When I declare a Protocol in Swift like this, say for a network module:
Protocol MyNetworkProtocol {
func reportError(error: Int) // <-- do you add prefix for this?
}
How do you name the functions inside a protocol? Do you add prefix for it?
I ask this because if I name the function as shown above, the caller would be like:
class MyClass: MyNetworkProtocol {
// some code for MyClass
func reportError(error: Int) {
// some code to handle it
}
}
If I view the code in MyClass solely, reportError cannot be identified as a the implementation of a protocol of another class immediately (unless I add some comment in the code).
I am thinking to name the functions in protocols like this:
Protocol MyNetworkProtocol {
func myNetworkProtocol_reportError(error: Int)
}
So that in MyClass, I can quickly know it's a implementation of a protocol for a certain class. Is this necessary? Do I misunderstand some concept?
Don't do that. Instead segregate the protocol methods into a separate extension.
class MyClass {
}
extension MyClass: MyNetworkProtocol {
func reportError(error: Int) {
// some code to handle it
}
}
In protocol function start by protocol name with arguments of protocol class & additional
Protocol MyNetworkProtocol {
func myNetworkProtocol(_ myNetwork: MyNetworkProtocol, _reportErrot: Int)
}
}
Like this function will help to debug easily
Related
studying Protocols here and I'd like to know why that code does not print. Thanks in advance.
import Foundation
protocol PrintSomething {
func printMyname(name: String)
}
class Display {
var myProtocol: PrintSomething?
func somethingToDisplay() {
myProtocol?.printMyname(name: "Ygor")
}
}
let display = Display()
display.somethingToDisplay()
Because you never instantiated an actual instance of PrintSomething. You have a reference to an optional which defaults to nil. Are you perhaps trying to have Display conform to the protocol? You need an actual type to conform to the protocol and then instantiate that to be able to call it.
struct Printer: PrintSomething {
func printMyname(name: String) {
print(name)
}
}
class Display {
var myProtocol: PrintSomething = Printer()
func somethingToDisplay() {
myProtocol?.printMyname(name: "Ygor")
}
}
Now you have a real type (a struct) implementing the protocol, you're instantiating it so that the reference isn't nil, and then you're calling it.
You can do better, though, by having Display implement the protocol directly.
class Display: PrintSomething {
func printMyname(name: String) {
print(name)
}
}
let display = Display()
display.printMyname(name: "whatever")
You can use your code like below
protocol PrintSomething {
//declaration
func printMyname(name: String)
}
class Display: PrintSomething {
//defination
func printMyname(name: String) {
print(name)
}
var myProtocol: PrintSomething?
func somethingToDisplay() {
myProtocol?.printMyname(name: "Ygor")
}
}
And finally you can call it from any where like below
let display = Display()
display.somethingToDisplay()
You can read more about Protocol here
The documentation is pretty clear about what a protocol is:
A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.
For starters, you need an object to conform to your protocol which provides an actual implementation of the blueprint, because that is what a protocol is. Imagine you're building a house. You have the blueprint for it, but you still have to implement it in the real world.
protocol PrintSomething {
func printMyname(name: String)
}
class Display: PrintSomething {
func printMyname(name: String) {
print(name)
}
}
let display = Display()
display.printMyname(name: "My name")
If you wish to understand protocols in Swift I strongly recommend you begin with the documentation and start practicing the examples.
You have defined a property to be of a protocol type but you never assign something to that property and for that you need a concrete type, a struct or a class, to implement your protocol,
struct Example: PrintSomething {
func printMyname(name: String) {
print("This is my name: \(name)")
}
then you need to use it
let display = Display()
display.myProtocol = Example()
Now it will print when you call display.somethingToDisplay()
How to resolve in swift (2.0) that case: we have two protocols:
protocol SuperDelegate{
func handleSuccess(rsp: BaseRsp)
}
protocol ChildOfSuperDelegate: SuperDelegate {
func handleSuccess(rsp: ChildOfBaseRsp)
}
and some place in code where we want do:
class A{
var delegate:SuperDelegate?
func foo(){
delegate?.handleSuccess(childOfBaseRspObj)
}
}
class Xyz:ChildOfSuperDelegate{
func handleSuccess(rsp: ChildOfBaseRsp){
print("Great!")
}
}
but compiler not founding inheritence in protocols (handleSuccess have function with base and child parameters) there are compile error:
Type 'Xyz' does not conform to protocol 'SuperDelegate'
. How to resolve them? What is the best way to only implements ChildOfSuperDelegate methods but class "A" use SuperDelegate variable.
Meybe I must use generic types to have somethink like inheritance in protocols.
Here your protocol ChildOfSuperDelegate is inherited from SuperDelegate and though it is a protocol it doesn't implement its methods. But when you inherit from ChildOfSuperDelegate you have to implement method of SuperDelegate as well ChildOfSuperDelegate.
So do like this:
class Xyz:ChildOfSuperDelegate{
func handleSuccess(rsp: BaseRsp) {
//do stuff
}
func handleSuccess(rsp: ChildOfBaseRsp){
print("Great!")
}
}
I have a protocol that has a static method with a default parameter. I want to change the default value in a class that implements the protocol. Essentially doing what is easily done with classes and super.
I only have a solution when the Protocol has no associated type.
The following code works, but as soon as you uncomment the associated type declaration, it doesn't compile.
protocol Protocol {
// associatedtype AssociatedType
}
extension Protocol {
func sayHello(name: String = "World") {
print("Hello, \(name)!")
}
}
class Class<T>: Protocol {
typealias AssociatedType = T
func sayHello(name: String = "Stack Overflow") {
// Uncommenting the Protocol.AssociatedType causes:
// Protocol can only be used as a generic constraint because it has associated type requirements
(self as Protocol).sayHello(name)
}
}
Class<()>().sayHello()
I do understand why it doesn't compile: Protocol has no concrete type for AssociatedType.
So maybe the question should read "Can I explicitly specialize a protocol?", to which I believe the answer is no.
I have a partial workaround. But even when it works, it sucks.
Especially when you consider that I'm writing a library where sayHello is public, so the following workaround forces me to have a second protocol, which has to be public, but is useless.
Here's the workaround:
protocol Parent {}
protocol Protocol: Parent {
associatedtype AssociatedType
}
extension Parent {
func sayHello(name: String = "World") {
print("Hello, \(name)!")
}
}
class Class<T>: Protocol {
typealias AssociatedType = T
func sayHello(name: String = "Stack Overflow") {
(self as Parent).sayHello(name)
}
}
Class<()>().sayHello()
But this doesn't work for me, because my sayHello uses the associated type. So it can't be extracted to another protocol.
Just to be sure I'm clear, here's what I'd like, only substituting the class for a protocol:
class Protocol<T> {
func sayHello(name: String = "World") {
print("Hello, \(name)!")
}
}
class Class<T>: Protocol<T> {
override func sayHello(name: String = "Stack Overflow") {
super.sayHello(name)
}
}
Class<()>().sayHello()
You're trying to reinvent inheritance in protocols, and there is no such thing. But it is trivial to get what you're talking about; just say what you mean. You don't mean "I want to do the thing I inherited." You mean "I want to do some common behavior." Just provide a name for that common behavior. This removes all ambiguity about which one you mean.
protocol Protocol {
associatedtype AssociatedType
}
extension Protocol {
// Put the default behavior on the protocol, not on the instance
// Of course you could also put it on the instance if that were convenient.
static func defaultSayHello(_ name: String = "World") {
print("Hello, \(name)!")
}
// If you want a default on the instance, too, provide one that we an override
func sayHello(_ name: String = "World") {
Self.defaultSayHello(name)
}
}
class Class<T>: Protocol {
typealias AssociatedType = T
func sayHello(name: String = "Stack Overflow") {
// Now the default behavior lives on my type
Class.defaultSayHello(name)
}
}
// But other types can get default behavior
class OtherClass<T>: Protocol {
typealias AssociatedType = T
}
Class<()>().sayHello() // Hello, Stack Overflow!
OtherClass<()>().sayHello() // Hello, World!
The one frustrating part about this is that Swift provides no way to limit defaultSayHello to implementers of Protocol. So technically anyone can call it. It can sometimes be worth prefixing it with an _ to indicate that outsiders shouldn't. This is a basic access control problem in protocols, having nothing to do with this specific question; it comes up all the time when you want "things my implementers can use on themselves, but shouldn't be called randomly." Swift doesn't have a solution for that today.
Inspired by Rob Napier's answer, here's what I went with; good old overloading for defaults:
protocol Protocol {
associatedtype AssociatedType
}
extension Protocol {
func sayHello(name: String = "World") {
print("Hello, \(name)!")
}
}
class Class<T>: Protocol {
typealias AssociatedType = T
func sayHello() {
self.sayHello("Stack Overflow")
}
}
Class<()>().sayHello() // Hello, Stack Overflow!
Class<()>().sayHello("you") // Hello, you!
This does fit my needs, but doesn't answer the question. So I'm not 100% satisfied.
I believe Rust gets this one right by allowing traits/protocols to be generic both using X<T> and associated types.
Is there a point to declaring a static function on a protocol? The client using the protocol has to call the function on a type conforming to the protocol anyway right? That breaks the idea of not having to know the type conforming to the protocol IMO. Is there a way to call the static function on the protocol in a way where I don't have to know the actual type conforming to my protocol?
Nice question. Here is my humble point of view:
Is there a point to declaring a static function on a protocol?
Pretty much the same as having instance methods declared in a protocol.
The client using the protocol has to call the function on a type conforming to the protocol anyway right?
Yes, exactly like instance functions.
That breaks the idea of not having to know the type conforming to the protocol IMO.
Nope. Look at the following code:
protocol Feline {
var name: String { get }
static func createRandomFeline() -> Feline
init()
}
extension Feline {
static func createRandomFeline() -> Feline {
return arc4random_uniform(2) > 0 ? Tiger() : Leopard()
}
}
class Tiger: Feline {
let name = "Tiger"
required init() {}
}
class Leopard: Feline {
let name = "Leopard"
required init() {}
}
let feline: Feline = arc4random_uniform(2) > 0 ? Tiger() : Leopard()
let anotherFeline = feline.dynamicType.createRandomFeline()
I don't know the real type inside the variable feline. I just know that it does conform to Feline. However I am invoking a static protocol method.
Is there a better way to do this?
I see, you would like to call a static method/function declared in a protocol without creating a value that conforms to the protocol.
Something like this:
Feline.createRandomFeline() // DANGER: compiler is not happy now
Honestly I don't know the reason why this is not possible.
yes this is possible:
Swift 3
protocol Thing {
static func genericFunction()
}
//... in another file
var things:[Thing] = []
for thing in things {
type(of: thing).genericFunction()
}
Thank you #appzYourLife for the help! Your answer inspired my answer.
#appzYourLife answered my question. I had an underlying issue I was trying to resolve and the following code resolves my issue, so I'll post this here, maybe it helps someone with my same underlying question:
protocol MyProtocol {
static func aStaticFunc()
}
class SomeClassThatUsesMyProtocolButDoesntConformToIt {
var myProtocolType: MyProtocol.Type
init(protocolType: MyProtocol.Type) {
myProtocolType = protocolType
}
func aFunction() {
myProtocolType.aStaticFunc()
}
}
I created another solution for this case. IMHO this is quite clean and simple.
First, create a protocol for accessing instance type.
protocol TypeAccessible {
func type() -> Self.Type
}
extension TypeAccessible {
func type() -> Self.Type {
return Swift.type(of: self)
}
}
then create your concrete class as here. The point is your protocol should conform to TypeAccessible protocol.
protocol FooProtocol: TypeAccessible {
static func bar()
}
class Foo: FooProtocol {
static func bar() { }
}
On call site use it as
let instance: FooProtocol = Foo()
instance.type().bar()
For further use cases, just make sure your protocols conform to TypeAccessible and that's all.
A little late to the party on this one.
Here's my solution for "adding" static properties/functions/types to a protocol using typealias.
For example:
enum PropertyScope {
case all
case none
}
struct PropertyNotifications {
static var propertyDidChange =
Notification.Name("propertyDidChangeNotification")
}
protocol Property {
typealias Scope = PropertyScope
typealias Notifications = PropertyNotifications
var scope: Scope { get set }
}
Then you can do this anywhere in your code:
func postNotification() {
let scope: Property.Scope = .all
NotificationCenter.post(name: Property.Notifications.propertyDidChange,
object: scope)
}
Using protocols like Java interfaces is rarely a good idea. They are meta types, meant for defining contracts, which is an entirely different kind of thing.
That being said, just for the point of understanding, I find the most simple and effective way for creating the equivalent of a static factory method of a protocol to write a free function.
It should contain the protocol's name, hoping that that will prevent name clashes, and improve discoverability.
In other languages, createP would be a static member of P, named create and be called as P.create(...), which would drastically improve discoverability and guarantee to prevent name clashes.
In swift, though, this is not an option for protocols, so if protocols are for some reason really actually used as a replacement for interfaces, at least including the protocol's name in the function's name is an ugly workaround that's still slightly better than nothing.
P.S. in case the goal is actually to achieve something like an inheritance hierarchy with structs, union style enums are the tool that's meant to serve that purpose :)
protocol P
{
var x: Int { get }
}
func createP() -> P
{
if (todayIsMonday())
{
return A()
}
else
{
return B()
}
}
class A: P
{
var x = 5
}
class B: P
{
var x = 7
}
This isn't an answer so much as it is an extension to the question. Say I have:
#objc public protocol InteractivelyNameable: Nameable {
static func alertViewForNaming(completion:#escaping((_ success: Bool, _ didCancel: Bool, _ error: Error?) -> Void)) -> UIAlertController?
}
And I have a generic view controller that manages various types (generic type is .fetchableObjectType... basically NSFetchResult). I need to check if a specific object type conforms to the protocol, and if so, invoke it.
something like:
// valid swift code
if self.dataSource.fetchableObjectType is InteractivelyNameable {
// not valid swift code
if let alert = (self.dataSource.fetchableObjectType as InteractivelyNameable).alertViewForNaming(....)
}
I had a situation where I need to create same DomainModel object from 2 different response. so this (static method in protocol helped me) approach helped me.
protocol BaseResponseKeyList: CodingKey {
static func getNameKey()->Self
}
enum FirstResponseKeyList: String, BaseResponseKeyList {
case name
func getNameKey()->FirstResponseKeyList {
return .name
}
}
enum SecondResponseKeyList: String, BaseResponseKeyList {
case userName
func getNameKey()->SecondResponseKeyList {
return .userName
}
}
struct MyDomainModel<T:BaseResponseKeyList> : Decodable {
var name:String?
required init(from d:Decoder) {
do {
let container = try d.container(keyedBy:T.self)
name = try container.decode(String.self, forKey:T.getNameKey())
}catch(_) {
print("error")
}
}
}
let myDomainModel = try JSONDecoder().decode(MyDomainModel <FirstResponseKeyList>.self, from: data)
let myDomainModel2 = try JSONDecoder().decode(MyDomainModel <SecondResponseKeyList>.self, from: data2)
I have some swift structs for which protocol compliance is generated with individual extensions with equal methods names which just differ in their return types which are struct dependent. On top of That I want to use them in a generic function which Calls a protocol conforming function for a generic type).
I tried to accomplish this like that:
//: Playground - noun: a place where people can play
import UIKit
protocol FooProt {
typealias T;
static func createMe<T>()->T;
}
struct FooStruct{
}
extension FooStruct: FooProt{
typealias T = FooStruct;
static func createMe () -> FooStruct{
return FooStruct();
}
}
class Creator{
fun createOne<T where T:FooProt>(type:T.Type){
let instance = T.createMe();
}
}
Unfortunately I get the following error :
/var/folders/sn/78_zvfd15d74dzn01mdv258h0000gq/T/./lldb/3741/playground6.swift:7 :17: note: protocol requires function 'createMe()' with type ' () -> T' (aka '<τ_1_0> () -> τ_1_0')
static func createMe()->T;
What exactly doesn't comply here and is there a workaround ?
There are several problems with your code. On the one hand you have defined a protocol with an associated type. However, you define your createMe() method as a generic which uses some other type. I don't think that was your intent. I think your intent was to have a createMe() method that returns the same type as the protocol's associated type. In this case you need to remove the from the createMe() method. Also, the name createMe() implies that you aren't just returning any type, but the type of the object on which this method is being called. In this case, you don't even need an associated type protocol. You just need a protocol with a Self constraint which allows your code to be a bit simpler. In your Creator's createOne method, your type constraint is more complex than needed.
I think you want the following code:
protocol FooProt {
static func createMe()->Self;
}
struct FooStruct{
}
extension FooStruct: FooProt {
static func createMe() -> FooStruct {
return FooStruct();
}
}
class Creator{
func createOne<T:FooProt>(type: T.Type) -> T {
return T.createMe()
}
}
let foo = Creator().createOne(FooStruct.self)
Here is an alternate solution using an initializer in the protocol instead of a static method.
protocol FooProt {
init()
}
struct FooStruct{
}
extension FooStruct: FooProt {
}
class Creator{
func createOne<T:FooProt>(type: T.Type) -> T {
return T.init()
}
}
let foo = Creator().createOne(FooStruct.self)