Why is it recommended to prefix property requirements with static in Swift protocols? - swift

Question

In the Swift documentation section about Protocols, it is written that :
Always prefix type property requirements with the static keyword when you define them in a protocol. This rule pertains even though type property requirements can be prefixed with the class or static keyword when implemented by a class
I don't understand if you should always do that or just in specific cases (because later in the docs, static isn't always there).
Why is it recommended to use static? What does it mean concretely?
Thank you.
EDIT : The same question applies to methods requirements.

Answer 1

later in the docs, static isn't always there
This section talks about type requirement, i.e. a requirement of the conforming type to have a static property of the specific name and type. In other words, when you write this
protocol MyProtocol {
static var myProperty: Int { get set }
}
the conforming class must do this
class MyClass : MyProtocol {
static var myProperty: Int
}// ^^^^^^
It also has an option to do this:
class MyClass : MyProtocol {
class var myProperty: Int
}// ^^^^^
but there is no such option when defining a protocol.
When static is not used, as happens later in the docs, the requirement becomes an instance requirement, not a type requirement:
protocol MyProtocol2 {
var myProperty2: Int { get set }
}
Now the class must provide an instance variable:
class MyClass2 : MyProtocol2 {
var myProperty2: Int
}
Note that there is a similar limitation on declaring instance requirements in protocols: you always use var, even though an implementation may use let to satisfy the requirement:
protocol MyProtocol3 {
var myProperty3: Int { get }
}// ^^^
class MyClass3 : MyProtocol3 {
let myProperty3 = 42
}// ^^^