Improve prop validator API

[znck]

What problem does this feature solve?

• Allows to write validation logic depending on multiple props.
• Allows to provide custom validation messages.

What does the proposed API look like?

type Validator = (value: any, key: string, props: Object, warn: (message: string) => void) => boolean

Example:

export default {
  props: {
    foo: {
      validator(value, key, props, warn) {
        if ('bar' in props && 'foo' in props) warn('Foo and Bar cannot be used together.')
        return true
      }
....

[damianstasik]

Should not the warn function be returned to stop the rest of the validator code from executing?

[kayandra]

I'm a fan of adding the prop as an argument. I think we can return a string from the validator function and let Vue treat that as an error.

My counter proposal is

type ValidatorOutput = boolean | string
type Validator = (value: any, key: string, props: Object) => ValidatorOutput

[znck]

Closing in favor of #9467. Also, I think changing arguments of prop validator function would require an RFC now.

[dietergeerts]

For a global workaround, see #9467 (comment)