How do I cast an __NSMallocBlock__ to its underlying type in Swift 3?

Question

I had a trick to help test UIAlertController that worked in Swift 2.x:

extension UIAlertController {

    typealias AlertHandler = @convention(b         


        

Answer 1

My answer is based on @Robert Atkins's, but shorter. The problem here is that, valueForKey returns a Any typed object, and because in Swift,

MemoryLayout.size == 32
MemoryLayout.size == 8

an assertion will be triggered in unsafeBitCast when casting between types of different sizes.

One work-around is to create an intermediate wrapper and transform back to raw pointer, which satisfies MemoryLayout.size == 8.

A much simpler way is to create an indirect reference directly using protocol AnyObject, relying on the fact that MemoryLayout.size == 8, we can write following valid code:

typealias AlertHandler = @convention(block) (UIAlertAction) -> Void

func tapButton(atIndex index: Int) {
    if let block = actions[index].value(forKey: "handler") {
        let handler = unsafeBitCast(block as AnyObject, to: AlertHandler.self)
        handler(actions[index])
    }
}