Meta-annotations for validation not (yet) supported by Fusion client side

[markhm]

Description of the bug / feature

It seems that trivial self-defined pattern meta-annotations are not brought to the client side by Vaadin Fusion.

Minimal reproducible example / Actual behavior

Please see an example of such a validation. This validation works correctly on the server side, but not on the client side. We use such a self-defined annotation so we don't have to duplicate the pattern in multiple locations.

Expected behavior

It would be good if 'meta-annotation'-based validation patterns can be brought to the client side, since the field input can be fully validated there.

Versions:

- Vaadin / Flow version: 19.0.3
- Java version: OpenJDK 15.0.2
- OS version: macOS 11.2
- Browser version (if applicable): Brave / Chrome

Thanks...!

[platosha]

Solution idea:

• Add a generator template for emitting custom TypeScript validators
• When processing for TypeScript form models, the generator should walk the annotations and search for constraints
• Emit custom constraints as custom TypeScript validators that compose with other builtin and emitted custom validators
• Use custom validators in the generated model