Struct binding fails to reload with slice field

[drognisep]

Describe the bug:

Whenever there is a slice of struct in a bound struct, the Reload method will always panic. I tried a few different permutations to see if it mattered whether the slice was of value or pointer, or if it was initialized or not, and all failed. However, reloading a bound struct with no slices in it works fine.

Based on the stack trace, the panic is originating here.
https://github.com/fyne-io/fyne/blob/master/data/binding/mapbinding.go#L292

To Reproduce:

Steps to reproduce the behaviour:

1. Copy down and run the tests in this playground link
   https://play.golang.org/p/xvfx58--S66

Example code:

See playground link:
https://play.golang.org/p/xvfx58--S66

Device (please complete the following information):

• OS: Windows 10
• Version: 21H1 19043.1288
• Go version: go version go1.16 windows/amd64
• Fyne version: fyne.io/fyne/v2 v2.1.1

[drognisep]

@andydotxyz Looking at the surrounding code it looks like only singular, primitive types are supported for struct binding. Is this expected or just something that hasn't surfaced yet?

For example, I can try comparing the previous value with reflect.DeepEqual if f.Kind() == reflect.Slice, but the following switch statement doesn't copy the new value if it's changed because there's no case for reflect.Slice.

[andydotxyz]

I have fixed the panic in the v2.1.2 bugfix release. Adding true support for slice content in struct mapping will require a bit more work, so keeping this open for the main work.
But panics should never occur :).

[andydotxyz]

Because the panic is fixed this will be de-prioritised a little.
Full slice support needs to be added but is not blocking 2.2