LINE Improves iOS Large-Scale Configuration System by Adopting AttributedString Architecture

LINE introduced a new system that adopts the AttributedString architecture from Swift Foundation to resolve circular dependency, type safety, and thread safety issues faced by its existing monolithic Service Configuration system.

The existing system required declaring approximately 700 configuration keys in a single file and was structured so that configuration modules returned only Raw Strings to avoid reverse reference issues caused by module-specific type definitions. As a result, consumer modules had to perform parsing every time, leading to significant maintenance burdens such as decoding errors for "Y"/"N" strings from the server and duplicate definitions of default values.

Application of AttributedString Architecture

The new system referenced four core type structures: AttributedStringKey, AttributeScope, AttributeDynamicLookup, and AttributeContainer. By utilizing @dynamicMemberLookup and KeyPath routing, it infers types at compile time and performs only safe Dictionary read/write operations at runtime. This allows each module (e.g., photo) to independently define its Key and Value types, thereby eliminating circular dependencies.

Extensions Tailored to Domain Requirements

Beyond simple adoption, LINE extended the design to fit actual service environments. It was designed as a Reference type for server synchronization, and thread safety was ensured by isolating mutable state with OSAllocatedUnfairLock using Swift 6 Sendable checks. To prevent decoding errors, a protocol was introduced to automatically convert "Y"/"N" to Bool. Additionally, ValueSource priority (overridden > server > default) and Debug override features were integrated for QA purposes.

Currently, this system is being applied incrementally, improving test reliability through MockConfigurationService and resolving issues from the existing system.