Implement a dropdown FormField which gets its source via AJAX

[GuySartorelli]

As a follow-on from silverstripe/silverstripe-cms#2901, we need a form field that uses AJAX requests to lazy-load its data.

Required functionality

• Initial list isn't injected into the front end - if it's a DataList, no DB query is initiated at all when sending the initial form to the front-end
• When user click on the field, an AJAX request is fired to get some initial results. The initial results is limited to some sane number.
• As user types into the field, AJAX requests are fired with the current field value. The response is a subset of the original list, filtered by the user input
• If user types the same value a second time, a second AJAX request is not sent - in-memory cache is used instead to fetch the results that were previously received for that search term
• in-memory cache is per instance of the field - it is not shared between even identical fields, and it is immediately discarded when the field is de-rendered.

Acceptance criteria

• Either a new field is implemented with the above functionality, or the above functionality is added to an existing field (e.g. ListboxField)
• The above functionality works in both react and entwine contexts
• The temporary workaround from 5.1: Can't open settings tab of SiteTree objects, if you have a lot of members silverstripe-cms#2901 is removed
• The new implementation is in use for all fields that fetch Member records by default (e.g. the "only these users" fields in cms, asset-admin, and siteconfig)
• The new implementation is used instead of dropdown field for auto-scaffolding has_one relation fields (NOTE: We only do this if it's 100% backwards compatible with the existing form field, in terms of its public API)
  • The DBForeignKey.dropdown_field_threshold configuration property is deprecated
• Validate the UX of the new field with our design team
• There's a field that allows you to select a single value and there's a field that allows you to select multiple values.

Notes

• The silverstripe/tagfield implementation has this functionality - we can likely grab a lot of that code and reuse it in core.
• The TreeDropDown field that uses a similar approach as well.
• Might be worthwhile reviewing which fields have AJAX search capabilities.

New issues created

• Consolidate Dropdown components silverstripe-framework#11080

PRs

• NEW SearchableDropdownField silverstripe-framework#11057
• API Use correct param types silverstripe-framework#11073
• NEW SearchableDropdownField #1625
• ENH Use SearchableMultiDropdownField to select Members silverstripe-cms#2906
• ENH Use SearchableMultiDropdownField silverstripe-asset-admin#1424
• DOC Add SearchableDropdownField to 5.2.0 changelog developer-docs#412
• ENH Add SearchableDropdownField webpack-config#64
• MNT Add ID attribute to input #1635
• ENH Use SearchableMultiDropdownField silverstripe-siteconfig#149
• FIX Set z-index of list #1636
• ENH Use SearchableDropdownField for autoscaffolded has_one relationships silverstripe-framework#11071
• DOC Auto scaffolding has_one relations developer-docs#425

[emteknetnz]

Yup

Have added PR to this issue - #1636

[GuySartorelli]

@emteknetnz Please resolve silverstripe/silverstripe-framework#11071 before reassigning to me

[GuySartorelli]

PRs merged