When developing iOS apps you rarely implement everything from the ground-up, because operating system as well as open source community offers large amount of functionality ready-to-use. Such pieces of functionality are usually packed in a distributable form known as a library. In this article let’s explore static and dynamic libraries and frameworks which are the two major types of building blocks in iOS and macOS projects.

Introduction

Frameworks and libraries are everywhere: UIKit, Foundation, WatchKit, GameKit, you name it — all of these are from Apple standard library and chances high that you are using lots of them in your current project. I’d venture to guess that you are also familiar with CocoaPods and Carthage that help you manage third parties in Xcode projects.

Despite most of iOS and macOS developers deal with libraries and frameworks on daily basis and intuitively understand what they are, there is lack of understanding how they work under the hood and how they differ.

Throughout the article we’ll answer that questions along with these ones:

  • What are libraries and frameworks?
  • What types of frameworks and libraries exist?
  • Which kind of libraries should you use in your project?
  • How frameworks and libraries affect your app startup time?

What is a Library?

Libraries are files that define pieces of code and data that are not a part of your Xcode target.

The process of merging external libraries with app’s source code files is known as linking. The product of linking is a single executable file that can be run on a device, say iPhone or Mac.

Besides linking, every Xcode project undergoes 4 more phases to produce an executable application. In Understanding Xcode Build System I will walk you through these steps.

Libraries fall into two categories based on how they are linked to the app:

  • Static libraries — .a
  • Dynamic libraries — .dylib

Additionally, a special kind of libraries exists:

  • Text Based .dylib stubs — .tbd

Let’s explore each type in more details.

What is a Framework?

Framework is a package that can contain resources such as dynamic libraries, strings, headers, images, storyboards etc. With small changes to its structure, it can even contain other frameworks. Such aggregate is known as umbrella framework.

Frameworks are also bundles ending with .framework extension. They can be accessed by NSBundle / Bundle class from code and, unlike most bundle files, can be browsed in the file system that makes it easier for developers to inspect its contents. Frameworks have versioned bundle format which allows to store multiple copies of code and headers to support older program version. You can learn about bundles structure in Bundle Programming Guide by Apple.

Static Library

Static libraries are collections of object files. In its turn, object file is just a name for a file that comes out of a compiler and contains machine code.

Static libraries are ending with .a suffix and are created with an archiver tool. If it sounds very similar to a ZIP archive, then it’s exactly what it is. You can think of a static library as an archive of multiple object files.

.a is an old format originally used by UNIX and its ar tool. If you want to give it a deep dive, I suggest reading the man page.

Object files have Mach-O format which is a special file format for iOS and macOS operating systems. It is basically a binary stream with the following chunks:

  • Header: Specifies the target architecture of the file. Since one Mach-O contains code and data for one architecture, code intended for x86-64 will not run on arm64.
  • Load commands: Specify the logical structure of the file, like the location of the symbol table.
  • Raw segment data: Contains raw code and data.

An attentive eye might have noticed that Mach-O files support single architecture. Then how can a Swift app with lots of static libraries run on all devices and even the simulator?

The answer is lipo tool. It allows to package multiple single architecture libraries into a universal one, called fat binary, or vice-versa. Here you can read more about lipo.

Dynamic Library

Dynamic libraries, as opposed to the static ones, rather than being copied into single monolithic executable, are loaded into memory when they are actually needed. This could happen either at load time or at runtime.

Dynamic libraries are usually shared between applications, therefore the system needs to store only one copy of the library and let different processes access it. As a result, invoking code and data from dynamic libraries happens slower than from the static ones.

All iOS and macOS system libraries are dynamic. Hence our apps will benefit from the future improvements that Apple makes to standard library frameworks without creating and shipping new builds.

Text Based .dylib Stubs

When we link system libraries, such as UIKit or Foundation, we don’t want to copy their entirety into the app, because it would be too large. Linker is also strict about this and does not accept shared .dylib libraries to be linked against, but only .tbd ones. So what are those?

Text-based .dylib stub, or .tbd, is a text file that contains the names of the methods without their bodies, declared in a dynamic library . It results in a significantly lower size of .tbd compared to a matching .dylib. Along with method names, it contains location of the corresponding .dylib, architecture, platform and some other metadata. Here is how a typical .tbd looks when opened in text editor:

--- !tapi-tbd-v3
archs:           [ x86_64 ]
uuids:           [ 'x86_64: 6FFAC142-415D-3AF0-BC09-336302F11934' ]
platform:        macosx
install-name:    /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libQuadrature.dylib
objc-constraint: none
exports:         
  - archs:           [ x86_64 ]
    allowable-clients: [ vecLib ]
    symbols:         [ _quadrature_integrate ]
...

Comparing Static vs. Dynamic Libraries

Let’s summarize pros and cons of static and dynamic libraries.


Static Libraries

Pros:

  • Static libraries are guaranteed to be present in the app and have correct version.
  • No need to keep an app up to date with library updates.
  • Better performance of library calls.

Cons:

  • Inflated app size.
  • Launch time degrades because of bloated app executable.
  • Must copy whole library even if using single function.

Dynamic Libraries

Pros:

  • Can benefit from library improvements without app re-compile. Especially useful with system libraries.
  • Takes less disk space, since it is shared between applications.
  • Faster startup time, as it is loaded on-demand during runtime.
  • Loaded by pieces: no need to load whole library if using single function.

Cons:

  • Can potentially break the program if anything changes in the library.
  • Slower calls to library functions, as it is located outside application executable.

Summary

Libraries and frameworks are basic building blocks for creating iOS and macOS programs.

Libraries are collections of code and data, while frameworks are hierarchial directories with different kinds of files, including other libraries and frameworks.

Based on how libraries are linked, they can be static or dynamic. Each kind of linking comes with its pros and cons. Understanding them will help you to make the right choice between static and dynamic libraries for your project.


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