I have classes Alpha and Berry:
class Alpha { }
class Berry : Alpha { }
I have a function that using inheritance within it's generic:
func myFunc<T : Alpha>(v:T) -> T {
return T()
}
I call myFunc like this:
myFunc(Berry())
In my project, the object that gets returned is of type Alpha, and not of type Berry. Is this is a bug in the compiler, or if this is simply something I'm misunderstanding about generics?
What you trying to achieve is passing an instance of Berry and getting another instance of Berry?
If so, following code should work:
class Alpha {
required init() { } // ← YOU NEED THIS
func printme() {
println("I'm alpha")
}
}
class Berry : Alpha {
override func printme() {
println("I'm berry")
}
}
func myFunc<T:Alpha>(v:T) -> T {
return v.dynamicType()
}
// This also works:
/*
func myFunc<T: Alpha>(v:T) -> T {
return (T.self as T.Type)()
}
*/
let a = myFunc(Berry())
a.printme() // -> I'm berry
required init() { } is necessary to ensure all classes derived from Alpha have init() initializer.
Here is related Q/A: Swift generics not preserving type
If what you want is passing Berry as a type and get new instance of Berry, try this:
class Alpha {
required init() { }
func printme() {
println("alpha")
}
}
class Berry : Alpha {
override func printme() {
println("berry")
}
}
func myFunc<T:Alpha>(v:T.Type) -> T {
return v()
}
let a = myFunc(Berry)
a.printme()
Related
I wanna unittest with generics.
But I can't think of a way.
My code is as below, and I want to test the part prepare().
class A_ViewModel {
var useCase: UseCaseProtocol = UseCase()
var item: A_CellModel = .init()
// I want to unittest prepare()
func prepare() {
// logic....
// and call calcuate
useCase.calculate(item: item)
}
}
protocol TestableProtocol {
var testProperty: Bool { get set }
}
class A_CellModel: TestableProtocol {
var testProperty: Bool = false
}
protocol UseCaseProtocol {
func calculate<T: TestableProtocol>(item: T)
}
class UseCase: UseCaseProtocol {
func calculate<T: TestableProtocol>(item: T) {
// logic......
}
}
However, since calcaulte(item:) in usecase uses generic, it is necessary to hand over the clear type (A_CellModel).
In that case, there is a dependence between A_ViewModel and A_CellModel, which makes it difficult to test the unit.
In order to test prepare(), should calcaulte(item:) give up generic?
Should I use existential type?
I have a class attribute that points to one of the class functions. However when I try to initialize this variable with one of the functions, I get the following error:
'self' used in method call before all stored properties are initialized.
I'm able to initialize any other variable to those functions, but the error makes it sound like I'm calling the function even though I'm not.
import UIKit
import AudioToolbox
class BeatMaker {
// iPhone 7 and up use beat function, iPhone 6s use beatFallback
let hapticFunction: () -> ()
let impactGenerator = UIImpactFeedbackGenerator.init(style: .heavy)
init(supportsImpactGenerator: Bool) {
// error 1: 'self' used in method call 'beat' before all stored properties are initialized
// error 2: 'self' used in method call 'beatFallback' before all stored properties are initialized
self.hapticFunction = (supportsImpactGenerator) ? beat : beatFallback
}
private func beat() {
impactGenerator.impactOccurred()
}
private func beatFallback() {
AudioServicesPlaySystemSound(1520)
}
func makeABeat() {
hapticFunction()
}
}
In this specific case I want to make use of the Taptic Engine and simulate a click through the UIImpactFeedbackGenerator. The iPhone 6s doesn't support this engine, so I want to call a fallback function that produces a similar effect.
I also tried initializing the variable on the spot:
// this works
var hapticFunction: (BeatMaker) -> () -> () = beat
init(supportsImpactGenerator: Bool) {
if !supportsImpactGenerator {
// error: Cannot assign value of type '() -> ()' to type '(BeatMaker) -> () -> ()'
self.hapticFunction = beatFallback
// produces same error
self.hapticFunction = beatFallback.self
}
}
I know that I could make everything static or put everything out of the class, but this feels like it should work yet it doesn't. Am I missing something?
EDIT
Setting the type of type of hapticFunction to an optional seems to work, but this doesn't make any sense to me. What's the difference?
// this works
var hapticFunction: (() -> ())?
init(supportsImpactGenerator: Bool) {
self.hapticFunction = (supportsImpactGenerator) ? beat : beatFallback
}
It might be better to not use a Bool, but rather a nested Enum, which is also more extendible if you wanna add some other modes of haptic feedback later on.
I have a generalized solution for a generalized problem of your question. So either you do:
public class FunctionOwner {
private let mode: Mode
public init(`do` mode: Mode = .default) {
self.mode = mode
}
}
public extension FunctionOwner {
enum Mode {
case foo, bar
}
func fooOrBar() {
switch mode {
case .foo: foo()
case .bar: bar()
}
}
}
private extension FunctionOwner {
func foo() {
print("doing foo")
}
func bar() {
print("doing bar")
}
}
public extension FunctionOwner.Mode {
static var `default`: FunctionOwner.Mode {
return .foo
}
}
// USAGE
FunctionOwner(do: .bar).fooOrBar() // prints "doing foo"
FunctionOwner(do: .foo).fooOrBar() // prints "doing bar"
Or if you for some reason do want to keep the stored Mode, you can do this (might be relevant for your actual question on how you do a workaround of referencing self in the init.):
public class FunctionOwner {
private let _function: (FunctionOwner) -> Void
public init(`do` mode: Mode = .default) {
_function = { functionOwner in
switch mode {
case .foo: functionOwner.foo()
case .bar: functionOwner.bar()
}
}
}
}
public extension FunctionOwner {
enum Mode {
case foo, bar
}
func fooOrBar() {
_function(self)
}
}
// The rest of the code is the same as the example above
There are two method to fix this
You can:
Give hapticFunction a initial value like {}
or fix like:
class BeatMaker {
let impactGenerator = UIImpactFeedbackGenerator.init(style: .heavy)
let supportsImpactGenerator: Bool
init(supportsImpactGenerator: Bool) {
self.supportsImpactGenerator = supportsImpactGenerator
}
private func beat() {
if supportsImpactGenerator {
impactGenerator.impactOccurred()
} else {
AudioServicesPlaySystemSound(1520)
}
}
func makeABeat() {
beat()
}
}
How can we call class functions with a dynamic class name?
Assume the following example where I have two class with methods with same signature
class Foo{
class func doSomething()
}
class Foobar {
class func doSomething()
}
class ActualWork{
//call following method with a variable type so that it accepts dynamic class name
func callDynamicClassMethod(x: dynamicClass)
x.doSomething()
}
How can this be implemented so that x accepts values at run time
Edit: Sorry, I missed to mention that I was looking for any other ways other than protocol oriented approach. This is more of an exploratory question to explore if there is a more direct approach/pods/libraries to achieve this.
I liked this question, because it made me to think a lit'bit outside of the box.
I'll answer it, by dividing it into a few parts.
First
call class functions
Class function is basically a Type methods, which can be achieved using the static word inside the class context.
Taking that into account, you can get a simple solution, using protocol and passing the class reference (conforming to that protocol) like this:
protocol Aaa{
static func doSomething();
}
class Foo : Aaa{
static func doSomething() {
print("Foo doing something");
}
}
class FooBar : Aaa{
static func doSomething() {
print("FooBar doing something");
}
}
class ActualWork{
//Using class (static) method
func callDynamicClassMethod <T: Aaa> (x: T.Type) {
x.doSomething();
}
}
//This is how you can use it
func usage(){
let aw = ActualWork();
aw.callDynamicClassMethod(x: Foo.self);
aw.callDynamicClassMethod(x: Foo.self);
}
Second
In case you don't really need the method on the class context, you may consider using instance methods. In that case the solution would be even simpler, like this:
protocol Bbb{
func doSomething();
}
class Bar : Bbb{
func doSomething() {
print("Bar instance doing something");
}
}
class BarBar : Bbb{
func doSomething() {
print("BarBar instance doing something");
}
}
class ActualWork{
//Using instance (non-static) method
func callDynamicInstanceMethod <T: Bbb> (x: T){
x.doSomething();
}
}
//This is how you can use it
func usage(){
let aw = ActualWork();
aw.callDynamicInstanceMethod(x: Bar());
aw.callDynamicInstanceMethod(x: BarBar());
}
Third
If you need to use the class func syntax, as OP originally did:
class func doSomething()
You CANNOT simply use a protocol. Because protocol is not a class...
So compiler won't allow it.
But it's still possible, you can achieve that by using
Selector with NSObject.perform method
like this:
class ActualWork : NSObject{
func callDynamicClassMethod<T: NSObject>(x: T.Type, methodName: String){
x.perform(Selector(methodName));
}
}
class Ccc : NSObject{
#objc class func doSomething(){
print("Ccc class Doing something ");
}
}
class Ddd : NSObject{
#objc class func doSomething(){
print("Ccc class Doing something ");
}
#objc class func doOther(){
print("Ccc class Doing something ");
}
}
//This is how you can use it
func usage() {
let aw = ActualWork();
aw.callDynamicClassMethod(x: Ccc.self, methodName: "doSomething");
aw.callDynamicClassMethod(x: Ddd.self, methodName: "doSomething");
aw.callDynamicClassMethod(x: Ddd.self, methodName: "doOther");
}
Generics and Protocol oriented programming will do the job:
protocol Doable {
static func doSomething()
}
class Foo: Doable {
static func doSomething() {
debugPrint("Foo")
}
}
class Foobar: Doable {
static func doSomething() {
debugPrint("Foobar")
}
}
class ActualWork {
func callDynamicClassMethod<T: Doable>(x: T.Type) {
x.doSomething()
}
}
let work = ActualWork()
work.callDynamicClassMethod(x: Foo.self)
work.callDynamicClassMethod(x: Foobar.self)
you can achieve this with help of Protocol
protocol common {
static func doSomething()
}
class Foo : common{
static func doSomething() {
print("Foo")
}
}
class Foobar : common {
static func doSomething() {
print("Foobar")
}
}
class ActualWork{
//call following method with a variable type so that it accepts dynamic class name
func callDynamicClassMethod(x: common.Type) {
x.doSomething()
}
}
let fooObj : common = Foo()
let Foobarobj : common = Foobar()
let workObk = ActualWork()
workObk.callDynamicClassMethod(x:Foo.self)
workObk.callDynamicClassMethod(x:Foobar.self)
I think, there are three solutions. I shared an sample below.
Use "protocol" that has "doSomething()" function requirements.
Create a function which gets function definition as a parameter.
Use reflection. you can use EVReflection that is good Api for reflection.
sample code:
protocol FooProtocol {
static func doSomething()
}
class Foo: FooProtocol {
class func doSomething() {
print("Foo:doSomething")
}
}
class Foobar: FooProtocol {
class func doSomething() {
print("Foobar:doSomething")
}
}
class ActualWork {
func callDynamicClassMethod<T: FooProtocol>(x: T.Type) {
x.doSomething()
}
func callDynamicClassMethod(x: #autoclosure () -> Void) {
x()
}
func callDynamicClassMethod(x: () -> Void) {
x()
}
}
ActualWork().callDynamicClassMethod(x: Foo.self)
ActualWork().callDynamicClassMethod(x: Foobar.self)
print("\n")
ActualWork().callDynamicClassMethod(x: Foo.doSomething())
ActualWork().callDynamicClassMethod(x: Foobar.doSomething())
print("\n")
ActualWork().callDynamicClassMethod(x: Foo.doSomething)
ActualWork().callDynamicClassMethod(x: Foobar.doSomething)
Looks like you are searching for duck typing, and this is harder to achieve in a statically typed language (with some exceptions, listed in the linked Wikipedia page).
This is because dynamically calling a method requires knowledge about the layout of the target object, thus either inheritance of the class declaring the method, or conformance to a protocol that requires that method.
Starting with Swift 4.2, and the introduction of dynamic member lookup, there is another approach to solve your problem, however it also involves some ceremony:
// This needs to be used as base of all classes that you want to pass
// as arguments
#dynamicMemberLookup
class BaseDynamicClass {
subscript(dynamicMember member: String) -> () -> Void {
return { /* empty closure do nothing */ }
}
}
// subclasses can choose to respond to member queries any way they like
class Foo: BaseDynamicClass {
override subscript(dynamicMember member: String) -> () -> Void {
if member == "doSomething" { return doSomething }
return super[dynamicMember: member]
}
func doSomething() {
print("Dynamic from Foo")
}
}
class Bar: BaseDynamicClass {
override subscript(dynamicMember member: String) -> () -> Void {
if member == "doSomething" { return doSomething }
return super[dynamicMember: member]
}
func doSomething() {
print("Dynamic from Bar")
}
}
func test(receiver: BaseDynamicClass) {
receiver.doSomething()
}
test(receiver: Bar()) // Dynamic from Bar
To conclude, in the current Swift version there is no way to have both the argument and the method dynamic, some common ground needs to be set.
I have an extension on UIView implementing a protocol
protocol SomeProtocol {
var property : Int
}
extension UIView : SomeProtocol {
var property : Int {
get {
return 0
}
set {
// do nothing
}
}
}
in a concrete subclass I want to override this extension method:
class Subclass : UIView, SomeProtocol {
var _property : Int = 1
var property : Int {
get { return _property}
set(val) {_property = val}
}
}
I set breakpoints and see that the extension method is called and not the concrete subclass method:
var subclassObject = Subclass()
someObject.doSomethingWithConcreteSubclassObject(subclassObject)
// other code;
fun doSomethingWithConcreteSuclassObject(object : UIView) {
var value = object.property // always goes to extension class get/set
}
As others have noted, Swift does not (yet) allow you to override a method declared in a class extension. However, I'm not sure whether you'll ever get the behavior you want even if/when Swift someday allows you to override these methods.
Consider how Swift deals with protocols and protocol extensions. Given a protocol to print some metasyntactic variable names:
protocol Metasyntactic {
func foo() -> String
func bar() -> String
}
An extension to provide default implementations:
extension Metasyntactic {
func foo() -> String {
return "foo"
}
func bar() -> String {
return "bar"
}
}
And a class that conforms to the protocol:
class FooBar : Metasyntactic {
func foo() -> String {
return "FOO"
}
func bar() -> String {
return "BAR"
}
}
Swift will use dynamic dispatch to call the appropriate implementations of foo() and bar() based on each variable's runtime type rather than on the type inferred by the compiler:
let a = FooBar()
a.foo() // Prints "FOO"
a.bar() // Prints "BAR"
let b: Metasyntactic = FooBar()
b.foo() // Prints "FOO"
b.bar() // Prints "BAR"
If, however, we extend the protocol further to add a new method:
extension Metasyntactic {
func baz() -> String {
return "baz"
}
}
And if we override our new method in a class that conforms to the protocol:
class FooBarBaz : Metasyntactic {
func foo() -> String {
return "FOO"
}
func bar() -> String {
return "BAR"
}
func baz() -> String {
return "BAZ"
}
}
Swift will now use static dispatch to call the appropriate implementation of baz() based on the type inferred by the compiler:
let a = FooBarBaz()
a.baz() // Prints "BAZ"
let b: Metasyntactic = FooBarBaz()
b.baz() // Prints "baz"
Alexandros Salazar has a fantastic blog post explaining this behavior in depth, but suffice it to say that Swift only uses dynamic dispatch for methods declared in the original protocol, not for methods declared in protocol extensions. I imagine the same would be true of class extensions, as well.
I know this question has been asked a while ago. But this will be handy for someone who looking for an easier way. There is a way of overriding an extension methods. I know its bit hacky but it does the job beautifully.
If you declare your protocol with #objc
#objc protocol MethodOverridable {
func overrideMe()
}
In Extension
extension MainClass: MethodOverridable {
func overrideMe() {
print("Something useful")
}
}
Subclass - You can able to override it in your subclass. It works like a magic. Well, not really when adding #objc it exposes your protocol to Objective-C and its Runtime. That allows your subclass to override.
class SubClass: MainClass {
override func overrideMe() {
print("Something more useful")
}
}
Swift 5
class Class
{
#objc dynamic func make() { print("make from class") }
}
class SubClass: Class {}
extension SubClass {
override func make() {
print("override")
}
}
It looks like you can override property for 2nd super class property. For example, you can access UIView property by making extension to the UILabel wanting to override frame property of UIView. This sample works for me in Xcode 6.3.2
extension UILabel {
override public var frame: CGRect {
didSet {
println("\(frame)")
}
}
}
You can't do this through normal means.
It's in Apple's docs that you can't override a method in an extension in a subclass.
Also, extensions can add new functionality to a type, but they cannot override existing functionality.
https://docs.swift.org/swift-book/LanguageGuide/Extensions.html
I think you forgot to override the superclass property in your subclass:
class Subclass : UIView {
var _property : Int = 1
override var property : Int {
get { return _property}
set(val) {_property = val}
}
}
It is easy to override a method in Swift:
class A {
func innerValue() -> Int { return 5 }
func getInnerValue() -> Int { return innerValue() }
}
class B: A {
override func innerValue() -> Int { return 8 }
}
B().getInnerValue() //returns 8
However I don't know how to do the same when I declare innerValue() as static (using the class keyword):
class A {
class func innerValue() -> Int { return 5 }
func getInnerValue() -> Int {
return A.innerValue() //ok for compiler but returns 5 instead of 8
return self.innerValue() //error: 'A' doesn't have a member named 'innerValue'
return innerValue() //error: Use of unresolved identifier 'innerValue'
}
}
class B: A {
override class func innerValue() -> Int { return 8 }
}
B().getInnerValue()
So is it possible in Swift?
return A.innerValue() //ok for compiler but returns 5 instead of 8
From your comment, it sounds like what you want to do is refer the current instance's class polymorphically. If that's what you want, then don't send the innerValue() message to A; that means A only. And don't send it to self, because the way you've written this, getInnerValue is an instance method, while what you want to call is a class method. Send it to self.dynamicType, the class of the current instance.