Populating a Swift array in a C function callback

I’d like to add a property to UIBezierPath that returns an array of the path’s elements. So I need to call the CGPathApply() function, which takes a function pointer (a closure in Swift 2.0) and calls the closure for each element in the path. My problem is: how do I get the data about the path elements out of the callback closure and stored in a Swift array? The closure can’t capture any variables (because it corresponds to a C function pointer), so I think I have to pass a pointer to a Swift array as a parameter. Thankfully, CGPathApply() takes a parameter info: UnsafeMutablePointer and passes it to the closure.


My problem is in line 10: how do I turn the UnsafeMutablePointer back into a Swift array that I can operate on inside the closure?


extension UIBezierPath {
    var elements: [CGPathElementType] {
        // I'm trying to reserve enough memory for the array. 
        // I don't know how many elements the path has, but this should be good enough for testing.

        var elements = ContiguousArray()

        CGPathApply(self.CGPath, &elements) { (userInfo: UnsafeMutablePointer, elementPointer: UnsafePointer) -> Void in
            // I don’t know how to convert userInfo back into a Swift array
            var elements = ???
            let elementType = elementPointer.memory.type

        return Array(elements)


I tried var elements = UnsafeMutablePointer<[CGPathElementType]>(userInfo).memory. This compiles but when I run the code on a bezier path with 5 path elements, the resulting array is empty.

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