Abstract classes in Swift Language

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

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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.

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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.

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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()

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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?

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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
    }
}

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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")
        }
    }
}

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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

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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()

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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
}

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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;
  }
}

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