I'm trying to create a protocol that describes a property that is a subclass of a certain class instead of being of that class.
I would like to know if something like this is possible.
protocol {
var prop : T where T: SomeClass { get set} // something like this
}
versus
protocol {
var prop : SomeClass {get set}
}
Use associatedtype in your protocol definition
protocol pp {
associatedtype T where T: SomeClass
var prop : T { get set}
}
Related
I'm struggling with protocols in Swift.
I have defined a protocol like this:
protocol AProtocol {
var property : BProtocol {get set}
}
And I would like to conform to AProtocol in a class with a property that also conform to another protocol. I've tried in these two ways:
class AClass: AProtocol {
var property = BClass()
}
and:
class AClass: AProtocol {
var property: BProtocol & MyBClassType = BProtocol()
}
but none of them seems to work (BClass itself confirm to BProtocol)
This issue is a bit difficult to explain, I hope it was clear.
Is it a limitation of the Swift language?
Do you know a work around to this?
You have two issues: firstly, the property name must match that declared in the protocol, secondly you need to type annotate the variable to be of type BProtocol as Hamish explained in the comment.
protocol AProtocol {
var aProperty : BProtocol {get set}
}
protocol BProtocol {}
class BClass: BProtocol {}
class AClass: AProtocol {
var aProperty: BProtocol = BClass()
}
You should also conform to the Swift naming convention, which is lowerCamelCase for variable names, so I changed AProperty to its correct form, aProperty.
I have some protocol hierarchies on my code where I have protocols defining the objects I use and protocols defining functions to use with this objects.
The object protocols are inherited by other object protocols that add more functionality to the original protocols and so are the functions that use them. The problem is that I can't find a way to specialize the function to take only the inherited parameter.
Here's some code to clarify what I'm trying to do:
protocol A {
var foo: String { get set }
}
protocol B: A {
var bar: String { get set }
}
struct Test: B {
var foo: String = "foo"
var bar: String = "bar"
}
protocol UseAProtocol {
static func use<T: A>(_ obj: T)
}
protocol UseBProtocol: UseAProtocol {
}
extension UseBProtocol {
//If I change the requirement to <T: B> this won't conform to `UseAProtocol`.
static func use<T: A>(_ obj: T) {
print(obj.foo)
// print(obj.bar) - Since obj does not conform to `B` I can't access ".bar" here without a forced casting.
}
}
struct Manager: UseBProtocol {
}
Manager.use(Test())
What I want to do is make the use function on the UseBProtocol only accept objects that conform to B. B inherits from A, but when I change from <T:A> to <T:B> I got an error saying that Manager does not conform to UseAProtocol and I have to change it back to <T:A>.
I know I can do this using associatedtype and where clauses on the inherit protocols - that's what I use today - but I wanted to move the generic requirement to the method so I could group all of them together under the same struct (I have a lot of this hierarchies and by using associatedtype I must use one struct by hierarchy). When the Conditional Conformances came to Swift this would be possible with associatedtype, but until them...
I could also use as! to force the casting from A to B on the UseBProtocol implementation, but that's a really bad solution and the error would be throw only at runtime.
Is there any way to achieve what I'm looking for?
It seems like what you are actually looking for is an associatedType in UseAProtocol rather than making the use function generic.
By declaring an associated type in UseAProtocol and changing the function signature of use to static func use(_ obj: ProtocolType) your code compiles fine and you can access both foo and bar from Manager.
protocol AProtocol {
var foo: String { get set }
}
protocol BProtocol: AProtocol {
var bar: String { get set }
}
struct Test: BProtocol {
var foo: String = "foo"
var bar: String = "bar"
}
protocol UseAProtocol {
associatedtype ProtocolType
static func use(_ obj: ProtocolType)
}
protocol UseBProtocol: UseAProtocol {
}
extension UseBProtocol {
static func use(_ obj: BProtocol) {
print(obj.foo)
print(obj.bar)
}
}
struct Manager: UseBProtocol {
}
Manager.use(Test()) //prints both "foo" and "bar"
I have the following protocols declared.
protocol TypeAProtocol {
...
}
protocol TypeBProtocol {
...
}
protocol SomeProtocol {
associatedtype TypeA: TypeAProtocol
associatedtype TypeB: TypeBProtocol
var objA: TypeA? { get set }
var objB: TypeB? { get set }
}
Now if I want to create a class that implements SomeProtocol, I would have done this
class SomeClass: SomeProtocol {
var objA: ClassOfTypeAProtocol?
var objB: ClassOfTypeBProtocol?
}
The problem I am facing now is, I want to be able to create classes that implement SomeProtocol without var objB: TypeB? { get set }. I want objB to be optional. How can I achieve that?
In Objective-C you can declare an optional member (method/computed property) in a protocol.
In pure Swift this is not possible unless you use the #objc annotation (but in this case structs and enums won't be able to conform to your protocol so it's not the best way to go).
So, how can you solve your problem?
Let's split SomeProtocol into 3 protocols
These are your protocols
protocol TypeAProtocol { }
protocol TypeBProtocol { }
protocol SomeProtocol {
associatedtype TypeA: TypeAProtocol
associatedtype TypeB: TypeBProtocol
var objA: TypeA? { get set }
var objB: TypeB? { get set }
}
Now we'll split SomeProtocol into 3 protocols
protocol SomeProtocolBase {
associatedtype TypeA: TypeAProtocol
associatedtype TypeB: TypeBProtocol
}
protocol SomeProtocolPartA: SomeProtocolBase {
var objA: TypeA? { get set }
}
protocol SomeProtocolPartB: SomeProtocolBase {
var objB: TypeB? { get set }
}
That's it
Now you can write
class ClassOfTypeAProtocol: TypeAProtocol { }
class ClassOfTypeBProtocol: TypeBProtocol { }
class SomeClass: SomeProtocolPartA {
typealias TypeA = ClassOfTypeAProtocol
typealias TypeB = ClassOfTypeBProtocol
var objA: ClassOfTypeAProtocol?
}
The following contrived Swift 2 example from real-world code won't compile:
protocol SomeModelType { }
protocol SomeProtocol {
var someVar: SomeModelType? { get }
}
class ConcreteClass<T: SomeModelType>: SomeProtocol {
var someVar: T?
}
This doesn't make sense to me fully. I would assume in ConcreteClass that because I have T being constrained to SomeModelType and have T as the backing type for the someVar property, the compiler would be able to figure out that the SomeProtocol was being conformed to by ConcreteClass.
How should an example like this be structured? Is it possible to the Swift compiler to determine protocol conformance through generic type constraints?
protocol SomeModelType { }
protocol SomeProtocol {
associatedtype T: Any
var someVar: T? { get }
}
class ConcreteClass<T> :SomeProtocol where T: SomeModelType {
var someVar: T?
}
I have a protocol (ProtocolA) containing a single property conforming to a second protocol (ProtocolB).
public protocol ProtocolA {
var prop: ProtocolB? { get }
}
public protocol ProtocolB {
}
I'm trying to declare two classes that will implement those:
private class ClassA : ProtocolA {
var prop: ClassB?
}
private class ClassB : ProtocolB {
}
But I get an error:
Type 'ClassA' does not conform to protocol 'ProtocolA'
Protocol requires property 'prop' with type 'ProtocolB?'
Candidate has non-matching type 'ClassB?'
Which is annoying as ClassB conforms to ProtocolB.
in the good-old i'd probably just declare the property as:
#property (nonatomic) ClassB <ProtocolB> *prop;
but the only way it seems I can get around this in swift is by adding an ivar like:
private class ClassA : ProtocolA {
var _prop: ClassB?
var prop: ProtocolB? { return _prop }
}
Is there no way around this?
You need to declare a typealias of the type that conforms to the other protocol. The way you did it is that prop has to be exactly of type ProtocolB, but you don't actually want that, you want a type that conforms to it instead.
protocol ProtocolA {
typealias Prop : ProtocolB
var prop: Prop? { get }
}
protocol ProtocolB {}
class ClassA : ProtocolA {
var prop: ClassB?
}
class ClassB : ProtocolB {}