I have a class, lets call it SomeClass. Instances of SomeClass have an optional pointer to SomeOtherClass. In this way, instances of SomeClass can be instantiated, given a pointer to SomeOtherClass (or a subclass of SomeOtherClass), and then this pointer can be used to dynamically create instances of this SomeOtherClass belonging to SomeClass. Eg;
class SomeClass {
var classPointer: SomeOtherClass.Type?
}
class SomeOtherClass {
}
So far so good. Now, I have a protocol - lets call it SomeProtocol - that I want SomeOtherClass to conform to. This protocol has class functions in it:
protocol SomeProtocol {
static func someClassFunction()
}
extension SomeOtherClass : SomeProtocol {
class func someClassFunction() {
print("I am a class function being executed on SomeOtherClass")
}
}
As expected, I can then call this protocol class function on SomeOtherClass like so:
SomeOtherClass.someClassFunction() // Prints "I am a class function being executed on SomeOtherClass"
Here is the troublesome part. I want to dynamically determine if an instance of SomeClass' classPointer conforms to SomeProtocol, and if so execute the class function on it. So, I try to cast the pointer using as?:
// Create an instance of SomeClass and set it's classPointer to the SomeOtherClass class
let someInstance = SomeClass()
someInstance.classPointer = SomeOtherClass.self
// Check if the instance's classPointer class conforms to the SomeProtocol protocol
if let conformingClass = someInstance.classPointer as? SomeProtocol {
// If so, execute the class function in SomeProtocol on the instance's classPointer
conformingClass.someClassFunction() // Build fails "Static member someClassFunction cannot be used on instance of type SomeProtocol"
}
And the build fails with the error "Static member of someClassFunction cannot be used on instance of type SomeProtocol".
Is there a way to accomplish what I'm attempting? Currently if this doesn't work I can only think of these alternatives (none are preferable and they're all rather hacky):
For completeness, here is all of the code together that can be pasted into a Playground (it won't build due to the error I mentioned though):
class SomeClass {
var classPointer: SomeOtherClass.Type?
}
class SomeOtherClass {
}
protocol SomeProtocol {
static func someClassFunction()
}
extension SomeOtherClass : SomeProtocol {
class func someClassFunction() {
print("I am a class function being executed on SomeOtherClass")
}
}
// Create an instance of SomeClass and set it's classPointer to the SomeOtherClass class
let someInstance = SomeClass()
someInstance.classPointer = SomeOtherClass.self
// Check if the instance's classPointer class conforms to the SomeProtocol protocol
if let conformingClass = someInstance.classPointer as? SomeProtocol {
// If so, execute the class function in SomeProtocol on the instance's classPointer
conformingClass.someClassFunction() // Build fails "Static member someClassFunction cannot be used on instance of type SomeProtocol"
}
Thanks for any help you can provide, - Adam
Ahah! As usual, as soon as I make the SO post, I figure out the answer.
For those wondering, you must cast the classPointer as the protocol's Type, not as the protocol itself. The line:
if let conformingClass = someInstance.classPointer as? SomeProtocol {
Needs to be changed to:
if let conformingClass = someInstance.classPointer as? SomeProtocol.Type {
And you'll then be able to message conformingClass with the class functions declared in SomeProtocol. The complete working code is:
class SomeClass {
var classPointer: SomeOtherClass.Type?
}
class SomeOtherClass {
}
protocol SomeProtocol {
static func someClassFunction()
}
extension SomeOtherClass : SomeProtocol {
class func someClassFunction() {
print("I am a class function being executed on SomeOtherClass")
}
}
// Create an instance of SomeClass and set it's classPointer to the SomeOtherClass class
let someInstance = SomeClass()
someInstance.classPointer = SomeOtherClass.self
// Check if the instance's classPointer class conforms to the SomeProtocol protocol
if let conformingClass = someInstance.classPointer as? SomeProtocol.Type {
// If so, execute the class function in SomeProtocol on the instance's classPointer
conformingClass.someClassFunction()
}
And it works :).
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