[BUG]: Forms requires TagFactory

[niden]

The new implementation of HTML generators (TagFactory) is registered in the DI container using the name tag. It is required by the Forms component to start rendering components.

The constructor of Form accepts an entity and user data, stores that information and immediately calls initialize (with that data) if present in the child class.

Changing the signature of the constructor is going to break backwards compatibility for all applications that are being upgraded to v5. Since forms are a commonly used component, this will require a time consuming refactoring.

All the work that has been done in v4 and v5 was with the aim to no longer rely on a central DI container accessed as a singleton, rather have services injected into components by an new (future) container that will offer automatic injection of services.

However with the above scenario, this will not be feasible at least for Forms.

Steps to get the best balance for everyone:

• v6 can introduce a new Html\Forms component that will use injected services in the constructor (TagFactory in particular)
• v5 Forms (current) will:
  • Check if the TagFactory is defined (Element)
  • If not, check if there is a form attached and check the TagFactory there
  • If not present, use the static Di::getDefault() to resolve the tag service
  • If not present, instantiate a new TagFactory component

The above is not the best solution but it will help for this version, so that current applications will not need a huge refactoring effort.

[niden]

Fixed in #15849