A step-by-step approach to using Swift with Objective-C.
If you’ve got an existing app written in Objective-C, migrating it into Swift is an excellent exercise for learning Swift, experimenting with Swift, and deciding whether to adopt Swift on a full-time basis. I’ve performed this migration with several real apps, so here are some tips culled from my actual experience.
You’re surely not going to translate all your code into Swift at once; you’re much more likely to translate one class at a time. As soon as you add a Swift file to your Objective-C app target, you’ve got a hybrid target: some classes are written in Swift, while other other classes are written in Objective-C. Thus, declarations in each language will need to be visible to the other language. Before you start, it’s a good idea to understand how this visibility works.
Recall that an Objective-C class declaration is conventionally spread over two files: a header file (.h) containing the
@interface section, and a code file (.m) containing the
@implementation section. If a .m file needs to know about a class, it imports the corresponding .h file.
Visibility of Swift and Objective-C to one another depends upon this convention: it works through .h files. There are two directions of visibility, and they must be considered separately.