Down casting multiple protocol Array<protocol<P1, P2>> to Array<P1>

问题

So I have two arrays

var arrayOne:Array<protocol<P1,P2>>
var arrayTwo:Array<P1>

Where P1 and P2 are protocols.

Question is how to make downcasting operation

arrayTwo = arrayOne as Array<P1>

What i get from Xcode is:

Cannot convert value of type 'Array<protocol<P1, P2>>' to specified type 'Array<P1>'

回答1:

You need to cast the elements of the array, not the array itself.

arrayTwo = arrayOne.map { $0 as P1 }

Or as MartinR stated, there is even no need to cast the element.

arrayTwo = arrayOne.map { $0 }

回答2:

protocol P1{}
struct A: P1{}
// everybody knows, that
let a = A()
// (1)
let p: P1 = a
// or (2)
let p1 = a as P1

let arr: Array<A> = []
// has type
print(arr.dynamicType) // Array<A>
// (3) the array is converted to different type here !!
let arr1: Array<P1> = arr.map{ $0 }
print(arr1.dynamicType) // Array<P1>

// arr and arr1 are differnet types!!!
// the elements of arr2 can be down casted as in (1) or (2)
// (3) is just an equivalent of
typealias U = P1
let arr3: Array<U> = arr.map { (element) -> U in
    element as U
}
print(arr3.dynamicType) // Array<P1>


// the array must be converted, all the elements are down casted in arr3 from A to P1
// the compiler knows the type so ii do the job like in lines (1) or (2)

来源：https://stackoverflow.com/questions/34257678/down-casting-multiple-protocol-arrayprotocolp1-p2-to-arrayp1