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Static vs Dynamic iOS explained

Understanding Static and Dynamic linking

Here is a comparison that will help you understand the differences between static and dynamic linkers.

Static Linker (ld)

During the compile time, the static linker includes all the code from the static library into the final executable object file. This means that once the app is compiled, it contains all the necessary code, making it self-sufficient.

Dynamic Linker (dyld)

At load or run time, the dynamic linker finds the embedded framework using @rpath and links it to the executable. This allows the app to load the library only when it is needed, which can save memory and reduce the initial load time of the application.

File extensions

•   Static Libraries: .a
•   Dynamic Libraries: .dylib

Dynamic frameworks are particularly useful because they allow you to include resources such as fonts, images, Xibs, and storyboards. These resources can be accessed using the framework’s bundle identifier, making resource management much easier.

Defining framework types

When creating a framework, you need to decide whether it will be static or dynamic. Here’s how to distinguish between the two:

Static libraries

  • Linked at Compile time: All code from the static library is included in the app’s final binary at compile time. This results in faster runtime performance when accessing the library’s code.
  • Performance: Since everything is compiled into a single binary, the app runs faster as there is no need to load additional code at runtime.
  • Content: Static libraries contain only code. While it is technically possible to include resources in base64 format, it is not advisable due to the complexity and potential performance issues.

Dynamic libraries

  • Linked at runtime: The library code is loaded into memory only when needed, which can reduce the initial load time of the application.
  • Performance: While the app launches faster, accessing the code within a dynamic library is slower compared to a static library because of the additional overhead of loading the library at runtime.
  • Content: Dynamic libraries can include both code and resources. This makes them ideal for frameworks that need to bundle multiple types of assets.
  • Executable: All iOS applications fall under this category. The executable is the main file that runs when the app is launched.
  • Bundle: Used solely for storing resources such as fonts, images, Xibs, storyboards, and Core Data models. Bundles do not contain executable code.
  • Framework: It can be a combination of static libraries and bundles or dynamic libraries. This provides the flexibility to choose the best approach based on the needs of your project.

Pros and cons

Pros of dynamic libraries:

  • Module stability in Swift: Dynamic libraries support module stability, allowing you to release new versions of your framework without needing to recompile and redeploy your app, provided the public interfaces remain the same.
  • System frameworks: For libraries like Core Data, which are linked dynamically, you benefit from automatic updates when Apple releases new versions. This means your app will always use the latest version of Core Data without requiring a recompile.
  • Resource management: Dynamic frameworks can bundle resources, which simplifies access and organization. Resources are accessible via the framework’s bundle identifier, making it easy to manage assets across multiple projects.

Cons of static libraries:

  • Dependencies: Static libraries must be present at compile time, which means any dependencies must be resolved before the app is built.
  • Binary size: Embedded binaries tend to be larger because all the code is included in the final executable, potentially leading to larger app sizes.

Use cases and considerations

When deciding between static and dynamic frameworks, consider the following use cases:

  • Static frameworks: Best suited for performance-critical applications where the fastest possible runtime performance is required. They are also ideal for simple libraries that do not need to bundle resources or be updated independently.
  • Dynamic frameworks: Ideal for complex applications that require modular updates or where multiple resources need to be managed. They are also beneficial when you need to share common code across different projects or teams without duplicating resources.

Best practices

  • Minimize dependencies: Whether using static or dynamic libraries, aim to minimize dependencies to reduce complexity and potential conflicts.
  • Optimize resource management: For dynamic frameworks, ensure that resources are properly organized and accessed efficiently using the framework’s bundle identifier.
  • Consider app size: Be mindful of the impact on app size. While dynamic libraries can help reduce the initial load time, they can increase the overall size of the app package due to bundled resources.

In conclusion, the choice between static and dynamic frameworks depends on your project’s specific needs for performance, resource management, and update flexibility. Understanding the differences and carefully considering the pros and cons will help you make the best decision for your application.

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