[Authenticator] Allow additional customization of prebuilt forms (custom FormFields, Buttons, etc.)

[andr3i4o]

1. Currently, SignUpForm.custom allows only the predefined SignUpFormField.custom field, which is a text field only.
2. Actions on SignUpForm.custom are static constants and not parameters.

Case scenario:

Before signing up, we need a checkbox for agreeing to the terms and conditions.
So either SignUpFormField.custom should be of a dynamic type, or the actions should be a parameter, so we could inherit the SignUpButton and validate the checkbox before calling state.signUp().

[Jordan-Nelson]

Hello @andr3i4o - You can find an example of how to add a Terms and Condition check box here.

This example will set a custom user attribute in cognito that stores the value for their selection (true or false).

This example also customizes a few other screens.

[Jordan-Nelson]

I've marked this as a feature request. It should be possible to achieve your use case with the 'authenticatorBuilder' api, but I think there are certain use cases where an alternate API as you are suggesting could be easier to use.

[andr3i4o]

Great, that's excatly the needed case scenario, just it's not described in any documentation I've tried to find... Thanks!
I still think that this would be a usefull feature in order to simplify the developer experience.

[Jordan-Nelson]

@andr3i4o - Ideally common use cases would be easy to implement (via an API like SignUpForm.custom()) while more advanced use cases could be achieved with a bit more work (via an API like authenticatorBuilder). I agree that the developer experience could be improved for this use case since it is fairly common.

I am going to update this issue title/description to reflect a request for API / developer experience improvements for common use cases such as this.

There is an example in the docs for the authenticatorBuilder API, but the example that I linked to above is much more closely aligned to this use case.