Default selection in MultiItemSelector

[Drevsh]

I have a usecase where I want to offer multi selection with some values already pre selected. Afaik this is currently not supported right?

The only pre selection, which I can see, is a single selection via the void org.springframework.shell.component.support.AbstractSelectorComponent.setDefaultExpose(I defaultExpose). But this is only useful for the SingleItemSelector.

Do you think this use cases should be supported? I think it would make sense to either extend the setDefaultExpose to also support collections, or rather to have this information directly on the SelectorItem/SelectorItemWrapper. So in addition to enabled there could be another boolean field selected.

During the building of the ItemStateView this could then be extracted and the item could be selected.

I also think void org.springframework.shell.component.support.AbstractSelectorComponent.initialExpose(C context)/setDefaultExpose would then be obsolete.

[Drevsh]

I implemented the changes here:
https://github.com/Drevsh/spring-shell/commit/b5e26ab470c8a015a9da9d651e24a31e1292e991

I think it works, but I'm a bit unhappy about the constructor since overloading is now not possible. Maybe a builder pattern is now needed?

[jvalkeal]

Thx, do you want to PR these changes as it's then easier to try and comment.

I've also not been happy about AbstractSelectorComponent as it was created to have some shared features in one place but then single/multi have their own features. Probably one area to refactor with 3.x. Most likely reason why I didn't implement pre-selection was to avoid bad behaviour or bugs as it has no meaning with single selector.

[Drevsh]

Created a pull request. Since this is my first time contributing I hope there are not that many problems.