in your code - different parameterisations of a generic type, MyClass<SubTypeAOfBaseClass> and MyClass<SubTypeBOfBaseClass>, can't conform to a protocol differently. This is a limitation of Swift, unfortunately. See here for more info.
Related
Consider the following example:
class ManObj {
func baseFunc() {
print("ManObj baseFunc")
}
}
class SubObj: ManObj {
}
protocol Model {
}
extension Model { // This is protocol extension
func someFunc() { // Protocol extension default implementation
(self as! ManObj).baseFunc()
print("Model implementation")
}
}
extension SubObj: Model {
func someFunc() {
print("SubObj Implementation")
}
}
let list = SubObj()
list.someFunc() // static dispatching
let list2: Model = SubObj()
list2.someFunc() // dynamic dispatching
The output is nicely:
SubObj Implementation
ManObj baseFunc
Model implementation
But I dislike the casting in the line (self as! ManObj).baseFunc().
In fact, I only plan to apply Model protocol to subclasses of ManObj. (But not all subclasses of ManObj are Model though!) So, I tried to change Model to:
extension Model where Self: ManObj {
func someFunc() {
self.baseFunc() // No more casting needed!
print("Model implementation")
}
}
But I'm greeted with error:
list2.someFunc() <- error: 'Model' is not a subtype of 'ManObj'
So, is there a way for me to trigger Model.someFunc from list2 after I constrain Model to where Self: ManObj?
Create an empty class just for type casting
class ManObj {
func baseFunc() {
print("ManObj baseFunc")
}
}
class SubObj: ModelCaster {
func someFunc() {
print("SubObj Implementation")
}
}
protocol Model {
}
extension Model where Self: ModelCaster { // This is protocol extension
func someFunc() { // Protocol extension default implementation
print("Model implementation")
}
}
class ModelCaster: ManObj, Model{
}
let list = SubObj()
list.someFunc() //SubObj Implementation
let list2: ModelCaster = SubObj()
list2.someFunc() //Model implementation
I'm wondering is there a way to work with protocol default implementations in polymorphic style.
Example
protocol RockInterface {
}
extension RockInterface {
func foo() {
print("You Rock")
}
}
extension RockInterface where Self: Metal {
func foo() {
print("Metal")
}
}
extension RockInterface where Self: Grunge {
func foo() {
print("Grunge")
}
}
class Rock: RockInterface {
init() {
foo()
}
}
class Metal: Rock {
}
class Grunge: Rock {
}
let rock = Rock() //prints "You Rock"
let metal = Metal() //prints "You Rock"
let grunge = Grunge() //prints "You Rock"
I expected Metal() to print "Metal", and Grunge to print "Grunge". But it seems that default implementations are solved at compile time instead of runtime. Is my assumption right of wrong? How could I get expected behavior?
The are at least two factors that contribute to the behaviour you see, some within your control, and some not.
functions that are not part of the protocol requirements are statically dispatched. If you want dynamic dispatch, then you'll need to add the method to the protocol declaration:
protocol RockInterface {
func foo()
}
however, the above won't solve your problem, since subclasses inherit the protocol witness table of the parent class. See this excellent answer for more details about this.
I'd also argue you design is a good one, since you tightly coupled the protocol and the classes that conform to that protocol. If you really need the behaviour you described, then one solution would be to drop the protocol extensions, and implement the foo method within each class:
protocol RockInterface {
func foo()
}
class Rock: RockInterface {
init() {
foo()
}
func foo() {
print("You Rock")
}
}
class Metal: Rock {
override func foo() {
print("Metal")
}
}
class Grunge: Rock {
override func foo() {
print("Grunge")
}
}
let rock = Rock() //prints "You Rock"
let metal = Metal() //prints "Metal"
let grunge = Grunge() //prints "Grunge"
I have multiple protocols that have the same function name. Some protocols have associated types, where I can't figure out how to call the functions as I do in non-generic protocols. I get the error: Protocol 'MyProtocol1' can only be used as a generic contraint because it has Self or associated type requirements
Here's what I'm trying to do:
protocol Serviceable {
associatedtype DataType
func get(handler: ([DataType] -> Void)?)
}
struct PostService: Serviceable {
func get(handler: ([String] -> Void)? = nil) {
print("Do something...")
}
}
protocol MyProtocol1: class {
associatedtype ServiceType: Serviceable
var service: ServiceType { get }
}
extension MyProtocol1 {
func didLoad(delegate: Self) {
print("MyProtocol1.didLoad()")
}
}
protocol MyProtocol2: class {
}
extension MyProtocol2 {
func didLoad(delegate: MyProtocol2) {
print("MyProtocol2.didLoad()")
}
}
class MyViewController: UIViewController, MyProtocol1, MyProtocol2 {
let service = PostService()
override func viewDidLoad() {
super.viewDidLoad()
didLoad(self as MyProtocol1) // Error here: Protocol 'MyProtocol1' can only be used as a generic contraint because it has Self or associated type requirements
didLoad(self as MyProtocol2)
}
}
How can I specifically call the function from a generic protocol extension?
It's simple to achieve by turning the protocol into a generic (see below), or by creating a type eraser for these protocols, but this very strongly suggests that you have a design problem and you should redesign your classes and/or extensions. A collision like this suggests strongly that MyStruct is doing too many things itself because it's being pulled in multiple directions by MyProtocol1 and MyProtocol2. There should likely be two objects here instead. (Composition rather than inheritance.)
class MyStruct: MyProtocol1, MyProtocol2 {
let service = PostService()
func prot1Load<T: MyProtocol1>(t: T) {
t.didLoad()
}
func prot2Load<T: MyProtocol2>(t: T) {
t.didLoad()
}
init() {
prot1Load(self)
prot2Load(self)
}
}
To your particular example in the comments, I would use composition rather than inheritance. You're treating protocols like multiple-inheritance, which is almost never right. Instead compose out of things that conform to a protocol.
protocol LoadProviding {
func load()
}
struct MyLoader1: LoadProviding {
func load() {
print("MyLoader1.didLoad()")
}
}
struct MyLoader2: LoadProviding {
func load() {
print("MyLoader2.didLoad()")
}
}
protocol Loader {
var loaders: [LoadProviding] { get }
}
extension Loader {
func loadAll() {
for loader in loaders {
loader.load()
}
}
}
class MyStruct: Loader {
let service = PostService()
let loaders: [LoadProviding] = [MyLoader1(), MyLoader2()]
init() {
loadAll()
}
}
Of course you don't really have to have LoadProviding be a full struct. It could just be a function if that's all you need:
typealias LoadProviding = () -> Void
func myLoader1() {
print("MyLoader1.didLoad()")
}
func myLoader2() {
print("MyLoader2.didLoad()")
}
protocol Loader {
var loaders: [LoadProviding] { get }
}
extension Loader {
func loadAll() {
for loader in loaders {
loader()
}
}
}
class MyStruct: Loader {
let service = PostService()
let loaders: [LoadProviding] = [myLoader1, myLoader2]
init() {
loadAll()
}
}
If you have time to wade through a video on the subject, you may be interested in the Beyond Crusty: Real World Protocols talk from dotSwift. It's about this and similar problems.
Is there a way to create an abstract class in the Swift Language, or is this a limitation just like Objective-C? I'd like to create a abstract class comparable to what Java defines as an abstract class.
There are no abstract classes in Swift (just like Objective-C). Your best bet is going to be to use a Protocol, which is like a Java Interface.
With Swift 2.0, you can then add method implementations and calculated property implementations using protocol extensions. Your only restrictions are that you can't provide member variables or constants and there is no dynamic dispatch.
An example of this technique would be:
protocol Employee {
var annualSalary: Int {get}
}
extension Employee {
var biweeklySalary: Int {
return self.annualSalary / 26
}
func logSalary() {
print("$\(self.annualSalary) per year or $\(self.biweeklySalary) biweekly")
}
}
struct SoftwareEngineer: Employee {
var annualSalary: Int
func logSalary() {
print("overridden")
}
}
let sarah = SoftwareEngineer(annualSalary: 100000)
sarah.logSalary() // prints: overridden
(sarah as Employee).logSalary() // prints: $100000 per year or $3846 biweekly
Notice that this is providing "abstract class" like features even for structs, but classes can also implement the same protocol.
Also notice that every class or struct that implements the Employee protocol will have to declare the annualSalary property again.
Most importantly, notice that there is no dynamic dispatch. When logSalary is called on the instance that is stored as a SoftwareEngineer it calls the overridden version of the method. When logSalary is called on the instance after it has been cast to an Employee, it calls the original implementation (it doesn't not dynamically dispatch to the overridden version even though the instance is actually a Software Engineer.
For more information, check great WWDC video about that feature: Building Better Apps with Value Types in Swift
Note that this answer is targeted at Swift 2.0 and above
You can achieve the same behaviour with protocols and protocol extensions.
First, you write a protocol that acts as an interface for all the methods that have to be implemented in all types that conform to it.
protocol Drivable {
var speed: Float { get set }
}
Then you can add default behaviour to all types that conform to it
extension Drivable {
func accelerate(by: Float) {
speed += by
}
}
You can now create new types by implementing Drivable.
struct Car: Drivable {
var speed: Float = 0.0
init() {}
}
let c = Car()
c.accelerate(10)
So basically you get:
Compile time checks that guarantee that all Drivables implement speed
You can implement default-behaviour for all types that conform to Drivable (accelerate)
Drivable is guaranteed not to be instantiated since it's just a protocol
This model actually behaves much more like traits, meaning you can conform to multiple protocols and take on default implementations of any of them, whereas with an abstract superclass you're limited to a simple class hierarchy.
I think this is the closest to Java's abstract or C#'s abstract:
class AbstractClass {
private init() {
}
}
Note that, in order for the private modifiers to work, you must define this class in a separate Swift file.
EDIT: Still, this code doesn't allow to declare an abstract method and thus force its implementation.
The simplest way is to use a call to fatalError("Not Implemented") into the abstract method (not variable) on the protocol extension.
protocol MyInterface {
func myMethod() -> String
}
extension MyInterface {
func myMethod() -> String {
fatalError("Not Implemented")
}
}
class MyConcreteClass: MyInterface {
func myMethod() -> String {
return "The output"
}
}
MyConcreteClass().myMethod()
After I struggled for several weeks, I finally realized how to translate a Java/PHP abstract class to Swift:
public class AbstractClass: NSObject {
internal override init(){}
public func getFoodToEat()->String
{
if(self._iAmHungry())
{
return self._myFavoriteFood();
}else{
return "";
}
}
private func _myFavoriteFood()->String
{
return "Sandwich";
}
internal func _iAmHungry()->Bool
{
fatalError(__FUNCTION__ + "Must be overridden");
return false;
}
}
public class ConcreteClass: AbstractClass, IConcreteClass {
private var _hungry: Bool = false;
public override init() {
super.init();
}
public func starve()->Void
{
self._hungry = true;
}
public override func _iAmHungry()->Bool
{
return self._hungry;
}
}
public protocol IConcreteClass
{
func _iAmHungry()->Bool;
}
class ConcreteClassTest: XCTestCase {
func testExample() {
var concreteClass: ConcreteClass = ConcreteClass();
XCTAssertEqual("", concreteClass.getFoodToEat());
concreteClass.starve();
XCTAssertEqual("Sandwich", concreteClass.getFoodToEat());
}
}
However I think Apple did not implement abstract classes because it generally uses the delegate+protocol pattern instead. For example the same pattern above would be better done like this:
import UIKit
public class GoldenSpoonChild
{
private var delegate: IStomach!;
internal init(){}
internal func setup(delegate: IStomach)
{
self.delegate = delegate;
}
public func getFoodToEat()->String
{
if(self.delegate.iAmHungry())
{
return self._myFavoriteFood();
}else{
return "";
}
}
private func _myFavoriteFood()->String
{
return "Sandwich";
}
}
public class Mother: GoldenSpoonChild, IStomach
{
private var _hungry: Bool = false;
public override init()
{
super.init();
super.setup(self);
}
public func makeFamilyHungry()->Void
{
self._hungry = true;
}
public func iAmHungry()->Bool
{
return self._hungry;
}
}
protocol IStomach
{
func iAmHungry()->Bool;
}
class DelegateTest: XCTestCase {
func testGetFood() {
var concreteClass: Mother = Mother();
XCTAssertEqual("", concreteClass.getFoodToEat());
concreteClass.makeFamilyHungry();
XCTAssertEqual("Sandwich", concreteClass.getFoodToEat());
}
}
I needed this kind of pattern because I wanted to commonize some methods in UITableViewController such as viewWillAppear etc. Was this helpful?
There is a way for simulating abstract classes using Protocols.
This is an example:
protocol MyProtocol {
func doIt()
}
class BaseClass {
weak var myDelegate: MyProtocol?
init() {
...
}
func myFunc() {
...
self.myDelegate?.doIt()
...
}
}
class ChildClass: BaseClass, MyProtocol {
override init(){
super.init()
self.myDelegate = self
}
func doIt() {
// Custom implementation
}
}
One more way how you can implement abstract class is to block initializer.
I've done it this way:
class Element:CALayer { // IT'S ABSTRACT CLASS
override init(){
super.init()
if self.dynamicType === Element.self {
fatalError("Element is abstract class, do not try to create instance of this class")
}
}
}
It's a really old question but still… Here's a snippet of actual code that compiles on Swift 5.2 and works as intended:
protocol Context {
init() throws
func out(_ aStr: String) throws
// Other stuff
}
class AbstractContext: Context {
required init() throws {
if Self.self === AbstractContext.self {
preconditionFailure("Call to abstract method \(Self.self).\(#function)")
}
}
func out(_ aStr: String) throws {
preconditionFailure("Call to abstract method \(Self.self).\(#function)")
}
// Other stuff
}
class CompileContext: AbstractContext {
required init() throws {}
override func out(_ aStr: String) throws {
print(aStr)
}
// Other stuff
}
And here's what I get once I remove CompileContext.out:
Fatal error: Call to abstract method CompileContext.out(_:): file swiftpg/contexts.swift, line 28
With the limitation of no dynamic dispatch, you could do something like this:
import Foundation
protocol foo {
static var instance: foo? { get }
func prt()
}
extension foo {
func prt() {
if Thread.callStackSymbols.count > 30 {
print("super")
} else {
Self.instance?.prt()
}
}
}
class foo1 : foo {
static var instance : foo? = nil
init() {
foo1.instance = self
}
func prt() {
print("foo1")
}
}
class foo2 : foo {
static var instance : foo? = nil
init() {
foo2.instance = self
}
func prt() {
print("foo2")
}
}
class foo3 : foo {
static var instance : foo? = nil
init() {
foo3.instance = self
}
}
var f1 : foo = foo1()
f1.prt()
var f2 : foo = foo2()
f2.prt()
var f3 : foo = foo3()
f3.prt()
I was trying to make a Weather abstract class, but using protocols wasn't ideal since I had to write the same init methods over and over again. Extending the protocol and writing an init method had it's issues, especially since I was using NSObject conforming to NSCoding.
So I came up with this for the NSCoding conformance:
required init?(coder aDecoder: NSCoder) {
guard type(of: self) != Weather.self else {
fatalError("<Weather> This is an abstract class. Use a subclass of `Weather`.")
}
// Initialize...
}
As for init:
fileprivate init(param: Any...) {
// Initialize
}
Move all references to abstract properties and methods of Base class to protocol extension implementation, where Self constraint to Base class. You will gain access to all methods and properties of Base class. Additionally compiler check implementation of abstract methods and properties in protocol for derived classes
protocol Commom:class{
var tableView:UITableView {get};
func update();
}
class Base{
var total:Int = 0;
}
extension Common where Self:Base{
func update(){
total += 1;
tableView.reloadData();
}
}
class Derived:Base,Common{
var tableView:UITableView{
return owner.tableView;
}
}
Is there a standard way to make a "pure virtual function" in Swift, ie. one that must be overridden by every subclass, and which, if it is not, causes a compile time error?
You have two options:
1. Use a Protocol
Define the superclass as a Protocol instead of a Class
Pro: Compile time check for if each "subclass" (not an actual subclass) implements the required method(s)
Con: The "superclass" (protocol) cannot implement methods or properties
2. Assert in the super version of the method
Example:
class SuperClass {
func someFunc() {
fatalError("Must Override")
}
}
class Subclass : SuperClass {
override func someFunc() {
}
}
Pro: Can implement methods and properties in superclass
Con: No compile time check
The following allows to inherit from a class and also to have the protocol's compile time check :)
protocol ViewControllerProtocol {
func setupViews()
func setupConstraints()
}
typealias ViewController = ViewControllerClass & ViewControllerProtocol
class ViewControllerClass : UIViewController {
override func viewDidLoad() {
self.setup()
}
func setup() {
guard let controller = self as? ViewController else {
return
}
controller.setupViews()
controller.setupConstraints()
}
//.... and implement methods related to UIViewController at will
}
class SubClass : ViewController {
//-- in case these aren't here... an error will be presented
func setupViews() { ... }
func setupConstraints() { ... }
}
There isn't any support for abstract class/ virtual functions, but you could probably use a protocol for most cases:
protocol SomeProtocol {
func someMethod()
}
class SomeClass: SomeProtocol {
func someMethod() {}
}
If SomeClass doesn't implement someMethod, you'll get this compile time error:
error: type 'SomeClass' does not conform to protocol 'SomeProtocol'
Another workaround, if you don't have too many "virtual" methods, is to have the subclass pass the "implementations" into the base class constructor as function objects:
class MyVirtual {
// 'Implementation' provided by subclass
let fooImpl: (() -> String)
// Delegates to 'implementation' provided by subclass
func foo() -> String {
return fooImpl()
}
init(fooImpl: (() -> String)) {
self.fooImpl = fooImpl
}
}
class MyImpl: MyVirtual {
// 'Implementation' for super.foo()
func myFoo() -> String {
return "I am foo"
}
init() {
// pass the 'implementation' to the superclass
super.init(myFoo)
}
}
You can use protocol vs assertion as suggested in answer here by drewag.
However, example for the protocol is missing. I am covering here,
Protocol
protocol SomeProtocol {
func someMethod()
}
class SomeClass: SomeProtocol {
func someMethod() {}
}
Now every subclasses are required to implement the protocol which is checked in compile time. If SomeClass doesn't implement someMethod, you'll get this compile time error:
error: type 'SomeClass' does not conform to protocol 'SomeProtocol'
Note: this only works for the topmost class that implements the protocol. Any subclasses can blithely ignore the protocol requirements. – as commented by memmons
Assertion
class SuperClass {
func someFunc() {
fatalError("Must Override")
}
}
class Subclass : SuperClass {
override func someFunc() {
}
}
However, assertion will work only in runtime.
This is what I usually do, to causes the compile-time error :
class SuperClass {}
protocol SuperClassProtocol {
func someFunc()
}
typealias SuperClassType = SuperClass & SuperClassProtocol
class Subclass: SuperClassType {
func someFunc() {
// ...
}
}
You can achieve it by passing function into initializer.
For example
open class SuperClass {
private let abstractFunction: () -> Void
public init(abstractFunction: #escaping () -> Void) {
self.abstractFunction = abstractFunction
}
public func foo() {
// ...
abstractFunction()
}
}
public class SubClass: SuperClass {
public init() {
super.init(
abstractFunction: {
print("my implementation")
}
)
}
}
You can extend it by passing self as the parameter:
open class SuperClass {
private let abstractFunction: (SuperClass) -> Void
public init(abstractFunction: #escaping (SuperClass) -> Void) {
self.abstractFunction = abstractFunction
}
public func foo() {
// ...
abstractFunction(self)
}
}
public class SubClass: SuperClass {
public init() {
super.init(
abstractFunction: {
(_self: SuperClass) in
let _self: SubClass = _self as! SubClass
print("my implementation")
}
)
}
}
Pro:
Compile time check for if each subclassimplements the required method(s)
Can implement methods and properties in superclass
Note that you can't pass self to the function so you won't get memory leak.
Con:
It's not the prettiest code
You can't use it for the classes with required init
Being new to iOS development, I'm not entirely sure when this was implemented, but one way to get the best of both worlds is to implement an extension for a protocol:
protocol ThingsToDo {
func doThingOne()
}
extension ThingsToDo {
func doThingTwo() { /* Define code here */}
}
class Person: ThingsToDo {
func doThingOne() {
// Already defined in extension
doThingTwo()
// Rest of code
}
}
The extension is what allows you to have the default value for a function while the function in the regular protocol still provides a compile time error if not defined