Fix crash in label with binding

[andydotxyz]

Moved to a different setup model where we don't recreate listeners all the time.
Fixes #2125

If this is a desirable change we could move other Bind/Unbind widget to match.

Checklist:

• Tests included. <- race
• Lint and formatter run with no errors.
• Tests all pass.

[fpabl0]

LGTM. I will try to explore the connected flag in other PR to see if it is viable :)

[fpabl0]

It seems that there is something more we have to do here, sometimes I got an error inside convert.go:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x42541c6]

goroutine 8 [running]:
fyne.io/fyne/v2/data/binding.(*stringFromFloat).Get(0x0, 0xc000048f78, 0x42545fa, 0xc0003bcdd8, 0x40638e0)
        .../fyne/data/binding/convert.go:109 +0x26
fyne.io/fyne/v2/widget.(*Label).createListener.func1()
        .../fyne/widget/label.go:71 +0x45
fyne.io/fyne/v2/data/binding.(*listener).DataChanged(0xc00018e9c0)
        .../fyne/data/binding/binding.go:63 +0x2e
fyne.io/fyne/v2/data/binding.processItems()
        .../fyne/data/binding/queue.go:56 +0x94
created by fyne.io/fyne/v2/data/binding.init.0
        .../fyne/data/binding/queue.go:15 +0x35
exit status 2

[andydotxyz]

It seems that there is something more we have to do here, sometimes I got an error inside convert.go:

Those line numbers don't mat this PR... do you have local changes?

[fpabl0]

Yes, but they are commented. I copied your changes and paste in a local branch. I will try to reproduce it, by cloning directly your branch, although I think it would be the same 🤔. I will investigate. However it seems that this PR is ok and this can be solved in a different one?

[fpabl0]

Ok, I have tested your branch without any change, this is the error trace:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x4254c46]

goroutine 8 [running]:
fyne.io/fyne/v2/data/binding.(*stringFromFloat).Get(0x0, 0xc0002b0508, 0xc00009a470, 0xc0002b05b8, 0x40643a0)
        /Users/pablofuentes/Desktop/fyne/data/binding/convert.go:109 +0x26
fyne.io/fyne/v2/widget.(*Label).createListener.func1()
        /Users/pablofuentes/Desktop/fyne/widget/label.go:118 +0x45
fyne.io/fyne/v2/data/binding.(*listener).DataChanged(0xc000338008)
        /Users/pablofuentes/Desktop/fyne/data/binding/binding.go:55 +0x28
fyne.io/fyne/v2/data/binding.processItems()
        /Users/pablofuentes/Desktop/fyne/data/binding/queue.go:56 +0x94
created by fyne.io/fyne/v2/data/binding.init.0
        /Users/pablofuentes/Desktop/fyne/data/binding/queue.go:15 +0x35

[fpabl0]

This is really weird, it seems that there is a chance that s could be nil here:

func (s *stringFromFloat) Get() (string, error) {
	val, err := s.from.Get()
	if err != nil {
		return "", err
	}

	return fmt.Sprintf(s.format, val), nil
}

I added a fmt.Printf("%p\n", s) just before the s.from.Get(), and when the crash occurs I get 0x0.

[fpabl0]

I have found something interesting that I didn't know before (probably it was logical but I hadn't realized it). Nil objects can call its methods without actually crashing the whole program:
https://play.golang.org/p/tOzpRaZTt3m

[fpabl0]

Same change should be done on Unbind method.