How to deal with feature toggling when a new feature requires massive changes in codebase?

Question

Feature toggling works perfectly for a new functionality - in most cases the functionality should be simply surrounded with some UI container that will be visible or not based on feature configuration. But how feature toggling could be used if a new feature requires massive changes in existing code base? I can not use 'copy & change', because obviously it is much worse than feature branching - I'll not get any merge conflicts. So what are the best practices for such requirements?

Answer 1

You can combine feature toggling with an IoC container to load different implementations of your components. For example, toggle "A" loads implementation "X" while toggle "B" loads implementation "Y". This approach would permit you to maintain parallel implementations and turn on legacy and new functionality based on a single toggle.