Downcast from Any to a protocol

问题:

I have the following code. How can I resolve the error in the last line?

protocol Animal {     func walk() }  struct Cat: Animal {     func walk() {}      init() { } }  var obj: Any = Cat() var cat = obj as Animal // ERROR: cannot downcast from Any to unrelated type Animal 

回答1:

Update: This has been fixed in Swift 1.2+ (Xcode 6.3+). The Xcode 6.3 beta release notes say:

Dynamic casts (“as!", “as?" and “is”) now work with Swift protocol types, so long as they have no associated types.

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You can only check for protocol conformance (which includes is, as, and as?) with an @objc protocol. Animal is not @objc.

See the Checking for Protocol Conformance section of the Swift book.

NOTE

You can check for protocol conformance only if your protocol is marked with the @objc attribute

回答2:

You can workaround it by doing

 var cat = obj as Cat as Animal 

but this workaround is almost useless... because you need to know the type of obj first

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Edit:

As @newacct point out, it is not bug, see his answer for more information

xcrun swift Welcome to Swift!  Type :help for assistance.   1>   2> @objc protocol Animal {   3.         func walk()   4. }  @objc class Cat: Animal {     func walk() {}      init() { } }  var obj: AnyObject = Cat()  var cat = obj as Animal   5>   6> @objc class Cat: Animal {   7.         func walk() {}   8.   9.         init() { }  10. }  11>  12> var obj: AnyObject = Cat() obj: Cat = {}  13>  14> var cat = obj as Animal cat: Cat = {}  15> 

Animal protocol need @objc attribute and Cat need to be @objc class.

回答3:

My approach

@objc protocol Animal {     func walk() }  @objc class DummyAnimal: Animal {     func walk() {      } }  @objc class Cat: DummyAnimal {     override func walk() {         print("meow")     }      override init() { } }  var obj: Any = Cat() var cat = obj as DummyAnimal cat.walk() 

回答4:

As far as I have been able to observe, AnyObject can only be downcastAny can only be downcast to types (class or otherwise), it is not possible to cast them to a protocol.

I don't know if I would qualify this as a bug or not, if this holds then perhaps it was implemented this way for a good reason - but @Bryan's workaround (casting to a type first and then to a protocol) at least "resolves the error"!

文章来源: Downcast from Any to a protocol