How does Swift bridge with Objective-C?
The foundation: two different runtimes talking to each other
Objective-C uses dynamic message dispatch (objc_msgSend); Swift uses static/witness-table dispatch. Bridging translates between these models at compile time and, in some cases, at runtime.
1. The bridging header — importing Objective-C into Swift
A bridging header (MyApp-Bridging-Header.h) lists Objective-C headers you want visible to Swift.
2. The generated interface — the reverse direction
A Swift framework generates a -Swift.h header with Objective-C-compatible declarations for anything marked @objc. Pure Swift structs, generics, enums with associated values generally cannot bridge.
| Swift construct | Bridges to ObjC? |
|---|---|
class subclassing NSObject, @objc methods | Yes |
struct/enum | No |
| Enum with associated values | No |
| Generic class/function | No |
Protocol without associated types, @objc | Yes |
| Closures | Yes, bridged to blocks |
3. Automatic type bridging
| Swift type | Objective-C type |
|---|---|
String | NSString |
Array | NSArray |
Dictionary | NSDictionary |
Int, Double, Bool | NSNumber |
Implemented via _ObjectiveCBridgeable. This bridging isn’t always free — Array/Dictionary bridging can be lazy/no-op when storage is compatible, but bridging a [CustomStruct] forces element-by-element conversion.
4. Memory management reconciliation
Both use ARC, but naming conventions (init, copy, new, alloc) still matter for correct retain/release insertion at the boundary.
5. Nullability and Optionals
nullable/nonnull annotations tell the Swift importer whether to expose T, T?, or T!. Un-annotated legacy headers are why implicitly-unwrapped optionals still show up at interop boundaries.
6. dynamic and message dispatch
Features like KVO/swizzling need dynamic, forcing objc_msgSend-style dispatch.
7. Selectors and #selector
button.addTarget(self, action: #selector(handleTap), for: .touchUpInside)Compile-time checked, closing off typo’d-selector-string footguns.
One-liner if pressed: “Swift bridges to Objective-C through compiler-generated headers in both directions, automatic value-type conversion via a compiler-known bridging protocol, and the @objc/dynamic attributes that opt Swift declarations into Objective-C’s message-dispatch model — with nullability annotations closing the gap between ObjC’s unchecked pointers and Swift’s Optionals.”