CODEWITHSUNDEEP
swiftgenericscasting
Tod Cunningham
Tod Cunningham

Reputation: 3709

Swift why does as? require AnyObject vs Any

I'm getting a compiler error at the as?. Type 'Type' does not conform to protocol 'AnyObject'. Why would as? be requiring AnyObject?

func listForKey<Type>(key: String) -> [Type] {

    guard let fullList = (itemList as NSArray).valueForKey( key ) as? NSArray else {
        return [Type]()
    }

    // Filter out any values not matching the expected type such as if nil was used (value wasn't supplied)!
    let typeOnlyList = fullList.filter( {$0 as? Type != nil} )
    guard let foundList = typeOnlyList as? [Type] else {     // <== at as?, Compiler Error 'Type' does not conform to protocol AnyObject
        return [Type]()
    }

    return foundList
}

If I change the declaration to the following, it will compile:

func listForKey<Type:AnyObject>(key: String) -> [Type] {

However, it then won't work with String objects as Strings are of type any. Any thoughts on what's going on?

enter image description here

I thought I had a potential solution after reading Anton's comment by casting to Any. That resolved the compile time error, but now I get a runtime error (fatal error: array cannot be bridged from Objective-C):

extension NSArray {
    public func toSwiftArray<Type>() -> [Type] {
        // Filter out any values not matching the expected type such as nil
        let typeOnlyList : [AnyObject] = self.filter( {$0 is Type} )
        let typeOnlyAnyList : [Any] = typeOnlyList as [Any]                // <== Runtime error EXC_BAD_INSTRUCTION
        guard let foundList : [Type] = typeOnlyAnyList as? [Type] else {
            return [Type]()
        }

        return foundList
    }
}

Why would casting from [AnyObject] to [Any] cause a runtime error? I figured [Any] was the super set of [AnyObject].

Upvotes: 2

Views: 419

Answers (3)

Sulthan
Sulthan

Reputation: 130092

If you want to work with NSArray in Swift, you will have to work with classes only because NSArray cannot hold anything else.

extension NSArray {
    public func toSwiftArray<Type: AnyObject>() -> [Type] {

        let typeOnlyList : NSArray = self.filter( {$0 is Type} )
        return typeOnlyList as! Array<Type>
    }
}

let nsArray: NSArray = [10, 20, "my-text"]
print("NSArray: \(nsArray)")

let swiftArray: [NSString] = nsArray.toSwiftArray()
print("Swift array: \(swiftArray)")

In short: use NSString instead of String. You can also convert [NSString] to [String] as the second step.

Upvotes: 0

Tod Cunningham
Tod Cunningham

Reputation: 3709

I guess this doesn't really answer the original question. However, it is a workaround that gets the job done. I still don't understand why the solution above doesn't work (especially the fatal error when casting from [AnyObject] to [Any]), but I took a different approach which works great:

extension NSArray {
    public func toSwiftArray<Type>() -> [Type] {
        var swiftArray = [Type]()

        for value in self {
            if let valueOfType = value as? Type {
                swiftArray.append( valueOfType )
            }
        }

        return swiftArray
    }

    public func toSwiftArray<Type>() -> ([Type], [Any]) {
        var swiftTypeArray = [Type]()
        var unknownTypeArray = [Any]()

        for value in self {
            if let valueOfType = value as? Type {
                swiftTypeArray.append( valueOfType )
            } else {
                unknownTypeArray.append( value )
            }
        }

        return (swiftTypeArray, unknownTypeArray)
    }
}

Not sure why I couldn't use .filter to do this, but this is a very straightforward solution to the problem, and it also allows for a version that returns a list of the values that couldn't be converted. This is very handy routine for converting to NSArray with full type safety.

Upvotes: 0

0x416e746f6e
0x416e746f6e

Reputation: 10136

Here:

guard let fullList = ... as? NSArray

... you declare your fullList variable to be NSArray.

Therefore typeOnlyList is also NSArray (it's a result of filtering of fullList).

NSArray's elements are AnyObject's (bridged from Objective-C).

Therefore, when you try to cast typeOnlyList as? [Type] Swift expects Type to conform to AnyObject protocol.

Upvotes: 1

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