ios - 将值传递给闭包？

Question

我想在处理完最后一项后执行额外的逻辑，但终端显示 i 始终与 c 具有相同的值。知道如何传入循环变量吗？

let c = a.count
for var i=0; i<c; i++ {

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

        // ..

        dispatch_async(dispatch_get_main_queue(), {

            println("i \(i) c \(c)")
            if i == c-1 {

                // extra stuff would come here
            }
        })
    })
}

Answer 1

当你的闭包被执行时，for 循环已经完成并且 i = c。在 for 循环中需要一个辅助变量:

let c = a.count
for var i=0; i<c; i++ {
    let k = i
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {

        // ..

        dispatch_async(dispatch_get_main_queue(), {

            println("k \(k) c \(c)")
            if k == c-1 {

                // extra stuff would come here
            }
        })
    })
}