Mastering Swift‑Objective‑C Mixed Compilation in JD’s iOS App: A Practical Guide

Background

Since Swift was introduced, JD has gradually migrated its iOS app from Objective‑C to Swift. After ABI stability, module stability and library evolution became available, JD launched the first mixed‑language version of the JD App in July 2020 and completed component‑level mixed compilation.

ABI Stability

Swift 5.0 provides ABI stability, allowing binaries compiled with Swift 5.0+ to run on any Swift 5.0+ runtime. This reduces startup time, improves performance and lowers memory usage. ABI stability is present in iOS 12.2, macOS 10.14.4 and later.

Module Stability

Swift 5.1 introduces module stability, enabling modules built with different compiler versions to be used together. The .swiftmodule file is replaced by a textual .swiftinterface that describes public APIs, making cross‑version integration seamless.

Library Evolution

Swift 5.1 also adds library evolution, allowing binary frameworks to evolve without breaking existing clients. Public or open symbols must be marked @objc and inherit from an Objective‑C class when accessed from ObjC.

Mixed‑Compilation Approaches

Project‑level mixed compilation

JD’s app is a CocoaPods‑based componentized project. Two scenarios are considered: the main project mixing Swift and ObjC, and each component mixing Swift and ObjC.

Swift calls ObjC (project)

Create a bridging header (

-Bridging-Header.h

) and set

SWIFT_OBJC_BRIDGING_HEADER

to its path.

Add the required ObjC headers to the bridging header.

ObjC calls Swift (project)

Enable

DEFINES_MODULE = YES

in build settings.

Import the generated

ProjectName‑Swift.h

(

#import <ProjectName/ProjectName‑Swift.h>

for frameworks or

#import "ProjectName‑Swift.h"

for static libraries).

Mark Swift classes as

public

/

open

and expose methods with

@objc

.

Component‑level mixed compilation

Add Swift source files to the component without a bridging header, list them in the podspec’s

source_files

, and configure ObjC‑to‑Swift and Swift‑to‑ObjC communication as described above.

Build Settings for Swift Support

Static linking issues are solved by adding the Swift library paths to

Library Search Paths

:

$(TOOLCHAIN_DIR)/usr/lib/swift/$(PLATFORM_NAME)

$(TOOLCHAIN_DIR)/usr/lib/swift-5.0/$(PLATFORM_NAME)

Dynamic loading problems on iOS 12.2+ require adding

/usr/lib/swift

to the first entry of

Runpath Search Paths

. On iOS <12.2, set

Always Embed Swift Standard Libraries

to

YES

.

Summary of Communication Schemes

All communication patterns—Swift↔ObjC inside a component, between components, and in the main project—are summarized in the final diagram.