Placeholder support in async validations

It would be nice to have placeholder support for the attribute name inside the error message returned for the callback in an async validation. This is posible with synchronous validations right now.

Actual behavior

Validator.registerAsync(
  'unique',
  (value, args, attribute, passes) => {
    // database check
    passes(false, 'The :attribute has already been taken.'); // It doesn't work!
  }
);

Expected behavior

Validator.registerAsync(
  'unique',
  (value, args, attribute, passes) => {
    // database check
    passes(false, 'The :attribute has already been taken.'); // The username has already been taken.
  }
);

[MatiasOlivera]

@haitm-eas I wrote an example showing how to register an async rule in #302.

You can generate the error message dynamically before passing it to passes function.

Validator.registerAsync(
  'unique',
  (value, args, attribute, passes) => {
    // database check
    passes(false, `The ${attribute} has already been taken.`);
  }
);

You need to pass a callback to passes function if the validator has one or more async rules. Related with “Use the same API for sync and async validations” in #303

if (validator.hasAsync) {
  validator.passes(() => {
    // success
  });
  validator.fails(() => {
    // failure
    const errors = validator.errors.all();
  });
}

[MatiasOlivera]

@haitm-eas The example is showing how to make an async rule using promises, not async/await. The callback is a normal function, not an async function.