Implement `xf:copy` element

[ebruchez]

This would be useful for the workflow UI (#3463).

[ebruchez]

Scenario: checkboxes for the choice of workflow operations, which looks like this:

<accessTo type="array">
    <_ type="NewPage">
        <initialStage type="object">
            <name>initial</name>
        </initialStage>
    </_>
    <_ type="SummaryPage"/>
    <_ type="ViewPage"/>
</accessTo>

What would it take?

• [x] static analysis
• [x] Itemset
• [x] runtime
• [x] handlers
• [x] evaluateExternalValue/translateExternalValue

[ebruchez]

Question: can it make sense to mix and match xf:value and xf:copy for a same selection control?

• xf:select1: could make sense since the content of the element is entirely replaced
• xf:select: hard to make sense, suggesting we prohibit it

XForms 2 says " It can only be used within itemset elements" but that's not a requirement and supporting xf:item would make sense too. In any case, you could have multiple xf:itemset elements so the problem is the same.

XForms 1.1 said that xf:copy must return a single element(). This makes sense. But it could be relaxed.

• xf:select1
  • any sequence of item (including attributes) could be supported
  • issue of removing attributes might be an issue as it's not as easy as clearing the element content
• xf:select
  • must be a single element
  • could support attributes too

In a first implementation:

• prohibit it mix and match of xf:value and xf:copy for xf:select
• only support element()

[ebruchez]

XFormsItemUtilsevaluateItemset is not great currently:

• [x] it's unclear when Item.value can be null
  • it seems to indicate that a value obtained with xf:itemset is non-relevant because it returns an empty sequence
  • but this doesn't appear to be the case for xf:item?
  • and this is transient, as pruneNonRelevantChildren() removes those
• [x] an xf:choice is represented with an Item and its value is set to ""
• [x] Items can contain other items in the case of hierarchical itemsets
  • It's ok. Clarified and refactored code.

And I think there is more. The bottom line is that it's hard to understand.

[ebruchez]

• [ ] SelectionControlTrait.evaluateStaticItemset and XFormsItemUtils.evaluateItemset should reduce duplication

[ebruchez]

• [ ] error handling in evaluateItemset: what's the semantic?
• [x] doc
  • [x] add that we support xf:copy; do we have a place for that?
• [ ] unit tests
  • [ ] xf:select1/(xf:itemset|xf:item)/xf:copy
  • [ ] xf:select/(xf:itemset|xf:item)/xf:copy
  • [ ] use entire item sequence for xf:select1
  • [ ] first-item rule for xf:select
  • [ ] value trimming for xf:select
  • [ ] static itemsets

[ebruchez]

Some documentation