Access to parsed DateFormat pattern data #665
Description
Is your feature request related to a problem? Please describe.
We are building an AngularDart implementation of the Material Datepicker for web which includes a free text field for users to type in a date:
https://m2.material.io/components/date-pickers#anatomy
The component needs to work for any locale and we are using DateFormat#parseLoose to handle the text, which works well enough and is certainly better than reinventing the parsing wheel.
The spec also requires hint text that guides the correct use for free-text entry; for example which would be "MM/DD/YYYY" for en_US and "DD/MM/YYYY" for en_UK. Since text is parsed using the date format's pattern, we'd like to be able to reliably produce the correct hint text based on that same pattern.
Describe the solution you'd like
Something like the test-only / internal-only parsePattern would mostly meet our needs but with public types for inspecting which kind of pattern token something is (literal vs quoted vs pattern). Using this we could build out the correct hint text.
Describe alternatives you've considered
Since we're using dart/i18n this felt like the natural fit. We could rewrite a simpler parser since we're only dealing with DateFormat#yMd but I'm worried about divergent code between DateFormat and a custom solution
Additional context
https://m2.material.io/components/date-pickers#anatomy
https://github.com/dart-lang/i18n/blob/main/pkgs/intl/lib/src/intl/date_format.dart#L915