In Java I can do the following:
interface SomeCallback {
void onDone();
}
then I can create a function like this:
void test(SomeCallback callback) {
...
}
To call this function I do:
test(new SomeCallback() {
#Override
void done() {
...
}
});
I want to do something similar in Swift. I could create a protocol
protocol SomeCallback : class {
func done()
}
and a function like this
func test(callback: SomeCallback) {
...
}
I am still struggling with the call of this function.
Edit: Since I use an external API which requires a delegate I cannot use a Closure.
Is it possible to create some kind of anonymous inner class like I did it in the Java example to call test()?
Update: If you can't use a closure/function, the direct answer is: no, there are no anonymous inner classes. However, as Mike M points out, you'll have to use a class, and this class may be nested/inner to prevent polluting the global namespace. This class may well have a closure for every method it needs to implement and just call through to those closures.
The Swift-y way of doing this as long as you just need one method is to just use a lambda/closure.
For example, see NSComparator, which is typealiased since it is used all over the place and you are meant to recognize it.
In your example, specifying a function type inline will do fine. So for example:
func test(callback: () -> Void) {
...
callback()
}
// called as:
test({ in
...
})
// or even (since it's the last parameter)
test { in
...
}
Just to clarify the syntax because what you wrote is a little confusing.
#objc protocol SomeCallback {
func done() -> Void
}
No need to inherit from class as you wrote. Also, don't forget the #objc even if you do not want to bridge the protocol to that language. It helps with compiler complaints later on (might be a bug at the moment)
You cannot instantiate a protocol in Swift. You can however have an internal class that inherits from NSObject (root object) and implements this.
class External: NSObject {
class Internal : SomeCallback {
func done() {
// does something
}
}
let int = Internal()
func test(callback : SomeCallback) {
// additional work
callback.done()
}
}
Related
So I'm new to iOS development and have been working on minor changes to an app at my internship that has a relatively large objective-c code base. I've been learning swift from Treehouse(Wow, love them!) and I just learned about protocols. Currently, they should be used in certain instances and the instructor used this example.
Say you have a company with two different types of employees: Salary and Hourly(Pretty common). Now, they both would inherit from a super class called Employee and both would have to call a function called "pay" which would pay the employee. How do you enforce these classes to implement that function? Sure, use a protocol but that would require you to remember to add that to the function declaration. Is there a way to just add the protocol to the super class "Employee" and then whatever inherits from that class would have to follow that protocol that's part of that superclass. Is there another way to do this? Thanks!
What you are looking for is an abstract class. The purpose of an abstract class is to behave as a base class for concrete classes to inherit from, but an abstract class cannot be instantiated directly.
If Employee was an an abstract class then any attempt to actually instantiate an instance of Employee would be reported as an error by the compiler. You would need to instantiate a concrete subclass of Employee, such as SalariedEmployee or HourlyEmployee.
The definition of the Employee class would include that the calculatePay method was required and again a compile time error would occur if a concrete subclass did not implement that method.
Now, the bad news. Neither Objective-C nor Swift supports abstract classes.
You can provide a similar kind of class by providing an implementation of a method that throws an exception if it isn't overridden by a subclass. This gives a runtime error rather than a compile time error.
e.g.
class Employee {
var givenName: String
var surname: String
...
init(givenName: String, surname: String) {
self.givenName = givenName
self.surname = surname
}
func calculatePay() -> Float {
fatalError("Subclasses must override calculatePay")
}
}
class SalariedEmployee: Employee {
var salary: Float
init(givenName: String, surname: String, annualSalary: Float) {
salary = annualSalary
super.init(givenName: givenName, surname: surname)
}
override func calculatePay() -> Float {
return salary/12 // Note: No call to super.calculatePay
}
}
Whether the calculatePay is part of the base class or assigned to the base class through an extension that adds conformance to a protocol, the result is the same;
The Employee class will need a default implementation of the function that generates some sort of error
Failure of a subclass to implement the method will not cause a compile time error
You could assign a protocol, say, Payable to each subclass individually, but then as the protocol was not part of the base class, you couldn't say something like:
var employees[Employee]
for e in employees {
let pay = e.calculatePay()
}
You would have to use the slightly more complicated:
for e in employees {
if e is Payable {
let pay = e.calculatePay()
}
}
Unfortunately abstract functions are not yet supported. A possible workaround is to launch a fatalError when such function is not overridden by a subclass, doing so:
protocol YourProtocol {
func pay()
}
class Employee: YourProtocol {
func pay() {
fatalError("Must Override")
}
}
class SubEmployee: Employee {
func pay() {
print("stuff here")
}
}
My approach to this is to include the delegate as a parameter in the class initializer. See the code below:
protocol ProtocolExample {
func somethingNeedsToHappen()
}
// typical class example with delegate property for the required protocol
class ClassExampleA {
var delegate: ProtocolExample!
init() {
}
func aCriticalMethodWithUpdates() {
delegate.somethingNeedsToHappen()
}
}
// use class example in a view controller. Can easily forget to invoke the delegate and protocol
class MySampleViewControllerA: UIViewController {
var classExampleA : ClassExampleA!
func loadMyData() {
classExampleA = ClassExampleA()
}
}
// an alternative approach for the class is to include the delegate parameter in the initializer.
class ClassExampleB {
var delegate: ProtocolExample!
init(delegateForUpdates: ProtocolExample) {
delegate = delegateForUpdates
}
func doSomething() {
delegate.somethingNeedsToHappen()
}
}
// go to use it and you're reminded that the parameter is required...
class MySampleViewControllerB: UIViewController {
var classExampleB: ClassExampleB!
func loadMyData() {
classExampleB = ClassExampleB() // error: Missing argument for parameter 'delegateForUpdates' in call
}
}
// so to avoid error:
class MySampleViewControllerC: UIViewController {
var classExampleB: ClassExampleB!
func loadMyData() {
classExampleB = ClassExampleB(delegateForUpdates: <#ProtocolExample#>)
}
}
New to IOS programming but just wondering where is the best place to put functions that I would use throughout my code. For example, I want to write a few functions to perform a POST request to a web service and return a dictionary. Maybe another function to do some calculations. Is it best to create another .swift file and put all my functions there. And what would be a good name to give the file if so?
public func postRequest() -> [String:String] {
// do a post request and return post data
return ["someData" : "someData"]
}
The best way is to create a helper class with static functions, like this:
class Helper{
static func postRequest() -> [String:String] {
// do a post request and return post data
return ["someData" : "someData"]
}
}
Now every time you need to use postRequest you can just use like so: Helper.postRequest()
I usually create a separate class if I have functions that will be used by multiple classes, especially for the ones involving network operations.
If you just have separate functions that will be used, you can simply create static functions inside that class so it is easily accessible by other classes in a static way:
class DataController {
static func getData() -> [String:String] {
// do some operations
return ["someData" : "someData"]
}
}
let data = DataController.getData() // example
However, what often has been the case for me (especially if it involves more complicated operations) was that these network operations needed to establish an initial connection beforehand or required some initial setups, and they also performed asynchronous operations that needed to be controlled. If this is the case and you will often be calling such methods, you might want to create a singleton object that you could use throughout different classes and functions. This way, you could do the initial setup or establish an initial connection just once, and then do the rest as needed with the other functions, instead of doing them every time the function gets called.
Creating a singleton object is pretty simple in Swift:
class DataController {
static let sharedInstance = DataController() // singleton object
init() {
// do initial setup or establish an initial connection
}
func getData() -> [String:String] {
// do some operations
return ["someData" : "someData"]
}
}
let data = DataController.sharedInstance.getData() // example
For the name of the class, I usually name it something like DataController or DataHelper, but anything that makes sense as a "helper" class would work.
Hope this helps :)
For reusable functions it depends what I decide to use. For this specific case I use a separate file, because posting to a backend will become more complicated when the application evolves. In my app I use a backend class, with all kinds of helper classes:
struct BackendError {
var message : String
}
struct SuccessCall {
var json : JSON
var containsError : Bool {
if let error = json["error"].string {
return true
}
else {
return false
}
}
}
typealias FailureBlock = (BackendError) -> Void
typealias SuccessBlock = (SuccessCall) -> Void
typealias AlamoFireRequest = (path: String, method: Alamofire.Method, data: [String:String]) -> Request
typealias GetFunction = (path: String , data: [String : String], failureBlock: FailureBlock, successBlock: SuccessBlock) -> Void
class Backend {
func getRequestToBackend (token: String )(path: String , data: [String : String], failureBlock: FailureBlock, successBlock:
}
For other cases I often use extensions on Swift classes. Like for getting a random element from an Array.
extension Array {
func sampleItem() -> T {
let index = Int(arc4random_uniform(UInt32(self.count)))
return self[index]
}
}
This very old question but I would like to chirp some more points.
There are a few option, basically you can write your utility functions in Swift -
A class with static function. For example
class CommonUtility {
static func someTask() {
}
}
// uses
CommonUtility.someTask()
Also, you can have class method's as well instead of static method but those functions can be overridden by subclasses unlike static functions.
class CommonUtility {
class func someTask() {
}
}
// uses
CommonUtility.someTask()
Secondly, you can have Global functions as well, that are not part of any class and can be access anywhere from your app just by name.
func someTask() {
}
Though, selecting one over other is very subjective and I thing this is ok to make a class with static function in this particular case, where you need to achieve networking functionality but if you have some functions which perform only one task than Global function is a way to go because Global functions are more modular and separate out single tasks for a single function.
In case of static functions, if we access one of the static member, entire class gets loaded in memory. But in case of global function, only that particular function will be loaded in mem
You can create a separate swift class, might name it WebServicesManager.swift, and write all methods related to web requests in it.
You can use class methods, or singleton pattern to access the methods.
I have a protocol called Social Service, declared as follows:
protocol SocialService: class {
class func testFunc()
}
A class that follows the protocol may look like this:
class Twitter: SocialService {
class func testFunc() {
}
}
I want to have a method which returns a class that follows this protocol, so calling it would look like this:
let socialService = socialServiceForServiceType(serviceType: String)
I'm not sure what I need to put as the return value type of this function. For example, this:
func socialServiceForServiceType(serviceType: String) -> SocialService.Type
doesn't give an error right here, but trying to call it as above, gives an error:
Accessing members of protocol type value 'SocialService.Type' is
unimplemented
EDIT: I don't want an instance of that type, I want a class of that type. So I want a Twitter class, so I can call the class methods from the SocialService protocol on it.
Like the error says, this feature is unimplemented. However...
I don't want an instance of that type, I want a class of that type. So I want a Twitter class, so I can call the class methods from the SocialService protocol on it.
I'm not sure what you think you're getting from avoiding instances like this. Bear in mind classes don’t need to have member variables, and without them are essentially just collection of function pointers – which is what you seem to be looking for.
If you implement a Twitter class that has no properties and that conforms to a protocol, then calling methods on that protocol will dynamically dispatch to the implementations of that instance:
protocol SocialService: class {
func testFunc()
}
class Twitter: SocialService {
func testFunc() {
println("Testing Twitter!")
}
}
func socialServiceForServiceType(serviceType: String) -> SocialService {
return Twitter()
}
let service = socialServiceForServiceType("blah")
// prints "Testing Twitter!"
service.testFunc()
If your concern is that you want to put member variables in the Twitter class, but don’t want the overhead of that for some features, then this probably suggests you want to decompose this functionality into two different classes. Alternatively, if you want a singleton instance (to handle the connectivity for example) then there are other patterns to handle this.
Use simply
func socialServiceForServiceType(serviceType: String) -> SocialService
A protocol can be the return type of a function.
Totally agree with Airspeed Velocity, but I'd like to expand on one of his points:
I'm not sure what you think you're getting from avoiding instances like this. Bear in mind classes don’t need to have member variables, and without them are essentially just collection of function pointers – which is what you seem to be looking for.
I assume you're trying to do something like this:
func socialServiceForServiceType(serviceType: String) -> SocialService.Type
...
let cls = socialServiceForServiceType("twitter")
let conn = cls.connect(user)
Or something like that. You don't need classes to achieve that. You can just return functions.
typealias Connect = User -> Connection
func connectorForServiceType(serviceType: String) -> Connect {
switch serviceType {
case "twitter": return Twitter.Connect
...
}
}
let connect = connectorForServiceType("twitter")
let conn = connect(user)
If you have a whole bundle of functions that you want to package together, just use a struct.
struct ServiceHandlers {
let connect : User -> Connection
let ping : () -> Bool
let name: () -> String
}
func standardPinger(host: String) -> () -> Bool {
return { host in
// perform an ICMP ping and return Bool
}
}
func handlersForServiceType(serviceType: String) -> ServiceHandlers {
switch serviceType {
case "twitter":
return ServiceHandlers(connect: Twitter.connect,
ping: standardPinger("www.twitter.com"),
name: { "Twitter" })
...
}
}
let service = handlersForServiceType("twitter")
let conn = service.connect(user)
In some ways this is duplicative with class methods, but (a) the features you need for class methods aren't implemented, and (b) this is much more flexible. You can return any collection of functions you want; they don't have to all be class methods. It's easier to have default behaviors (which are hard in Swift when you use inheritance). It's easier to extend because you don't necessarily have to extend all the classes (see my use of standardPinger, which is some function I've made up that returns another function; it doesn't have to be a class method).
Breaking free of class/inheritance thinking and just passing around functions can be a major benefit in Swift. Sometimes a struct is better than a protocol.
Use a Factory pattern to achieve the same.
class SocialFactory : NSObject
{
class func socialServiceForServiceType(serviceType: String) -> SocialService?
{
switch serviceType
{
case "Twitter":
return Twitter();
case "Facebook":
return Facebook()
default:
return nil;
}
}
}
In my application written in Swift, I have the following class structure. Class A has a static method which does some stuff, but in a very simple form it looks like the code below.
class A {
class func create<T: A>() -> T? {
println(NSStringFromClass(T));
return nil;
}
}
Class B is subclassed from class A.
class B : A {
}
Now, when I execute the following code, the println command outputs A instead of B.
var myVar:B? = B.create();
I am not sure what I am doing wrong here, but I would expect it to output B.
When debugging and putting a breakpoint in the create method, the value $swift.type.T is defined as a Builtin.RawPointer MyApp.A instead of B.
Your generic class method on A doesn't make sense to me. Instead I would actually use something like the code below. This way it creates an instance of Self, which is whatever class you call it on. No need for generics in this case.
class A {
required init() {}
class func create() -> Self {
return self()
}
func test() -> String {
return "A"
}
}
class B : A {
override func test() -> String {
return "B"
}
}
let b = B.create() // "{A}" according to the playground, but it is a "B" instance!
b.test() // "B"
Please note that A needs a required initializer because the use of Self. When doing it in playground, the created instance is shown as {A} on the right. This is an error in Xcode I believe, the actual type is correct.
Edit:
I believe the code above isn't what you were looking for exactly, now I do get what you're trying to do. I would suggest not doing that by depending on the actual class name, but using a generic class to create the instances for you:
protocol Entity {
init()
class func entityName() -> String
}
class EntityFactory<T : Entity> {
class func newEntity() -> T? {
var entity: T?
// ... create entity here using T.entityName()
return entity
}
}
class Person : Entity {
required init() {}
class func entityName() -> String {
return "Person"
}
}
let person = EntityFactory<Person>.newEntity()
Think this is a more elegant solution, which moves the responsibility of creating an entity to a separate generic class. This results in code that is maintainable and testable. You can even abstract it out further for i.e. unit testing purposes, but that seems a bit out of scope here.
I want to lazy/inline implement a protocol in Swift.
So in the point of the implementation I will have access to variables outside the protocol scope ,
Same as implementing a interface in Java without declaring a class:
class MyClass:UIView {
var someComponent:SomeInnerComponent = SomeInnerComponent();
var count:Int = 0;
var a = :SomeProtocol { //<----- IS THIS POSSIBLE, IF YES HOW ?
func a0() {MyClass.count--}
func a1() {MyClass.count++}
}
someComponenet.delegate = a;
}
protocol SomeProtocol {
func a0()
func a1()
}
editing----
thanks i look at this solution, and i didn't see how to access a variable of the parent class.
all the examples show an Anonymous class but no one of the examples is accessing the parent variables .
What you're looking for is an inner class (not necessarily an anonymous one), declared in a scope that lets it access the count variable of a MyClass instance, and that adopts a protocol defined at a different scope. Right now Swift has a few of those pieces, but it doesn't look like you can put them all together in any way that's as concise as what you might be looking for.
You might think about declaring an inner class:
class MyView: UIView {
let someComponent = SomeInnerComponent() // type SomeInnerComponent is inferred
var count = 0 // type Int is inferred
class Helper: SomeProtocol {
func a0() { count-- } // ERROR
// ...
}
init() {
someComponent.delegate = Helper()
}
}
But that won't work, because count is implicitly self.count, where self is a Helper instance, not the MyView instance that "owns" the Helper instance. And there isn't a way to reference that MyView instance (or its properties) from within a Helper's methods, because you could just as well construct a MyView.Helper() without having an existing MyView instance. Inner classes (or nested types in general) in Swift nest only in lexical scope, not in existential ownership. (Or to put it another way, since you referenced Java: all inner classes in Swift are like static inner classes in Java. There's no non-static inner class.) If that's a feature you'd like, though, it's probably worth telling Apple you want it.
You could also try declaring Helper inside MyView.init() -- in Swift you can nest type definitions anywhere, including inside functions or methods of other types. Defined there, it can refer to MyView's properties. However, now the type information for Helper is only visible inside of MyView.init(), so when you assign it to someComponent.delegate (whose type is just SomeProtocol), you can't make use of it... this crashes the compiler, even. (That's another bug to report, but it's hard to say whether the bug is really "compiler crashes on valid usage" or "code is bad, but compiler crashes instead of producing error".)
The closest solution I can come up with looks something like this:
class SomeInnerComponent {
var delegate: SomeProtocol?
}
protocol SomeProtocol {
func a0()
func a1()
}
class MyClass {
var someComponent = SomeInnerComponent()
var count = 0
struct Helper: SomeProtocol {
var dec: () -> ()
var inc: () -> ()
func a0() { dec() }
func a1() { inc() }
}
init() {
someComponent.delegate = Helper(
dec: { self.count -= 1 }, // see note below
inc: { self.count += 1 }
)
}
}
How it works:
Helper is an inner struct (could be a class, but a struct is simpler)
It implements the a0 and a1 methods, satisfying the requirements of SomeProtocol
The implementations of a0 and a1 call through to the closures dec and inc, which are stored properties (aka instance variables) of the Helper struct
You write (or otherwise specify) these closures when you construct a Helper instance (using the default member-wise initializer, Helper(dec: (Void -> Void), inc: (Void -> Void)))
Because you can write the closures when initializing a Helper, those closures can capture variables where you're calling the initializer, including the implicit self that refers to the MyClass instance creating the Helper.
You need both a0/a1 and dec/inc because you need closures (the latter), not methods, for capturing the enclosing state. And even though closures and funcs/methods are in many ways interchangeable, you can't create a method/func implementation by assigning a closure to a method/func name. (It'd be a different story if SomeProtocol required closure properties instead of methods, but I'm assuming SomeProtocol isn't something under your control.)
Anyway, this is kind of a lot of boilerplate and a layer of abstraction that you might not really need, so it's probably worth looking into other ways to architect your code.
Note: my example uses the closure { self.count -= 1 } where you might expect { self.count-- }. The latter doesn't work because that's an expression with a value, so Swift will interpret it as shorthand for the closure's return value. Then it'll complain that you assigned a () -> Int closure to a property that expects a () -> () (aka Void -> Void) closure. Using -= 1 instead works around this issue.
I would go for a different approach, I know this a pretty old topic but just in case someone else struggles with this issue:
class MyClass:UIView {
var someComponent:SomeInnerComponent = SomeInnerComponent();
var count:Int = 0;
init(){
// Assign the delegate or do it somewhere else to your preference:
someComponenet.delegate = ProtocolImplementation(myClass: self);
}
private class ProtocolImplementation: SomeProtocol {
let selfReference: MyClass
init(myClass: MyClass){
selfReference = myClass
}
public func a0(){
selfReference.count--
}
public func a1(){
selfReference.count++
}
}
}
protocol SomeProtocol {
func a0()
func a1()
}
By following this approach it's also possible to include the same protocol multiple times, lets say your Protocol supports a generic and you want to implement it twice. SomeProtocol< SomeObject > and SomeProtocol< OtherObject > could be both used this way if needed.
Kind regards