Assigning proper value from for loop in GUIs

Question

I have been learning the Fyne library for making GUIs in Go, and have run into a problem.

func createResponses(content *fyne.Container){
for i:=0;i<address.childrenCount;i++{
    content.Add(widget.NewButton(address.children[i].text,func(){
        fmt.Println(i)
    }))
}}

This is supposed to iterate through the children of a node, add a button with the text stored in that node, and then when clicked, print the value of i when it was created to the console. However, while the text of the button, the first parameter (address.children[i].text), displays correctly, the value of i always prints out the same, as 3, the number that causes the loop to terminate.

At first I though this was a specific problem to Fyne, or my poor understanding of how to use Go in general, but I recall having a similar problem when I first learned JavaFX. Is there something about GUIs that causes this behavior to emerge? More importantly, what is the proper way of dealing with this problem? Thank you!

Aaaand...I figured this out. Albeit, in a hacky way, but it works. I created a new variable at the beginning of the for loop called num, and used that instead of i. This seems to work. If anyone with experience in this type of problem could explain to me why this solves it, I would be very grateful and pick that as the answer. Thanks! — Aaron, Mar 14 '21 at 02:48
Does this answer your question? [Captured Closure (for Loop Variable) in Go](https://stackoverflow.com/questions/26692844/captured-closure-for-loop-variable-in-go) — Brits, Mar 14 '21 at 04:09

score 1 · Accepted Answer · answered Mar 14 '21 at 09:33

This is to do with using a delayed function inside the for loop. By the time the code executed the i variable will be at the last value. Solution is to “capture” the value before passing it to the callback.

for i:=0;i<address.childrenCount;i++{
    index := i
    content.Add(widget.NewButton(address.children[i].text,func(){
        fmt.Println(index)
    }))
}

Assigning proper value from for loop in GUIs

1 Answers1