Credit Memo number generator requires refactor

[diimpp]

Hi,

There are multiple problems with given number generator, functionally, styling and idiomatic wise.

• NumberGenerator interface's generate method doesn't have arguments, like OrderNumberGeneratorInterface's generate have $order, which leads to problem by which data to generate the number.
• https://github.com/Sylius/RefundPlugin/blob/master/src/Generator/NumberGenerator.php NumberGenerator interface doesn't have Interface part in it's name
• Sylius traditional placement is src/NumberGenerator, name should be CreditMemoNumberGeneratorInterface
• SequentialNumberGenerator uses DateTime('now') as source of the date, but it should take date from CreditMemo IssuedAt. This is outright bug and was done so, because NumberGenerator interface's generate doesn't have CreditMemo as argument I've noticed how CreditMemo is instanced and why CreditMemo is not supplied to number generator, but issue still holds true: generate(OrderInterface $order, \DateTimeInterface $issuedAt) Something like this maybe?
• Entity\SequenceInterface::getId should be nullable Done
• Entity\SequenceInterface should be named as CreditMemoSequenceInterface, this plugin is not a place to offer generic sequence interface Done
• Needs strict comparison Done
• DI definition is not friendly for extension. Interface should be used as base service name or regular non-fqcn string name.

Some of the entities of this plugin, including CreditMemoSequence don't implement ResourceInterface. Is it some kind of policy? I would have preferred them to be regular resources, whenever possible.