System Design for iOS Apps (HLD & LLD)

In iOS development with Swift, HLD (High-Level Design) and LLD (Low-Level Design) represent different stages in the design process for an application or system architecture. Here’s how they are typically defined and applied

1. High-Level Design (HLD)

HLD provides a broad, system-wide view of the project. It outlines the architecture and how different modules will interact. In the context of iOS and Swift development, HLD focuses on the overall structure of the app.

Key Components in HLD:

• Architecture Selection: Choosing the architecture pattern, such as MVC, MVVM, VIPER, or Coordinator Pattern, that best fits the project requirements.
• Module Interaction: Defining how different components of the app (like Networking, UI, Data Persistence) will communicate.
• Third-Party Services: Identifying external frameworks and libraries (e.g., Alamofire for networking, Realm for databases) and how they’ll be integrated into the app.
• API Architecture: Outlining the REST or GraphQL services your app will consume, and the structure of requests and responses.
• Overall System Flow: Describing the flow of the app, such as how users will navigate between views and screens.

HLD Diagram Example:

2. Low-Level Design (LLD)

LLD focuses on the implementation details and specific design choices for individual components or modules. It deals with classes, methods, data structures, and the precise logic to be followed.

Key Components in LLD:

• Class Design: Defining the structure and relationships between specific classes or structs.
• Data Structures: Choosing appropriate data structures for storing and manipulating data.
• Detailed Algorithm: Implementing detailed logic for handling business rules and operations.
• Method Definitions: Outlining the specific methods each class will contain and how they will interact with each other.
• Error Handling: Defining strategies for managing errors, retries, and failures at a granular level.

LLD Diagram Example:

Example of HLD and LLD in Swift (iOS App):

High-Level Design (HLD)

• Architecture: Using MVVM for better separation of UI and business logic.
• Modules:Networking (for API requests),Database (for CoreData or Realm),UI (for Views and ViewModels),Authentication (for login/sign-up),UserProfile (for user data management).
• External Services: Using Alamofire for networking and Realm for local storage.

Low-Level Design (LLD)

• Class Design:UserViewModel: Handles transforming user data for the UI.UserService: Contains fetchUsers() method to retrieve data from API.UserModel: Stores user data fetched from API.
• Method Details:fetchUsers() -> [User]: This method fetches users, handles pagination, sorts the result alphabetically, and manages errors.
• Data Flow:User taps “Fetch Users,” triggering the UserViewModel to call UserService, which fetches user data and passes it back to the ViewModel.

Conclusion:

Both HLD and LLD are crucial for designing robust, maintainable, and scalable iOS applications. HLD ensures a well-structured architecture, while LLD dives deep into the actual implementation details, ensuring that the design is translated into clean and efficient code.