Fatal error when casting array of types to protocols: cannot be bridged from Objective-C

问题

There's similar questions out there, but this one is on the latest Swift 2.2 version. Hopefully there's a solution by now because this seems like a big obstacle to Protocol-Oriented Programming in my mind.

The below fails on assigning to let results with an error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0X0).

protocol P: class {
    var value:Int {get}
}

class X: P {
    var value = 0

    init(_ value:Int) {
        self.value = value
    }
}

func getItems() -> [P] {
    let items: [X] = [X(1), X(2), X(3)]
    return items
}

let results: [P] = getItems()

Is there any way to treat an array of classes as an array of protocols that it conforms to? This seems like a really simple and natural request for a language especially one that is heavily protocol-oriented.

I don't want to use @objc or flatMap because of vast implications on the dependency chain and performance - this would be a hack. I'd like this to work natively or it's a bug that we can hopefully formulate and present to Apple / Swift open-source team.

回答1:

may be i don't understand your question, but this works

protocol P: class {
    var value:Int {get}
}

class X: P {
    var value = 0

    init(_ value:Int) {
        self.value = value
    }
}

func getItems() -> [P] {
    let items: [P] = [X(1), X(2), X(3)]
    return items
}

let results = getItems()
results.forEach { (p) in
    print(p.value)
}
/*
 1
 2
 3
 */

why casting [X] as [P] doesn't work? See the next example!

protocol P: class {
    var value:Int {get}
}
protocol P1: class {
    var value: Double { get }
}
protocol Z {}
class X: P,Z {
    var value = 0

    init(_ value:Int) {
        self.value = value
    }
}
class X1: P1,Z {
    var value = 0.0

    init(_ value:Double) {
        self.value = value
    }
}

func getItems() -> [Z] {
    // the only common type of all items is protocol Z  !!!!
    let items: [Z] = [X(1), X(2), X(3), X1(1), X1(2)]
    return items
}

let results = getItems()
print(results.dynamicType)
results.forEach { (p) in
    if let p = p as? P {
        print("P:", p.value)
    }
    if let p = p as? P1 {
        print("P1:", p.value)
    }
}
/*
 Array<Z>
 P: 1
 P: 2
 P: 3
 P1: 1.0
 P1: 2.0
*/

that is the reason, why using flatMap is good idea, if you would like to separate X and X1 type items from results

let arrX = results.flatMap { $0 as? P }
let arrX1 = results.flatMap { $0 as? P1 }
print(arrX, arrX1) // [X, X, X] [X1, X1]

来源：https://stackoverflow.com/questions/36379934/fatal-error-when-casting-array-of-types-to-protocols-cannot-be-bridged-from-obj