In Swift 2.0, how can I do the equivalent of #property (nonatomic, strong) NSManagedObject*<SomeProtocol> model?
Basically, I’m trying to define a property on my class that must both be a subclass of NSManagedObject AND conform to SomeProtocol (I will be calling methods defined by both).
I saw this: https://stackoverflow.com/a/25826948/363789, but I'm not sure how I can apply this syntax to property definition...
Swift 4
This is now possible in Swift 4 using the following syntax:
var myObject: NSManagedObject & SomeProtocol
Unfortunately Swift doesn't support such type composition yet.
Three reasonable good solutions as workaround (the third one is probably the best):
1.
You can make another type with has those properties and all types have to inherit from it in order to be used as model.
class SomeManagedObject: NSManagedObject, SomeProtocol {
// conforming to protocol
func someMethod()
}
// type declaration
var model: SomeManagedObject
2.
A more static way to solve this problem is to use generics:
class Aclass<T: NSManagedObject where T: SomeProtocol > {
var model: T
}
Note: You don't have to care about another type which has to be the superclass but this solution is not as dynamic and abstract as the one before.
3.
You could also make your own protocol and make NSManagedObject conform though an extension:
protocol ManagedProtocol {
// if you want to access some methods/properties directly from the type
func method()
var variable: Int { get }
// otherwise call all methods/properties through "managedObject"
// this property also reduces casting if you want to have the object itself
var managedObject: NSManagedObject { get }
}
extension NSManagedObject: ManagedProtocol {
var managedObject: NSManagedObject { return self }
}
Now the type is abstract and can be written as:
var model: protocol<ManagedProtocol, SomeProtocol>
Related
Context
I am currently working with Protocols and AssociatedTypes and encountered the following new Xcode Warning I don't really understand.
Non-final class 'SomeCustomComponent' cannot safely conform to protocol 'CustomComponent', which requires that 'Self.C.CC' is exactly equal to 'Self'; this is an error in Swift 6
Code
protocol Component {
associatedtype CC: CustomComponent where CC.C == Self
var customComponent: CC { get }
}
protocol CustomComponent {
associatedtype C: Component where C.CC == Self
var component: C { get }
}
enum SomeComponent: Component {
var customComponent: { ... }
}
// Please note, that SomeCustomComponent is an NSManagedObject conforming to CustomComponent.
extension SomeCustomComponent: CustomComponent { // I get the Xcode Warning in this Line.
var component: C { ... }
}
Question
How can I handle this Xcode Warning? Is it possible to mark an NSManagedObject as final? And how do I do it since it is defined in the background?
You can mark an NSManagedObject subclass as final but this of course requires that you manage the code for the subclass yourself, that is set Codegen to Manual for this entity in your core data model.
Regarding the final requirement, this is because if you would create a subclass the mapping between the two protocols might break.
Consider if you create a subclass of SomeCustomComponent,
class SubCustomComponent: SomeCustomComponent {}
with no other changes that concern the protocols. Then the component property of the subclass would point to the enum SomeComponent but the enum would not point back to the new class SubCustomComponent but to its superclass.
So if you would use this for some kind of conversion or mapping in both directions your would go from SubCustomComponent to SomeComponent and back to SomeCustomComponent which is surely a bug.
This question already has answers here:
Why can't a get-only property requirement in a protocol be satisfied by a property which conforms?
(3 answers)
Closed 5 years ago.
(if someone can suggest a better title, please do)
The following code does not compile with an error Type 'ObserverClass' does not conform to protocol 'Observer', and compiler suggests a fix by declaring var object: ObservedObject.
class ObservedObject {}
class ObservedObjectSubclass: ObservedObject {}
protocol Observer {
var object: ObservedObject { get }
}
class ObserverClass: Observer { // Type 'ObserverClass' does not conform to protocol 'Observer'
// suggested:
// var object: ObservedObject
var object: ObservedObjectSubclass = ObservedObjectSubclass()
}
The way i see it - ObservedObjectSubclass is ObservedObject, and so object property is guaranteed to be of type ObservedObject as required by the protocol.
(The same is true, if using protocol conformance instead of subclassing - below)
protocol ObservedObjectProtocol {}
protocol Observer {
var object: ObservedObjectProtocol { get }
}
class ObservedObject: ObservedObjectProtocol {}
class ObserverClass: Observer { // same error
var object: ObservedObject = ObservedObject()
}
Why is compiler unhappy? Is it current limitation, or the compiler is actually right and there is some logical constraint?
When you define a variable in a protocol and assign a type to it, that is going to be a concrete type, so you cannot change it to a subclass of that type when conforming to the protocol. The type of the variable declared in the conforming class must be the same type as declared in the protocol, it cannot be a covariant (inheritance related) type.
You can fix the second error by creating an associatedType for your Observer protocol, which inherits from ObserverObject then you can define object to be of the same type as your associated type. Then you can make your ObserverClass have a property object of type ObservedObjectSubclass.
class ObservedObject {}
class ObservedObjectSubclass: ObservedObject {}
protocol Observer {
associatedtype ObjectSubclass: ObservedObject
var object:ObjectSubclass { get }
}
class ObserverClass: Observer {
var object = ObservedObjectSubclass()
}
class SuperDelegate <T: AnyObject> {
func addDelegate(delegate: T)
{
}
}
My question is about T key, does it mean the same as id in Objective-c? I mean about case of uses.
how to understand the first line class SuperDelegate <T: AnyObject> Sorry I am new in Swift.
As Objective C program for me this line means that we make class to conform a protocol that has to implement all required method. But I don't understand func addDelegate(delegate: T) is this the same like
- (void)addDelegate:(id)delegate which is a property id <T> delegate.
Yes you are correct in your assumptions that AnyObject behaves like id:
You can call any Objective-C method and access any property on an
AnyObject value without casting to a more specific class type. This
includes Objective-C compatible methods and properties marked with the
#objc attribute.
but you have used it here as a generic type rather than as a concrete type that should be cast to. The class is requiring a type that adheres to the AnyObject protocol but it isn't forcing it to be AnyObject (see header files: cmd + click on AnyObject inside Xcode).
So your instance could be instantiated SuperDelegate<AnyObject> but it could also be instantiated SuperDelegate<NSDate>. This means that the whole subset of ObjC methods and properties cannot be guaranteed as they can with a cast to AnyObject as a concrete type because at runtime T might represent NSDate or NSNumber or any other class.
To achieve what you want you would need to write:
class SuperDelegate {
func addDelegate(delegate: AnyObject)
{
}
}
But Swift is a strongly-typed language and it would normally be the case that you had a delegate protocol and that the delegate for your type adhered to the delegate protocol:
protocol DelegateProtocol {
func somethingHappened()
}
struct MyTypeDelegate:DelegateProtocol {
func somethingHappened() {
print("Thanks for telling me!")
}
}
struct MyType {
var delegate:DelegateProtocol?
func tellDelegateSomethingHappened() {
delegate?.somethingHappened()
}
}
let del = MyTypeDelegate()
var type = MyType()
type.delegate = del
type.tellDelegateSomethingHappened()
I want to use a strategy pattern to register a set of objects that implement a protocol. When I set this up, I get a compile error when trying to set the delegate that is part of the protocol.
For discussion purposes, I have slightly reworked the DiceGame from the Swift eBook's Delegation chapter. The changes of significance are:
protocol DiceGame - declares a delegate
class SnakesAndLadders implements DiceGame (and therefore the protocol and delegate)
class Games holds 3 instances of SnakesAndLadders as
1) a concrete class of SnakesAndLadders
2) a 'let' constant of protocol DiceGame
3) a 'var' variable of protocol DiceGame
We can set the delegate fine if we use the concrete class (snakesAndLadders). However, there is a compile error if we use 'let' to hold it as a protocol (diceGameAsLet) but it compiles if we hold the variable as a 'var' (diceGameAsVar).
It is easy to work around, however, the delegate itself never changes so should be held as a 'let' constant, as it is only the internal property that changes. I must not understand something (possibly subtle but significant) about protocols and how they work and should be used.
class Dice
{
func roll() -> Int
{
return 7 // always win :)
}
}
protocol DiceGame
{
// all DiceGames must work with a DiceGameDelegate
var delegate:DiceGameDelegate? {get set}
var dice: Dice {get}
func play()
}
protocol DiceGameDelegate
{
func gameDidStart( game:DiceGame )
func gameDidEnd( game:DiceGame )
}
class SnakesAndLadders:DiceGame
{
var delegate:DiceGameDelegate?
let dice = Dice()
func play()
{
delegate?.gameDidStart(self)
playGame()
delegate?.gameDidEnd(self)
}
private func playGame()
{
print("Playing the game here...")
}
}
class Games : DiceGameDelegate
{
let snakesAndLadders = SnakesAndLadders()
// hold the protocol, not the class
let diceGameAsLet:DiceGame = SnakesAndLadders()
var diceGameAsVar:DiceGame = SnakesAndLadders()
func setupDelegateAsClass()
{
// can assign the delegate if using the class
snakesAndLadders.delegate = self
}
func setupDelegateAsVar()
{
// if we use 'var' we can assign the delegate
diceGameAsVar.delegate = self
}
func setupDelegateAsLet()
{
// DOES NOT COMPILE - Why?
//
// We are not changing the dice game so want to use 'let', but it won't compile
// we are changing the delegate, which is declared as 'var' inside the protocol
diceGameAsLet.delegate = self
}
// MARK: - DiceGameDelegate
func gameDidStart( game:DiceGame )
{
print("Game Started")
}
func gameDidEnd( game:DiceGame )
{
print("Game Ended")
}
}
DiceGame is a heterogeneous protocol that you're using as a type; Swift will treat this type as a value type, and hence (just as for a structures), changing its mutable properties will mutate also the instance of the protocol type itself.
If you, however, add the : class keyword to the DiceGame protocol, Swift will treat it as a reference type, allowing you to mutate members of instances of it, without mutating the instance itself. Note that this will constraint the protocol as conformable to only by class types.
protocol DiceGame: class { ... }
With the addition of the above, the mutation of immutable diceGameAsLet:s properties will be allowed.
In this context, it's worth mentioning that the : class keyword is usually used to constrain protocols used as delegates (e.g., DiceGameDelegate in your example) as conformable to only by class types. With this additional constraint, the delegates can be used as types to which the delegate owner (e.g. some class) only hold a weak reference, useful in contexts where a strong reference to the delegate could create a retain cycle.
See e.g. the 2nd part of this answer for details.
The issue is that when you store something as a Protocol, even if it is a class, swift considers them to be a value type, instead of the reference type you are expecting them to be. Therefore, no part of it is allowed to be changed. Take a look at this reference for more information.
In Objective-C, it's possible to write something like that:
#property(retain) UIView<Protocol1, Protocol2, ...> *myView;
But how can I write this code in swift?
I already know how to make a property conform to many protocols, but it does not work by using the inheritance:
var myView: ??? protocol<Protocol1, Protocol2, ...>
Edit:
I use many UIView subtypes like UIImageView, UILabel or others, and I need to use some of the UIView properties plus some methods defined in the protocols. In the worst case I could create a UIViewProtocol with the needed properties, but I would know if it is possible in Swift to declare a property/variable with a type and some protocol to conform with.
You can do this with a generic class using a where clause:
A where clause enables you to require that an associated type conforms
to a certain protocol, and/or that certain type parameters and
associated types be the same.
To use it, make the class your property is defined in a generic class with a type constraint to check if the type parameter for your property matches your desired base class and protocols.
For your specific example, it could look something like this:
class MyViewController<T where T: UIView, T: Protocol1, T: Protocol2>: UIViewController {
var myView: T
// ...
}
In Swift 4 it's finally possible. You can declare variable of some class conforming to protocol at the same time, like this:
class ClassA {
var someVar: String?
}
protocol ProtocolA {}
class ClassB {
var someOptional: (ClassA & ProtocolA)? // here is optional value
var some: ClassA & ProtocolA // here is non-optional value; need to provide init though :)
}
One and probably a bit ugly one of the ways to do that, is to create a wrapper protocol for UIView:
protocol UIViewRef {
var instance: UIView { get }
}
Now it is possible to create a protocol which implements Protocol1, Protocol2 and UIViewRef, which is going to be used to get the UIView itself:
protocol MyUIViewProtocol: UIViewRef, Protocol1, Protocol2 { }
And last step will be implementing UIViewRef protocols for your UIViews, which, in you case, as I understand, already implement Protocol1 and Protocol2:
// SomeOfMyViews already implements Protocol1 and Protocol2
extension SomeOfMyUIViews: MyUIViewProtocol {
var instance: UIView { return self }
}
As the result we have MyUIViewProtocol, implementers of which hold a reference to a UIView and each of them implement
Protocol1 and Protocol2. One caveat though - to get the UIView itself, we need to ask it's reference from instance
property. For example
// Lets say we're somewhere in a UIViewController
var views: [SomeOfMyUIView] = // Get list of my views
views.forEach { self.view.addSubview($0.instance) }