避免在闭包中捕获值副本的最佳方法

struct Foo {

    var i = 0 { didSet { println("Current i: \(i)") } }

    func delayedPrint() {
        dispatch_async(dispatch_get_main_queue(), { _ in
            println("Closure i: \(self.i)")
        })
    }

    mutating func foo() {
        delayedPrint()
        i++
    }
}
struct Foo {

    var i = 0 { didSet { println(