Code compatibility (.NET)

DD4T 2.*

Summary

In general, the approach is to include a separate DD4T-facade in DXA 2.0 which covers as much as possible of the current DD4T API. Every function that can be supported will be supported, but some will be marked as obsolete to steer the customers in the direction of DXA 2.0.

The facade must be supported at least throughout the life cycle of DXA 2. To remove it would require a new major release.

Compatibility issues that affect classes have a higher priority than issues that only affect views. If we need to choose, we should address the former before tending to the latter - but only if choosing is necessary because we should support as much functionality as possible.

Support for DD4T 1 is not required. Classes or methods that were marked as obsolete with the introduction of DD4T 2 will be removed.

Model	The generic model (IPage / IComponent / etc) should be ported to the new framework. Since the model service from a DXA perspective shouldn't use the generic model anyway, there is no conflict. This applies to the default implementation of the model as well as the contracts. In many implementations the implementation is used instead of the interface, because the interface is read-only.
Factories	DD4T 2 has factories that give access to generic types (IPage, IComponentPresentation, etc). This is indispensible, so we must include this in our facade.
ViewModels (factory + attributes)	The entire ViewModel layer must be included in the facade. That - combined with the fact that the PageFactory / ComponentPresentationFactory are also in the facade - will guarantee that the DD4T ViewModels will continue to work.
Caching	The ICacheAgent interface must still be supported as well as the DefaultCacheAgent and NullCacheAgent implementations. This should not be a problem. Should have: support for JMS listener approach (unless DXA 2 contains a better alternative for cache invalidation).
Configuration	The IDD4TConfiguration interface must be supported. The exposed properties (settings) must be mapped to the new settings in DXA 2.
Link resolving	ILinkResolver and DefaultLinkResolver must be supported.
Rich text resolving	IRichTextResolver and DefaultRichTextResolver must be supported.
Publication resolving	IPublicationResolver and DefaultPublicationResolver must be supported.
MVC	ModelControllerBase, TridionControllerBase and DD4TControllerBase must be supported (as these are the basis for most DD4T implementations) RenderHelper must be supported (essential for all views)
Binaries	The DD4T approach to SDL-managed binaries is to store them on the file system when they are requested the first time, and serve them from there on subsequent requests. This approach must continue to be supported. Support for the BinaryDistributionModule is optional (it's also fine if this is replaced by a better alternative).
Image resizing	ImageHelper.ResizeToWidth should be supported (or an alternative must be offered) → only affects views so lower prio
XPM	XpmMarkupService and XpmExtensions should be supported (only affects the views on some implementations) SiteEditHelper and SiteEditService are old and can be dropped
Providers	The Tridion/SDLWeb providers themselves are a nice to have, but the interface should be supported. I don't know of any custom implementations but it is conceivable.
Other stuff	TcmUri should be supported in the facade as a wrapper around whatever DXA will be using DependencyMappings could be supported (this gives access to DI-managed objects from a non-managed context) ~~Resource management (labels) should be supported if it doesn't require too much hacking~~ ~~Sitemap should be supported if it doesn't require too much hacking~~

Factories

Interface	Method	Explanation	Priority
IPageFactory
	IPage FindPage(string url);	Finds IPage (generic type) for given URL	M
	IPage GetPage(string tcmUri);	Retrieves IPage (generic type) for given TCM URI	M
	string FindPageContent(string url)	Retrieves raw content for page as string for given URL	S
	string GetPageContent(string tcmUri)	Retrieves raw content for page as string for given TCM URI	S
	IPage GetIPageObject(string pageStringContent)	Converts raw content (string) to IPage	S
	DateTime GetLastPublishedDateByUrl(string url)	Gets last publish date by URL (for custom caching purposes)	S
	DateTime GetLastPublishedDateByUri(string uri)	Gets last publish date by TCM URI (for custom caching purposes)	S
	string[] GetAllPublishedPageUrls(string[] includeExtensions, string[] pathStarts)	Gets all published page URLs	C
	TODO: FINISH (perhaps as an Excel sheet..)

DXA 1.*

There is no new DXA 2.0 API, only the R2 data model (there are a few breaking changes in the API which are documented).