The evolution of Fb’s iOS app structure

Fb for iOS (FBiOS) is the oldest cell codebase at Meta. For the reason that app was rewritten in 2012, it has been labored on by hundreds of engineers and shipped to billions of customers, and it will probably help a whole bunch of engineers iterating on it at a time.

After years of iteration, the Fb codebase doesn’t resemble a typical iOS codebase:

• It’s filled with C++, Goal-C(++), and Swift.
• It has dozens of dynamically loaded libraries (dylibs), and so many lessons that they’ll’t be loaded into Xcode directly.
• There may be virtually zero uncooked utilization of Apple’s SDK — every little thing has been wrapped or changed by an in-house abstraction.
• The app makes heavy use of code technology, spurred by Buck, our customized construct system.
• With out heavy caching from our construct system, engineers must spend a complete workday ready for the app to construct.

FBiOS was by no means deliberately architected this manner. The app’s codebase displays 10 years of evolution, spurred by technical selections essential to help the rising variety of engineers engaged on the app, its stability, and, above all, the consumer expertise.

Now, to have fun the codebase’s 10-year anniversary, we’re shedding some gentle on the technical selections behind this evolution, in addition to their historic context.

2014: Establishing our personal cell frameworks

Two years after Meta launched the native rewrite of the Fb app, Information Feed’s codebase started to have reliability points. On the time, Information Feed’s information fashions had been backed by Apple’s default framework for managing information fashions: Core Information. Objects in Core Information are mutable, and that didn’t lend itself effectively to Information Feed’s multithreaded structure. To make issues worse, Information Feed utilized bidirectional information circulate, stemming from its use of Apple’s de facto design sample for Cocoa apps: Mannequin View Controller.

Finally, this design exacerbated the creation of nondeterministic code that was very tough to debug or reproduce bugs. It was clear that this structure was not sustainable and it was time to rethink it.

Whereas contemplating new designs, one engineer investigated React, Fb’s (open supply) UI framework, which was turning into fairly well-liked within the Javascript neighborhood. React’s declarative design abstracted away the tough crucial code that brought on points in Feed (on net), and leveraged a one-way information circulate, which made the code a lot simpler to motive about. These traits appeared effectively fitted to the issues Information Feed was dealing with. There was just one downside.

There was no declarative UI in Apple’s SDK. 

Swift wouldn’t be introduced for just a few months, and SwiftUI (Apple’s declarative UI framework) wouldn’t be introduced till 2019. If Information Feed wished to have a declarative UI, the crew must construct a brand new UI framework. 

Finally, that’s what they did.

After spending just a few months constructing and migrating Information Feed to run on a brand new declarative UI and a brand new information mannequin, FBiOS noticed a 50 % efficiency enchancment. A couple of months later, they open-sourced their React-inspired UI framework for cell, ComponentKit. 

To today, ComponentKit remains to be the de facto alternative for constructing native UIs in Fb. It has supplied numerous efficiency enhancements to the app through view reuse swimming pools, view flattening, and background format computation. It additionally impressed its Android counterpart, Litho, and SwiftUI.

Finally, the selection to interchange the UI and information layer with customized infra was a trade-off. To attain a pleasant consumer expertise that might be reliably maintained, new staff must shelve their trade data of Apple APIs to be taught the customized in-house infra. 

This wouldn’t be the final time FBiOS must decide that balanced finish consumer expertise with developer expertise and velocity. Going into 2015, the app’s success would set off what we consult with as a characteristic explosion. And that introduced its personal set of distinctive challenges.

2015: An architectural inflection level

By 2015, Meta had doubled down on its “Mobile First” mantra, and the FBiOS codebase noticed a meteoric rise within the variety of every day contributors. As increasingly more merchandise had been built-in into the app, its launch time started to degrade, and folks started to note. Towards the top of 2015, startup efficiency was so sluggish (practically 30 seconds!) that it risked being killed by the cellphone’s OS.

Upon investigation, it was clear that there have been many contributing elements to degraded startup efficiency. For the sake of brevity, we’ll focus solely on those that had a long-term impact on the app’s structure:

• The app’s ‘pre-main’ time was rising at an unbounded charge, because the app’s dimension grew with every product.
• The app’s ‘module’ system gave every product ungoverned entry to all of the app’s resourcing. This led to a tragedy of the commons challenge as every product leveraged its ‘hook’ into startup to carry out computationally costly operations in order that preliminary navigation to that product could be snappy.

The modifications that had been wanted to mitigate and enhance startup would basically alter the best way product engineers wrote code for FBiOS.

2016: Dylibs and modularity

In line with Apple’s wiki about enhancing launch instances, a lot of operations need to be carried out earlier than an app’s ‘main’ perform could be known as. Typically, the extra code an app has, the longer this can take.

Whereas ‘pre-main’ contributed solely a small subset of the 30 seconds being spent throughout launch, it was a selected concern as a result of it might proceed to develop at an unbounded charge as FBiOS continued to amass new options.

To assist mitigate the unbounded development of the app’s launch time, our engineers started to maneuver massive swaths of product code right into a lazily loaded container referred to as a dynamic library (dylib). When code is moved right into a dynamically loaded library, it isn’t required to load earlier than the app’s most important() perform.

Initially, the FBiOS dylib construction appeared like this:

Two product dylibs (FBCamera and NotOnStartup) had been created, and a 3rd dylib (FBShared) was used to share code between the assorted dylibs and the primary app’s binary.

The dylib answer labored superbly. FBiOS was capable of curb the unbounded development of the app’s startup time. Because the years glided by, most code would find yourself in a dylib in order that startup efficiency stayed quick and was unaffected by the fixed fluctuation of added or eliminated merchandise within the app.

The addition of dylibs triggered a psychological shift in the best way Meta’s product engineers wrote code. With the addition of dylibs, runtime APIs like NSClassFromString() risked runtime failures as a result of the required class lived in unloaded dylibs. Since lots of the FBiOS core abstractions had been constructed on iterating by means of all of the lessons in reminiscence, FBiOS needed to rethink what number of of its core techniques labored.

Other than the runtime failures, dylibs additionally launched a brand new class of linker errors. Within the occasion the code in Fb (the startup set) referenced code in a dylib, engineers would see a linker error like this:

Undefined symbols for structure arm64:
  "_OBJC_CLASS_$_SomeClass", referenced from:
      objc-class-ref in libFBSomeLibrary-9032370.a(FBSomeFile.mm.o)

To repair this, engineers had been required to wrap their code with a particular perform that might load a dylib if obligatory:

All of a sudden:

int most important() 
  DoSomething(context);

Would appear to be this:

int most important() 
  FBCallFunctionInDylib(
    NotOnStatupFramework,
    DoSomething,
    context
  );

The answer labored, however had fairly just a few code smells:

• The app-specific dylib enum was hard-coded into numerous callsites. All apps at Meta needed to share a dylib enum, and it was the reader’s duty to find out whether or not that dylib was utilized by the app the code was working in.
• If the flawed dylib enum was used, the code would fail, however solely at runtime. Given the sheer quantity of code and options within the app, this late sign led to plenty of frustration throughout growth.

On high of all that, our solely system to safeguard in opposition to the introduction of those calls throughout startup was runtime-based, and plenty of releases had been delayed whereas last-minute regressions had been launched into the app.

Finally, the dylib optimization curbed the unbounded development of the app’s launch time, but it surely signified an enormous inflection level in the best way the app was architected. FBiOS engineers would spend the following few years re-architecting the app to clean a few of the tough edges launched by the dylibs, and we (ultimately) shipped an app structure that was extra strong than ever earlier than.

2017: Rethinking the FBiOS structure

With the introduction of dylibs, just a few key elements of FBiOS needed to be rethought:

• The ‘module registration system’ might not be runtime-based.
• Engineers wanted a method to know whether or not any codepath throughout startup might set off a dylib load.

To handle these points, FBiOS turned to Meta’s open supply construct system, Buck.

Inside Buck, every ‘target’ (app, dylib, library, and so forth.) is said with some configuration, like so:

apple_binary(
  title = "Fb",
  ...
  deps = [
    ":NotOnStartup#shared",
    ":FBCamera#shared",
  ],
)

apple_library(
  title = "NotOnStartup",
  srcs = [
    "SomeFile.mm",
  ],
  labels = ["special_label"],
  deps = [
    ":PokesModule",
    ...
  ],
)

Every ‘target’ lists all info wanted to construct it (dependencies, compiler flags, sources, and so forth.), and when ‘buck build’ is named, it builds all this info right into a graph that may be queried.

$ buck question “deps(:Facebook)”
> :NotOnStartup
> :FBCamera

$ buck question “attrfilter(labels, special_label, deps(:Facebook))”
> :NotOnStartup

Utilizing this core idea (and a few particular sauce), FBiOS started to supply some buck queries that might generate a holistic view of the lessons and capabilities within the app throughout construct. This info could be the constructing block of the app’s subsequent technology of structure.

2018: The proliferation of generated code

Now that FBiOS was capable of leverage Buck to question for details about code within the dependency, it might create a mapping of “function/classes -> dylibs” that might be generated on the fly.

  "capabilities": 
    "DoSomething": Dylib.NotOnStartup,
    ...
  ,
  "lessons": 
    "FBSomeClass": Dylib.SomeOtherOne
  

Utilizing that mapping as enter, FBiOS used it to generate code that abstracted away the dylib enum from callsites:

static std::unordered_map functionToDylib 
   "DoSomething", Dylib.NotOnStartup ,
   "FBSomeClass", Dylib.SomeOtherOne ,
  ...
;

Utilizing code technology was interesting for just a few causes:

• As a result of the code was regenerated based mostly on native enter, there was nothing to examine in, and there have been no extra merge conflicts! On condition that the engineering physique of FBiOS might double yearly, this was an enormous growth effectivity win.
• FBCallFunctionInDylib no-longer required an app-specific dylib (and thus might be renamed to ‘FBCallFunction’). As an alternative, the decision would learn from static mapping generated for every utility throughout construct.

Combining Buck question with code technology proved to be so profitable that FBiOS used it as bedrock for a brand new plugin system, which ultimately changed the runtime-based app-module system.

Transferring sign to the left

With the brand new Buck-powered plugin system. FBiOS was capable of substitute most runtime failures with build-time warnings by migrating bits of infra to a plugin-based structure.

When FBiOS is constructed, Buck can produce a graph to point out the situation of all of the plugins within the app, like so:

From this vantage level, the plugin system can floor build-time errors for engineers to warn: 

• “Plugin D, E could trigger a load of a dylib. This is not allowed, since the caller of these plugins lives in the app’s startup path.”
• “There is no plugin for rendering Profiles found in the app … this means that navigating to that screen will not work.”
• “There are two plugins for rendering Groups (Plugin A, Plugin B). One of them should be removed.”

With the previous app module system, these errors could be “lazy” runtime assertions. Now, engineers are assured that when FBiOS is constructed efficiently, it received’t fail due to lacking performance, dylibs loading throughout app startup, or invariants within the module runtime system.

The price of code technology

Whereas migrating FBiOS to a plugin system has improved the app’s reliability, supplied quicker alerts to engineers, and made it potential for the app to trivially share code with our different cell apps, it got here at a price:

• Plugin errors are usually not on Stack Overflow and could be complicated to debug.
• A plugin system based mostly on code technology and Buck is a far cry from conventional iOS growth. 
• Plugins introduce a layer of indirection to the codebase. The place most apps would have a registry file with all options, these are generated in FBiOS and could be surprisingly tough to seek out.

There is no such thing as a doubt that plugins led FBiOS farther away from idiomatic iOS growth, however the trade-offs appear to be price it. Our engineers can change code utilized in many apps at Meta and make certain that if the plugin system is completely happy, no app ought to crash due to lacking performance in a hardly ever examined codepath. Groups like Information Feed and Teams can construct an extension level for plugins and make certain that product groups can combine into their floor with out touching the core code.

2020: Swift and language structure

Whereas most of this text has centered on architectural modifications stemming from scale points within the Fb app, modifications in Apple’s SDK have additionally pressured FBiOS to rethink a few of its architectural selections.

In 2020, FBiOS started to see an increase within the variety of Swift-only APIs from Apple and a rising sentiment for extra Swift within the codebase. It was lastly time to reconcile with the truth that Swift was an inevitable tenant in FB apps. 

Traditionally, FBiOS had used C++ as a lever to construct abstraction, which saved on code dimension due to C++’s zero overhead precept. However C++ doesn’t interop with Swift (but). For most FBiOS APIs (like ComponentKit), some form of shim must be created to make use of in Swift — creating code bloat.

Right here’s a diagram outlining the problems within the codebase:

With this in thoughts, we started to type a language technique about when and the place numerous bits of code ought to be used:

Finally, the FBiOS crew started to advise that product-facing APIs/code mustn’t comprise C++ in order that we might freely use Swift and future Swift APIs from Apple. Utilizing plugins, FBiOS might summary away C++ implementations in order that they nonetheless powered the app however had been hidden from most engineers.

This kind of workstream signified a little bit of shift in the best way FBiOS engineers thought of constructing abstractions. Since 2014, a few of the greatest elements in framework constructing have been contributions to app dimension and expressiveness (which is why ComponentKit selected Goal-C++ over Goal-C).

The addition of Swift was the primary time these would take a backseat to developer effectivity, and we count on to see extra of that sooner or later.

2022: The journey is 1 %  completed

Since 2014, FBiOS structure has shifted fairly a bit:

• It launched numerous in-house abstractions, like ComponentKit and GraphQL.
• It makes use of dylibs to maintain ‘pre-main’ instances minimal and contribute to a blazing-fast app startup.
• It launched a plugin system (powered by Buck) in order that dylibs are abstracted away from engineers, and so code is definitely shareable between apps.
• It launched language pointers about when and the place numerous languages ought to be used and started to shift the codebase to replicate these language pointers.

In the meantime, Apple has launched thrilling enhancements to their telephones, OS, and SDK:

• Their new telephones are quick. The price of loading is far smaller than it was earlier than.
• OS enhancements like dyld3 and chain fixups present software program to make code loading even quicker.
• They’ve launched SwiftUI,  a declarative API for UI that shares plenty of ideas with ComponentKit.
• They’ve supplied improved SDKs, in addition to APIs (like interruptible animations in iOS8) that we might have constructed customized frameworks for.

As extra experiences are shared throughout Fb, Messenger, Instagram, and WhatsApp, FBiOS is revisiting all these optimizations to see the place it will probably transfer nearer to platform orthodoxy. Finally, we’ve seen that the simplest methods to share code are to make use of one thing that the app offers you without cost or construct one thing that’s just about dependency-free and might combine between all of the apps.

We’ll see you again right here in 2032 for the recap of the codebase’s 20-year anniversary!