Working with existing code.
WHY IT MATTERS
Enormous amounts of existing code and many system frameworks are written in the older language.
WHAT IS AUTOMATIC
Swift can use Objective-C types, and much of the older API appears translated.
WHAT TRANSLATION DOES
Renames methods to Swift conventions Converts certain types Marks values as optional where nullability is declared
WHAT NULLABILITY ANNOTATIONS DO
Tell Swift whether a value can be nothing.
WHAT HAPPENS WITHOUT THEM
Values arrive as implicitly unwrapped, which can crash unexpectedly.
WHAT TO DO IN MIXED PROJECTS
Add nullability annotations to older headers.
WHAT EXPOSING SWIFT TO OBJECTIVE-C REQUIRES
Marking types and members, and inheriting from the base object type where needed.
WHAT CANNOT BE EXPOSED
Swift-only features: enumerations with associated values, generics, structures.
WHAT THAT MEANS
An interface used from both must be expressible in both.
WHAT TO DO WHEN ADOPTING SWIFT GRADUALLY
Convert file by file, starting with self-contained code.
WHAT TO AVOID
Converting everything at once.
