通过价值关闭？

问题描述

我希望在处理完最后一项后做额外的逻辑，但终端显示 i 总是与 c 。知道如何传递循环变量吗？

I want to do extra logic after last item was processed, but terminal show that i has always the same value as c. Any idea how to pass the loop variable in?

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
            }
        })
    })
}




推荐答案

您可以使用捕获列表 [i]显式捕获 i 的值闭包中的
，那么你不需要将它复制到一个单独的变量中。
示例：

You can capture the value of i explicitly with a capture list [i]in the closure, then you don't need to copy it to a separate variable.Example:

let c = 5
for var i=0; i<c; i++ {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
        [i] in   // <===== Capture list
        dispatch_async(dispatch_get_main_queue(), { 

            println("i \(i) c \(c)")
        })
    })
}

输出：

i 0 c 5
i 1 c 5
i 2 c 5
i 3 c 5
i 4 c 5

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