SPIKE Investigate extending SiteTree/ContentController instead of Page/PageController

[emteknetnz]

Follow on issue from https://github.com/silverstripe/silverstripe-framework/issues/10466

Currently we have a bad piece of architecture where core and non-core module classes extend the Page and PageController classes which reside in project code.

Investigate the feasibility of changing:

• extends Page to extends SiteTree
• extends PageController to extends ContentController

The following core and non-core controllers were identified:

Core:

• RedirectorPageController
• ErrorPageController

Non-core:

• BlogController
• BlogPostController
• IFramePageController
• RegistryPageController
• TaxonomyDirectoryController
• UserDefinedFormController
• CKANRegistryPageController

Acceptance criteria

• [ ] Investigate potential impact on projects of changing the base classes
• [ ] Come up with options for projects can retain existing functional when upgrading to CMS 5 e.g. use of extensions
• [ ] Make a recommendation if this is worth doing or too hard or too risky

[michalkleiner]

As Guy hinted at in #10466, I would either leave the status quo as it's what developers are used to working with, or remove the concept of Page and PageController altogether, leaving SiteTree and ContentController as the building blocks and leaving it up to devs to create their own project-level reusable BasePage and BasePageController (like CWP had) or use extensions to apply custom behaviour to core and non-core page classes.

In short, I'd be against the hybrid where the project code has Page and PageController coming from the recipe but the core classes not extending that.

[maxime-rainville]

Yeah. I don't think we're going to do this in the short term. I remember us talking about how we could allow people to put Page/PageController into your project namespace at some stage but it was put into the too hard basket.

We can probably revisit this in a more holistic way once we start thinking about CMS 6

[maxime-rainville]

That's where we thought about this https://github.com/silverstripe/silverstripe-framework/issues/5844