Given this context where I have a class that implements a protocol's property where that property is prefixed with the property wrapper #Published.
protocol Manageable {
var foo: String { get }
}
class Manager: Manageable {
#Published var foo: String
}
I found myself in the following situation where I can't call the published property as $ using the protocol.
let manager: Manageable = Manager()
manager.$foo // <== Error
Because we can't declare the #Published in the protocol I found myself in the position where I have to cast it to the class that implements it but is not what I want.
What I did to solve it were declaring a new publisher type property that returns the $foo property in its implementation as follow.
protocol Manageable {
var foo: String { get }
var fooPublisher: Published<String>.Publisher { get }
}
class Manager: Manageable {
#Published var foo: String
var fooPublisher: Published<String>.Publisher {
return $foo
}
}
So now I call the publisher.
let manager: Manageable = Manager()
manager.fooPublisher // <== Works
I wonder if this is a proper way to solve this.
Have you guys found another way to do it ?
Related
I have this sample protocol having a default valued property with extension.
protocol SampleProtocol{
var sample:String?{get set}
}
extension SampleProtocol{
var sample:String?{ get { return nil } set{} }
}
Now, my TestClass implements SampleProtocol as below.
class TestClass:SampleProtocol {
var sample: String?{
return "TestClass"
}
}
And there's one helper method that prints a sample value from the SampleProtocol.
func printValue(_ value: SampleProtocol){
print(value.sample)
}
Now the problem is
let testObj = TestClass()
print(testObj.sample) // prints "TestClass"
printValue(testObj) // prints nil
From the above result, I need to understand why when the testObj is typecasted into SampleProtcol, it is considering default implementation from extension instead of the implementation from the TestClass?
The problem is that the TestClass is not implementing the SampleProtocol so when you pass it in the printValue method that takes a SampleProtocol, it will call the default implementation in the protocol extension(since the instance you pass does not implement the protocol method).
In the SampleProtocol you have defined the variable as {get set} but in the TestClass variable you have only provided a getter, this does not match the requirements (If you remove the protocol extension you will see an error that the class does not conform to the protocol).
It will work if you provide a setter:
class TestClass: SampleProtocol {
var sample: String? {
get {
return "TestClass"
} set { }
}
}
I'm trying to learn protocols and associatedtypes. I have couple of protocols which declare associatedtypes, starting with:
protocol MasterViewModel {
associatedtype Item: AWMediaItem
...
}
AWMediaItem is another protocol
protocol AWMediaItem {
var name: String { get }
var source: AWMediaSource { get }
}
And AWAlbum is yet another protocol which inherits from AWMediaItem
protocol AWAlbum: AWMediaItem {
var albumName: String { get }
...
}
For some reason, in a class implementing MasterViewModel protocol, I cannot set the AWAlbum to be the Item.
final class AlbumsMasterViewModel: MasterViewModel {
typealias Item = AWAlbum // Error
...
}
The warning I get is
Possibly intended match 'AlbumsMasterViewModel.Item' (aka 'AWAlbum') does not conform to 'AWMediaItem'
If I understand correctly, all AWAlbum's will implement AWMediaItem so why is this not working?
I think you meant to write
final class AlbumsMasterViewModel<Item: AWAlbum>: MasterViewModel {
}
I assume that when you write :
typealias Item = AWAlbum // Error
you want AlbumsMasterViewModel's item to conform to your AWAlbum protocol but you just create a typelias meaning that Item is just an alias for AWAlbum .
If you want to use a type alias you need a concrete type conforming to AWMediaItem, not a protocol inheriting from it. eg :
class ConcreteAlbum: AWAlbum {
var albumName: String
var name: String
var source: AWMediaSource
...
}
final class AlbumsMasterViewModel: MasterViewModel {
typealias Item = ConcreteAlbum // No Error
}
Edit
if you want to use AlbumsMasterViewModel with multiple Item types you can also declare it that way :
final class AlbumsMasterViewModel<Item: AWMediaItem>: MasterViewModel {
}
I'm confused when using get set in protocols. Using only get works fine, but the set part doesnt'.
protocol MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject { get set }
}
extension MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject {
get { return MainViewModel.instance.localDoor }
set { localDoor = newValue }
}
}
final class MainViewModel: MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject = LocalDoorCoreDataObject()
...
Then when I use it in the viewController
self.mainViewModel.localDoor = $0
But this gives me the error
Cannot assign to property: 'mainViewModel' is a get-only property
How do I set it up properly?
EDIT
Initiation of the viewModel is done with factory based dependency injection
protocol MainViewModelInjected {
var mainViewModel: MainViewModelProtocol { get }
}
extension MainViewModelInjected {
var mainViewModel: MainViewModelProtocol { return MainViewModel.instance }
}
It is totally depends on how you create object for mainViewModel.
Let's create some cases with your code:
import UIKit
typealias LocalDoorCoreDataObject = String
protocol MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject { get set }
}
extension MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject {
get { return MainViewModel.instance.localDoor }
set { localDoor = newValue }
}
}
final class MainViewModel: MainViewModelProtocol {
static let instance = MainViewModel()
var localDoor: LocalDoorCoreDataObject = LocalDoorCoreDataObject()
}
protocol MainViewModelInjected {
var mainViewModel: MainViewModelProtocol { get }
}
extension MainViewModelInjected {
var mainViewModel: MainViewModelProtocol { return MainViewModel.instance }
}
Case 1
Here we are creating an object and assigning object through getter as a closure.
So, here mainViewModel has only getter not setter i.e. it'a get-only property.
var mainViewModel: MainViewModelProtocol { MainViewModel.instance }
mainViewModel.localDoor = "assign some thing" // Error: Cannot assign to property: 'mainViewModel' is a get-only property
Case 2
Here we are directly assigning object to mainViewModelOther. So, this will be a normal property and you can make changes in properties of model.
var mainViewModelOther: MainViewModelProtocol = MainViewModel.instance
mainViewModelOther.localDoor = "assign some thing"
Case 3
You can also create a class that will hold your model object, and created another object of your class. You can make changes in properties of model.
class MyClass {
var mainViewModel: MainViewModelProtocol = MainViewModel.instance
}
let myClassObj = MyClass()
myClassObj.mainViewModel.localDoor = "assign some thing"
TL;DR
Mark your MainViewModelProtocol as being class-only (i.e. protocol MainViewModelProtocol: class { ... }) to solve the issue.
The long answer
To understand why marking your MainViewModelProtocol as class-only fixes the problem, we need to take couple steps back and look at how structs and classes are stored internally.
Case 1: MainViewModelProtocol is a reference-type (i.e. class)
First, let's consider the case where MainViewModel is a class: Classes are reference-types, which means that after you retrieve the your view model through the mainViewModel property, you have a pointer to the same view model that is stored inside your view controller. Modifying the referenced type will also modify the view model of the view itself (since they both point to the same object). As an example
/* ... */
class MainViewModel: MainViewModelProtocol { /* ... */ }
var viewModel = myViewController.mainViewModel
viewModel.localDoor = /* something */
modifies the view model that's shared between the local variable viewModel and the view controller. This is exactly what you want.
Case 2: MainViewModelProtocol is a value type (i.e. struct)
Now let's consider if the MainViewModel was a struct: structs are value-types, so retrieving the view model through the mainViewModel computed property essentially clones the view model. Now you might modify the retrieved view model as much as you like locally, but there is no way assign it back to your view controller
/* ... */
struct MainViewModel: MainViewModelProtocol { /* ... */ }
var viewModel = myViewController.mainViewModel
viewModel.localDoor = /* something */
just modifies the local copy of the view model stored in the viewModel variable. There is no way to assign the local variable back to myViewController.
Conclusion
I hope this illustrates why your pattern only works with reference-types and not value types.
Now the Swift compiler needs to be conservative and consider both cases since it doesn't know if all types conforming to MainViewModelProtocol will be classes or structs (consider public protocols vended as a library to which library-users can conform). If you add the class-constraint to the protocol, you tell the compiler that using the pattern from Case 1 is totally fine – just grab a shared instance and modify it – and that there is no need for a setter to modify the view model.
No need to mark MainViewModelProtocol as class only, when the compiler says :
Cannot assign to property: 'mainViewModel' is a get-only property
it's actually complaining about your view controller implementation. I assume mainViewModel is a computed property so you can't assign it.
I managed to reproduce your error with the following playground :
typealias LocalDoorCoreDataObject = String
protocol MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject { get set }
}
extension MainViewModelProtocol {
var localDoor: LocalDoorCoreDataObject {
get { return MainViewModel.instance.localDoor }
set { localDoor = newValue }
}
}
final class MainViewModel: MainViewModelProtocol {
static let instance = MainViewModel()
var localDoor: LocalDoorCoreDataObject = LocalDoorCoreDataObject()
}
final class FakeVC {
var mainViewModel: MainViewModelProtocol {
MainViewModel.instance
}
}
var viewController = FakeVC()
viewController.mainViewModel.localDoor = "foo" // Cannot assign to property: 'mainViewModel' is a get-only property
I got rid of the error by changing FakeVC implementation to :
final class FakeVC {
var mainViewModel: MainViewModelProtocol = MainViewModel()
}
protocol BasePresenterProtocol : class {}
protocol DashboardPresenterProtocol : BasePresenterProtocol {}
final class DashboardPresenter {
weak var view: DashboardPresenterProtocol?
init() {
self.view = DashboardViewController()
}
func test() {
print("Hello")
}
}
extension DashboardPresenter: DashboardViewProtocol { }
protocol BaseViewProtocol : class {
weak var view: BasePresenterProtocol? { get set }
}
protocol DashboardViewProtocol : BaseViewProtocol {
}
class DashboardViewController {
}
extension DashboardViewController: DashboardPresenterProtocol { }
In the above code, I get an error at following line
extension DashboardPresenter: DashboardViewProtocol { }
that, DashboardPresenter doesn't confirm to protocol DashboardViewProtocol, but I have declared weak var view: DashboardPresenterProtocol? in DashboardPresenter . Although I have declared
Why am I getting this error ? Please let me know what I am doing wrong in this code.
You cannot implement a read-write property requirement of type BasePresenterProtocol? with a property of type DashboardPresenterProtocol?.
Consider what would happen if this were possible, and you upcast an instance of DashboardPresenter to DashboardViewProtocol. You would be able to assign anything that conforms to BasePresenterProtocol to a property of type DashboardPresenterProtocol? – which would be illegal.
For this reason, a read-write property requirement has to be invariant (although it's worth noting that a readable-only property requirement should be able to be covariant – but this currently isn't supported).
After reading the docs about this I discovered
In Swift, as in Objective-C, protocol conformance is global—it is not possible for a type to conform to a protocol in two different ways within the same program.
So what is the purpose of the private prefix here
private protocol PartyFormViewControllerDelegate: class {
func partyFormViewController(controller: PartyFormViewController, cancelButtonPressed button: UIBarButtonItem)
}
class PartyFormViewController: GenericViewController {
//...
}
In Swift private means: visible within the current source file.
A private Type could conform to a private protocol, look here:
private protocol Animal { }
private class Dog: Animal { }
class Zoo {
private var animals = [Animal]()
var count : Int { return animals.count }
}
Here, Animal and Dog are visible only within the current files. However, they are used by Zoo which has internal visibility and exposes the count of the animals to the whole module.