Array element cannot be bridged to Objective-C

Question

I have this code that creates a view and applies a gradient to it.

import UIKit
import QuartzCore


let rect : CGRect = CGRectMake(0,0,320,100)

var vista : UIView = UIView(frame: rect)

let gradient : CAGradientLayer = CAGradientLayer()
gradient.frame = vista.bounds

let cor1 = UIColor.blackColor()
let cor2 = UIColor.whiteColor()

let arrayColors = [cor1.CGColor, cor2.CGColor]

gradient.colors = arrayColors

view.layer.insertSublayer(gradient, atIndex:0)

Xcode is giving me no compile error, but the code is crashing on the line

let arrayColors = [cor1.CGColor, cor2.CGColor]

with the message array element cannot be bridged to Objective-C

In fact I was expecting it to crash there, because I am not sure how I can create an array of CGColors on Swift. The surprise here is Xcode mentioning Objective-C. In my mind I was creating a CGColorRef in swift...

Any clues? Why is it mentioning Objective-C and how do I solve this?

Answer 1

The reason Objective-C is mentioned is because UIKit and QuartzCore are Objective-C frameworks. In particular, gradient.colors = arrayColors is calling an Objective-C method that expects an NSArray.

This seems like a bug, as Apple's documentation makes it sound like that the array should auto-bridge to an NSArray so long as the items in the array can be considered AnyObject:

When you bridge from a Swift array to an NSArray object, the elements in the Swift array must be AnyObject compatible. For example, a Swift array of type Int[] contains Int structure elements. The Int type is not an instance of a class, but because the Int type bridges to the NSNumber class, the Int type is AnyObject compatible. Therefore, you can bridge a Swift array of type Int[] to an NSArray object. If an element in a Swift array is not AnyObject compatible, a runtime error occurs when you bridge to an NSArray object.

You can also create an NSArray object directly from a Swift array literal, following the same bridging rules outlined above. When you explicitly type a constant or variable as an NSArray object and assign it an array literal, Swift creates an NSArray object instead of a Swift array.

For now, a work around would be either to declare arrayColors as an NSArray:

let arrayColors: NSArray = [cor1.CGColor, cor2.CGColor]

Or to declare it as taking AnyObject:

let arrayColors: Array <AnyObject> = [cor1.CGColor, cor2.CGColor]

Answer 2

This runtime error can also be triggered if it tries to bridge an array of type [MySwiftProtocol] over to Objective-C.

The solution is to mark your protocol with @objc:

@objc protocol MySwiftProtocol {
   // ...
}

Answer 3

I found that I could fix the problem by explicitly using CGColorRef rather than CGColor for my colours, e.g.:

    var bottomColour:CGColorRef = UIColor.redColor().CGColor
    var topColour:CGColorRef = UIColor(red: 255.0/255.0, green: 123.0/255.0, blue: 37.0/255.0, alpha: 1.0).CGColor

    gradientLayer.colors = [bottomColour, topColour]

...worked fine, without any NSArray or AnyObject casting. If I take out the explicit CGColorRef in the type declarations for the colours, I get the "array element cannot be bridged to Objective-C" error.

Answer 4

With Swift arrays, you can call bridgeToObjectiveC() on them, and they'll turn into NSArrays. The same is true of Swift dictionaries.