callback - 戈朗 : evaluate variable in callback declaration

Question

我正在尝试在 golang 中定义一个回调:

package main

func main() {
    x, y := "old x ", "old y"
    callback         := func() { print("callback: "        , x        , y       , "\n") }
    callback_bound   := func() { print("callback_bound: "  , x        , y       , "\n") }
    callback_hacked  := func() { print("callback_hacked: " , "old x " , "old y" , "\n") } 

    x, y = "new x ", "new y"

    callback()
    callback_bound()
    callback_hacked()
}

输出是:

callback: new x new y
callback_bound: new x new y
callback_hacked: old x old y

基本情况有效，但它使变量未绑定(bind)，即使用调用时的值。毫无疑问，这在某些情况下很有用，但我如何声明 callback_bound 以便使用声明时的值并且输出变为:

callback: new x new y
callback_bound: old x old y
callback_hacked: old x old y

Answer 1

例如，

package main

func callbackXY(x, y string) func() {
    return func() { print("callbackXY: ", x, y, "\n") }
}

func main() {
    x, y := "old x ", "old y"
    callback := callbackXY(x, y)
    x, y = "new x ", "new y"
    callback()
}

输出:

callbackXY: old x old y

或者

package main

func main() {
    x, y := "old x ", "old y"
    callback := func() {}
    {
        x, y := x, y
        callback = func() { print("callbackXY: ", x, y, "\n") }
    }
    x, y = "new x ", "new y"
    callback()
}

输出:

callbackXY: old x old y