I am taking a iOS course online provided by a famous university. I don't understand why the following code use override and it is legal.
According to the official definition, we use override to override superclass' methods. Where is the subclass and superclass in the following code?
What's been override and by what?
public override var description: String {
return "\(url.absoluteString) (aspect ratio = \(aspectRatio))"
}
Here is an example:
Your original class:
class Person {
func walk() {
//do something
}
}
Your subclass:
class Runner: Person {
override func walk() {
//do something that is different from Person's walk
}
}
In the Runner class, there is an override with the function walk. That is because it is a subclass of Person, and it can override Person's walk function. So If you instantiate a Runner:
var usainBolt = Runner()
And you call the walk function:
usainBolt.walk()
Then that will call the overriden function that you wrote in the Runner class. If you don't override it, it will call the walk function that you wrote in Person.
According to the official definition, we use override to override superclass' methods.
That's correct. The superclass in your example is the class that encloses the override of description property. This could be NSObject, some other class derived from it (directly or indirectly), or some class unrelated to NSObject that has var description: String property.
description is a property that Swift classes commonly have as a way to present themselves as a string, because description provides conformance to CustomStringConvertible protocol. This is similar to toString() method of Java, and to str() method of Python.
What's been override and by what?
The implementation of the property is what's being overridden. The class that has the implementation does the overriding.
Related
I've encountered a problem that is explained in the code below (Swift 3.1):
protocol MyProtocol {
func methodA()
func methodB()
}
extension MyProtocol {
func methodA() {
print("Default methodA")
}
func methodB() {
methodA()
}
}
// Test 1
class BaseClass: MyProtocol {
}
class SubClass: BaseClass {
func methodA() {
print("SubClass methodA")
}
}
let object1 = SubClass()
object1.methodB()
//
// Test 2
class JustClass: MyProtocol {
func methodA() {
print("JustClass methodA")
}
}
let object2 = JustClass()
object2.methodB()
//
// Output
// Default methodA
// JustClass methodA
So I would expect that "SubClass methodA" text should be printed after object1.methodB() call. But for some reason default implementation of methodA() from protocol extension is called. However object2.methodB()call works as expected.
Is it another Swift bug in protocol method dispatching or am I missing something and the code works correctly?
This is just how protocols currently dispatch methods.
A protocol witness table (see this WWDC talk for more info) is used in order to dynamically dispatch to implementations of protocol requirements upon being called on a protocol-typed instance. All it is, is really just a listing of the function implementations to call for each requirement of the protocol for a given conforming type.
Each type that states its conformance to a protocol gets its own protocol witness table. You'll note that I said "states its conformance", and not just "conforms to". BaseClass gets its own protocol witness table for conformance to MyProtocol. However SubClass does not get its own table for conformance to MyProtocol – instead, it simply relies on BaseClass's. If you moved the : MyProtocol down to the definition of SubClass, it would get to have its own PWT.
So all we have to think about here is what the PWT for BaseClass looks like. Well, it doesn't provide an implementation for either of the protocol requirements methodA() or methodB() – so it relies on the implementations in the protocol extension. What this means is that the PWT for BaseClass conforming to MyProtocol just contains mappings to the extension methods.
So, when the extension methodB() method is called, and makes the call out to methodA(), it dynamically dispatches that call through the PWT (as it's being called on a protocol-typed instance; namely self). So when this happens with a SubClass instance, we're going through BaseClass's PWT. So we end up calling the extension implementation of methodA(), regardless of the fact that SubClass provides an implementation of it.
Now let's consider the PWT of JustClass. It provides an implementation of methodA(), therefore its PWT for conformance to MyProtocol has that implementation as the mapping for methodA(), as well as the extension implementation for methodB(). So when methodA() is dynamically dispatched via its PWT, we end up in its implementation.
As I say in this Q&A, this behaviour of subclasses not getting their own PWTs for protocols that their superclass(es) conform to is indeed somewhat surprising, and has been filed as a bug. The reasoning behind it, as Swift team member Jordan Rose says in the comments of the bug report, is
[...] The subclass does not get to provide new members to satisfy the conformance. This is important because a protocol can be added to a base class in one module and a subclass created in another module.
Therefore if this was the behaviour, already-compiled subclasses would lack any PWTs from superclass conformances that were added after the fact in another module, which would be problematic.
As others have already said, one solution in this case is to have BaseClass provide its own implementation of methodA(). This method will now be in BaseClass's PWT, rather than the extension method.
Although of course, because we're dealing with classes here, it won't just be BaseClass's implementation of the method that's listed – instead it will be a thunk that then dynamically dispatches through the class' vtable (the mechanism by which classes achieve polymorphism). Therefore for a SubClass instance, we'll wind up calling its override of methodA().
A very short answer that a friend shared with me was:
Only the class that declares the conformance gets a protocol witness table
Meaning a subclass having that function has no effect on how the protocol witness table is setup.
The protocol witness is a contract only between the protocol, it's extensions, and the concrete class that implements it.
Well I suppose the subclass method A is not polymorphic because you can't put the override keyword on it, since the class doesn't know the method is implemented in an extension of the protocol and thus doesn't let you override it. The extension method is probably stepping on your implementation in runtime, much like 2 exact category methods trump each other with undefined behavior in objective C. You can fix this behavior by adding another layer in your model and implementing the methods in a class rather than the protocol extension, thus getting polymorphic behavior out of them. The downside is that you cannot leave methods unimplemented in this layer, as there is no native support for abstract classes (which is really what you're trying to do with protocol extensions)
protocol MyProtocol {
func methodA()
func methodB()
}
class MyProtocolClass: MyProtocol {
func methodA() {
print("Default methodA")
}
func methodB() {
methodA()
}
}
// Test 1
class BaseClass: MyProtocolClass {
}
class SubClass: BaseClass {
override func methodA() {
print("SubClass methodA")
}
}
let object1 = SubClass()
object1.methodB()
//
// Test 2
class JustClass: MyProtocolClass {
override func methodA() {
print("JustClass methodA")
}
}
let object2 = JustClass()
object2.methodB()
//
// Output
// SubClass methodA
// JustClass methodA
Also relevante answer here: Swift Protocol Extensions overriding
In your code,
let object1 = SubClass()
object1.methodB()
You invoked methodB from an instance of SubClass, but SubClass does not have any method named methodB. However its super class, BaseClass conform to MyProtocol, which has a methodB methodB.
So, it will invoke the methodB from MyProtocal. Therefore it will execute the methodA in extesion MyProtocol.
To reach what you expect, you need implement methodA in BaseClass and override it in SubClass, like the following code
class BaseClass: MyProtocol {
func methodA() {
print("BaseClass methodA")
}
}
class SubClass: BaseClass {
override func methodA() {
print("SubClass methodA")
}
}
Now, output would become
//Output
//SubClass methodA
//JustClass methodA
Although the method can reach what you expect, but I'm not sure this kind of code struct is recommended.
So what I am trying to do is make it easy for me to implement many custom UITableViewCells on one tableView, to do this I wanted to create a protocol that would facilitate filling out the function:
- (void)registerClass:(nullable Class)cellClass forCellReuseIdentifier:(NSString *)identifier
my protocol so far looks like this:
protocol TableViewCellClassReportingProtocol: class {
func reuseID() -> String
}
extension TableViewCellClassReportingProtocol {
static func classObject() -> AnyClass? {
return self.class
}
}
however I am having issues with getting the class type even thought I specify that this protocol must be implemented by a class. Any suggestions, I may be approaching this the wrong way
So this was close to the right answar and let me both elaborate and give credit to Charles A.
dynamicType is part of the answar! great call!
the function implamintation also had to change, because of subclassing and static really meaning "class final"
Protocol for class method
the override points must be
override class func reuseID() -> String
Why need to set public before override viewDidload in a public access control viewController
public class customViewController: UIViewController {
override public func viewDidLoad() {
super.viewDidLoad()
}
}
if I remove the public, Xcode will give an error warning!
The error message is fairly explicit:
Overriding instance method must be as accessible as the declaration it
overrides.
This means that a method must not have a lower access level than the method it overrides.
For example given this class:
public class Superclass {
internal func doSomething() {
...
}
}
You cannot then override doSomething with a method that is less accessible than interal. e.g.
public class Subclass : Superclass {
// error
private override func doSomething() {
}
}
You can however override a method and make it more accessible:
public class Subclass : Superclass {
public override func doSomething() {
// You can even call the internal method in the superclass
super.doSomething()
}
}
The reference documentation has lots of detail on this, but seems to leave this relationship to implication.
Took from here
Public access enables entities to be used within any source file from their defining module, and also in a source file from another module that imports the defining module. You typically use public access when specifying the public interface to a framework.
Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of that module. You typically use internal access when defining an app’s or a framework’s internal structure.
Private access restricts the use of an entity to the enclosing declaration. Use private access to hide the implementation details of a specific piece of functionality.
File-private access restricts the use of an entity to its own defining source file.
Do you need public modifier? You can write it like this:
class customViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
}
}
No warnings
You class declaration is:
public class customViewController: UIViewController
This opens the customViewController up to public access (publicly inherited)
Writing this gives you error:
override func viewDidLoad() {
super.viewDidLoad()
}
What happened here was an unpleasant interaction between the default access level for a method and how inheritance handles access levels.
A subclass must preserve the accessibility of its superclass’s methods
wherever the subclass might be used directly. Otherwise, we would
violate the substitution principle that allows us to treat all kinds
of UIViewController as just another UIViewController.
This is my inheritance structure
Protocols
protocol BaseProtocol {
}
protocol ChildProtocol: BaseProtocol {
}
Classes
class BaseClass: NSObject {
var myVar: BaseProtocol!
}
class ChildClass: BaseClass {
override var myVar: ChildProtocol!
}
I'm receiving a compiler error:
Property 'myVar' with type 'ChildProtocol!' cannot override a property with type 'BaseProtocol!'
What is the best approach to achieve this?
UPDATE
I updated the question trying to implement the solution with generics but it does not work :( This is my code (now the real one, without examples)
Protocols
protocol TPLPileInteractorOutput {
}
protocol TPLAddInteractorOutput: TPLPileInteractorOutput {
func errorReceived(error: String)
}
Classes
class TPLPileInteractor<T: TPLPileInteractorOutput>: NSObject, TPLPileInteractorInput {
var output: T!
}
And my children
class TPLAddInteractor<T: TPLAddInteractorOutput>: TPLPileInteractor<TPLPileInteractorOutput>, TPLAddInteractorInput {
}
Well, inside my TPLAddInteractor I can't access self.output, it throws a compiler error, for example
'TPLPileInteractorOutput' does not have a member named 'errorReceived'
Besides that, when I create the instance of TPLAddInteractor
let addInteractor: TPLAddInteractor<TPLAddInteractorOutput> = TPLAddInteractor()
I receive this other error
Generic parameter 'T' cannot be bound to non-#objc protocol type 'TPLAddInteractorOutput'
Any thoughts?
#tskulbru is correct: it can't be done, and this has nothing to do with your protocols. Consider the example below, which also fails…this time with Cannot override with a stored property 'myVar':
class Foo {
}
class Goo: Foo {
}
class BaseClass: NSObject {
var myVar: Foo!
}
class ChildClass: BaseClass {
override var myVar: Foo!
}
To understand why, let's reexamine the docs:
Overriding Properties
You can override an inherited instance or class property to provide
your own custom getter and setter for that property, or to add
property observers to enable the overriding property to observe when
the underlying property value changes.
The implication is that if you are going to override a property, you must write your own getter/setter, or else you must add property observers. Simply replacing one variable type with another is not allowed.
Now for some rampant speculation: why is this the case? Well, consider on the one hand that Swift is intended to be optimized for speed. Having to do runtime type checks in order to determine whether your var is in fact a Foo or a Bar slows things down. Then consider that the language designers likely have a preference for composition over inheritance. If both of these are true, it's not surprising that you cannot override a property's type.
All that said, if you needed to get an equivalent behavior, #tskulbru's solution looks quite elegant, assuming you can get it to compile. :)
I don't think you can do that with protocols
The way i would solve the problem you are having is with the use of generics. This means that you essentially have the classes like this (Updated to a working example).
Protocols
protocol BaseProtocol {
func didSomething()
}
protocol ChildProtocol: BaseProtocol {
func didSomethingElse()
}
Classes
class BaseClass<T: BaseProtocol> {
var myProtocol: T?
func doCallBack() {
myProtocol?.didSomething()
}
}
class ChildClass<T: ChildProtocol> : BaseClass<T> {
override func doCallBack() {
super.doCallBack()
myProtocol?.didSomethingElse()
}
}
Implementation/Example use
class DoesSomethingClass : ChildProtocol {
func doSomething() {
var s = ChildClass<DoesSomethingClass>()
s.myProtocol = self
s.doCallBack()
}
func didSomething() {
println("doSomething()")
}
func didSomethingElse() {
println("doSomethingElse()")
}
}
let foo = DoesSomethingClass()
foo.doSomething()
Remember, you need a class which actually implements the protocol, and its THAT class you actually define as the generic type to the BaseClass/ChildClass. Since the code expects the type to be a type which conforms to the protocol.
There are two ways you can go with your code, depending what you want to achieve with your code (you didn't tell us).
The simple case: you just want to be able to assign an object that confirms to ChildProtocol to myVar.
Solution: don't override myVar. Just use it in ChildClass. You can do this by design of the language Swift. It is one of the basics of object oriented languages.
Second case: you not only want to enable assigning instances of ChildProtocol, you also want to disable to be able to assign instances of BaseProtocol.
If you want to do this, use the Generics solution, provided here in the answers section.
If you are unsure, the simple case is correct for you.
Gerd
Is it possible to access and run a specific method/function from another class that can change dynamically as the app is run?
I’ll try to simplify the problem as much as possible.
SelectionPage.swift
Choose which class needs to be selected and accessed using an UIPickerView - 10 possible selections (Class1, Class2, Class3,…, Class10).
Class1.swift, Class2.swift, … Class10.swift
Each of the 10 classes has a single method that has exactly the same name but is programmed differently:
func runOnUpdate() { }
GameSceneViewController.swift
When a selection is made on the SelectionPage, the app segues to a GameSceneViewController where the specific selected function is run every time the update function is run:
override func update(currentTime: CFTimeInterval)
{
// run runOnUpdate() function here from selected class
}
Inside the update function, I would like to execute the runOnUpdate( ) function depending on which class was selected on the SelectionPage. Is this possible? Ideally I'd like to be able to assign the specific class/method in the:
override func didMoveToView(view: SKView)
so that I can access in other functions as well.
I’ve looked into lazy instantiation, creating delegates for each of the classes, #objc(Class1), arrays of [AnyClass], typealias, global variables in structs, singletons etc. but I’m unable to figure out how to make this work.
It seems like a fairly common problem so any help would be greatly appreciated! Thank you in advance!
You were correct in trying delegates as this is a case where you should make a protocol and a delegate. The protocol requires the function. From there you set the delegate property to an instance of a class that conforms to that protocol and then you call delegate?.someFunction() to call the function on the given object.
class ViewController: UIViewController {
var delegate: Updatable?
override func viewDidLoad() {
super.viewDidLoad()
let foo = Foo()
delegate = foo
delegate?.runOnUpdate() // prints do something
}
}
protocol Updatable {
func runOnUpdate()
}
class Foo: NSObject, Updatable {
func runOnUpdate() {
println("do something")
}
}