问题
I'm trying to build a protocol exposing a static property, then use that static property in an extension of that protocol, but it seems to work only if I define this static property in the protocol extension as well. Basically the code I'm trying to get to work:
protocol NibInstantiable: class {
static var bundle: Bundle? { get }
static var nibName: String { get }
}
extension NibInstantiable where Self: UIViewController {
// static var nibName: String {
// return ""
// }
static func instantiate() -> Self {
return Self(nibName: Self.nibName, bundle: Self.bundle ?? Bundle.main)
}
}
This used to work basically as-is in Swift 2, but it's no longer the case in Swift 3. I can get it to work by uncommenting the nibName
property in the protocol extension, but that would suppress compiler warnings if I forget to define this property in classes that implement this protocol.
Any idea what I'm missing ? Thanks !
EDIT: For reference, here is a Swift 2.3 version of that code that compiles and works without any issue:
protocol Instantiable {
static var bundle: NSBundle? { get }
static func instantiate() -> Self
}
extension Instantiable {
static var bundle: NSBundle? {
return NSBundle.mainBundle()
}
}
// MARK: With Nib
protocol NibInstantiable: Instantiable {
static var nibName: String { get }
}
extension NibInstantiable where Self: UIViewController {
static func instantiate() -> Self {
return Self(nibName: Self.nibName, bundle: Self.bundle ?? NSBundle.mainBundle())
}
}
回答1:
This looks like a bug to me (see the related bug report SR-2992) – the compiler thinks that there's a conflict between UIViewController
's nibName instance property and your NibInstantiable
protocol's nibName
static property requirement. A simpler reproducible example would be:
protocol Foo {
static var bar : String { get }
}
class Bar {
var bar = "" // commenting out this line allows the code to compile
}
extension Foo where Self : Bar {
static func qux() {
print(bar) // compiler error: Instance member 'bar' cannot be used on type 'Self'
}
}
A simple workaround would be to just rename your protocol's nibName
static property requirement.
来源:https://stackoverflow.com/questions/41329940/access-static-property-of-protocol-extension