Go: transfer var into anonymous function

Question

I am having trouble transferring a variable into an anonymous function. Is there a solution?

import  "github.com/lxn/walk"

***

var openAction [12]*walk.Action
for i := 0; i < 12; i++ {

    openBmp, err := walk.NewBitmapFromFile(_films[i][0])
    if err != nil {
        log.Printf("Open bitmap for buildBody() :%v\n", err)
    }
    openAction[i] = walk.NewAction()
    openAction[i].SetImage(openBmp)
    openAction[i].SetText(_films[i][2])
    openAction[i].Triggered().Attach( func(){
        exec(i)
    })
    mw.ToolBar().Actions().Add(openAction[i])
}

exec(i) where i always = 11

Answer 1

for i := 0; i < 12; i++ {
    i := i
    ...

Crazy as it looks, this is something you will see in Go code. It results from the way closures work and the way variables are scoped. Your anonymous function is a closure that captures i. Specifically, it is capturing a variable called i, not the current value of i, and it captures whatever i is in scope. In your original code this is the loop variable, which is the same variable for each iteration of the loop. All of your closures captured the same variable. The addition of i := i declares a new variable on each iteration. Now each closure will capture this new variable, and on each iteration it will be a different variable.

In a little more detail, the scope of the loop variable i is the for statement. This includes the loop block, but since the declaration of the loop variable i is outside of the block, declaring a new variable with the same name inside the block is legal and creates a new variable at that point in the block. The loop variable is then shadowed. Often a variable declared like this goes on the stack, but in this case compiler escape analysis sees that your closure is still referring to this block variable when it goes out of scope at the end of the block, and so the variable is placed on the heap. On each iteration, the block is reentered and a new variable i is placed on the heap.

Answer 2

I think that this will get you what you want:

openAction[i].Triggered().Attach(func(x int) func() {
    return func() { exec(x) }
}(i))

The trick is to have your anonymous function return an anonymous function, and each created function will enclose each of the values of i .

Answer 3

You are encountering a quirk of go's for loops. The i variable in the loop is not a new variable for each iteration. because of this all of your closures are closing over the same variable whose value is changing underneath them. When your code runs after the loop all of the functions see the value 11 for the i they closed over.

The solution is to pass the i into a function which then returns another function that closes over the functions arg. This is why Adam Crosslands solution works.