I have an abstract view controller that has some functions. Some view controllers will extend the abstract one and override some functions. I need to force some functions when any view controller override them, to call super.
I suggest that you make your public methods final and expose empty internal methods that do nothing in the abstract class but get called in your public final methods.
Something like this:
class AbstractViewController {
internal func willDoStuff() {
}
internal func didDoStuff() {
}
final public func doStuff() {
willDoStuff()
// Do the stuff
didDoStuff()
}
}
class RealViewController : AbstractViewController {
override internal func willDoStuff() {
print("Will do stuff")
}
}
let viewController = RealViewController()
viewController.doStuff()
Related
ArtistVC is a subclass of CategoryVC which is a subclass of BrowserVC.
Most of ArtistVC's UITableView Data Source methods wind up calling CategoryVC's implementation of those methods with return super.tableView(tableView, cellForRowAt: adjustedIndexPath)(for example), after some other things happen of course, like modifying the indexPath passed to the method.
But sometimes I actually want to bypass CategoryVC's implementation and use BrowserVC's implementation.
super.super.someMethod() apparently isn't the answer. Is this possible?
There's a (convoluted) way to do this but it requires that the intermediate class "contributes" to the bypassing of its override.
For example:
class A
{
func doIt(_ parameter:Int) { print("A.doIt(\(parameter))") }
}
class B:A
{
override func doIt(_ parameter:Int) { print("B.doIt(\(parameter))") }
var super_doIt:(Int)->() { return super.doIt }
}
class C:B
{
override func doIt(_ parameter:Int)
{ super.super_doIt(parameter) }
}
let c = C()
c.doIt(3) // A.doIt(3)
I have a super class
class Father {
public func doSomething() {
}
}
I want this child class to be
class Child: Father {
private override func doSomething() {
}
}
but Xcode says that
Overriding instance method must be as accessible as the declaration it
overrides
so, how can I override a public method in superclass be a private method in subclass
THANK YOU
You can't because that would violate the Liskov Substitution Principle.
Essentially, any code that can operate on an instance of a superclass must also be able to operate on an instance of your subclass.
So, if some other class has a method
class Unrelated {
func operateOnAThing(_ someThing:Father) {
someThing.doSomething()
}
}
then it still has to work when you do the following:
let aChild = Child()
unrelatedInstance.operateOnAThing(aChild)
If the doSomething method had more restrictive access in the Child class then you would get a runtime error. To prevent this, you cannot make access more restrictive in a subclass.
You can achieve that by marking the public method as unavailable using the #available attribute. Then, call the method using super in the private method.
Example:
class Father {
public func doSomething() {
print("doSomething privatly")
}
}
class Child: Father {
#available (*, unavailable)
override func doSomething() {
}
fileprivate func internal_doSomething() {
super.doSomething()
}
}
Child().internal_doSomething()
Child().doSomething() //'doSomething()' has been explicitly marked unavailable
(Tested using Swift 4.0!)
So I was thinking about a custom pattern in my project, but I can't get it to work. The main idea is to change the typealias on every subclass to get access to the subclass specific interface.
protocol InstanceInterface: class {
typealias Interface
var interface: Interface { get }
}
// Baseclass
protocol FirstClassInterface: class { /* nothing here for the example */ }
class FirstClass: InstanceInterface, FirstClassInterface {
typealias Interface = FirstClassInterface
var interface: Interface { return self }
}
// Subclass
protocol SecondClassInterface: FirstClassInterface {
func foo()
}
class SecondClass: FirstClass, SecondClassInterface {
typealias Interface = SecondClassInterface // <--- This does nothing :(
func foo() { print("hello world") } // Swift 2.0 here
}
// Lets say I want to call foo trough the interface
let test = SecondClass()
test.interface.foo() // 'Interface' does not have a member named 'foo'
Is there something I'm doing wrong or do I misunderstand some Swift concepts here?! I do need to subclass here to not to implement everything from super class' protocols over and over again. Is my little pattern even possible? I'd appreciate any help. :)
Would something like this work for your purposes?
class MyClass<T> {
}
class MySubclass1: MyClass<String> {
}
class MySubclass2: MyClass<Int> {
}
Unfortunately there is no good workaround for this problem.
The main idea to override the typealias would work in this case but consider the following:
protocol TakeAndGet {
typealias T
func take(value: T)
func get() -> T
}
class FirstClass: TakeAndGet {
typealias T = FirstClass
var property = 0
func take(value: T) {
value.property = 4
}
func get() -> T {
return FirstClass()
}
}
class SecondClass: FirstClass {
typealias T = SecondClass
var property2 = "hello"
}
If the typealias of the SecondClass overrides the other one the take method would work since it takes a subclass which can be treated as the superclass. But the get method cannot implicitly convert FirstClass to SecondClass. Therefore it is not possible to override a typealias.
Now if we want to override the get function with get() -> SecondClass it wouldn't work since it has not the same signature as the one in the superclass. In addition we inherit the get method which results in an ambiguous use:
SecondClass().get() // which type gets returned? SecondClass or FirstClass
So you have to try a different approach.
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