Introduction

Before the 7.0 release, all source code and binary Firebase SDKs are compiled as static frameworks. With Firebase 7.0, CocoaPods developers can control whether Firebase is linked statically or dynamically from the Podfile. Use the Podfile option use_frameworks! :linkage => :static to get the Firebase 6.x linkage behavior.

The 7.x update applies to all Firebase libraries except FirebaseAnalytics, which continues to be distributed as a binary static framework.

The zip and Carthage distributions continue to only be built for static linking.

The Swift Package Manager distribution follows the Swift Package Manager defaults which is currently static linking.

Using Firebase from a framework or a library

Most of the time you’ll link Firebase frameworks directly to your app targets, but in some cases it makes sense to link Firebase frameworks to your app indirectly, from another library or another framework. This note talks about some of the pitfalls of this technique, almost always hard-to-debug undefined behaviors, that all come down to code duplication and how static and dynamic linking work. Your framework itself may be either static or dynamic. Let's consider these two options in more detail.

Using Firebase SDKs from dynamic frameworks

A dynamic framework is a bundle containing dynamic libraries and other resources. The dynamic libraries bundled in the framework can themselves be compiled from static libraries, like Firebase core and product libraries, meaning all of the symbols of the static libraries are part of the dynamic framework bundle. What if your core app already directly links a static Firebase library when you link to the same library indirectly from a dynamic framework? Well, you end up with duplicate symbols in your app. This leads to undefined behavior (especially when different versions of the static framework are linked to the app and the dynamic framework). For example, a dispatch_once may or may not perform the correct initialization since there are now two entities to initialize. Here are a couple more examples of issues related to this undefined behavior: #4315, #5152.

In this case you will most likely see warnings like the following in the console:

objc[40943]: Class FIRApp is implemented in both
~/Library/Developer/Xcode/DerivedData/FrameworkTest-apqjxlyrxvkbhhafhaypsbdquref/Build/Products/Debug-iphonesimulator/DynamicFramework.framework/DynamicFramework
(0x10b2a87f8) and
~/Library/Developer/CoreSimulator/Devices/4821F959-24A6-4D78-A102-4C5703103D99/data/Containers/Bundle/Application/F017D210-113A-4DAF-9E17-BDE455E71E06/FrameworkTest.app/FrameworkTest
(0x10ad2d348). One of the two will be used. Which one is undefined.

This commonly leads to a crash with the following error message:

The default FirebaseApp instance must be configured before the defaultFirebaseApp instance can be initialized.

Figure 1: Using Firebase SDKs from dynamic framework

Conclusions:

The Firebase SDKs may be used from an embedded dynamic framework in your project (e.g. for code reuse purposes) only when Firebase is not used from the app directly.
The Firebase SDKs should never be used from vendor dynamic frameworks because the version of Firebase compiled into the dynamic framework will conflict with the versions compiled into the app or included in any app bundles.

Using Firebase SDKs from static frameworks

A static framework is a bundle containing static libraries and other resources. When a static binary is built, it is not necessary to link any static or dynamic dependencies into the binary because presence of the dependencies will be verified when the whole app is linked. This means that both the static framework/library and your app will be able to "share" symbols (which is basically what we need).

The main downside of this approach arises when the static framework using Firebase is used from, for example, an app and its extension. In this case, in contrast to a dynamic embedded framework, a copy of the static framework will be added to both the app and each extension. It doesn't lead to any symbol collisions, but it does increase the download size of your app.

Figure 2: Firebase SDKs from static framework

Conclusions:

Using the Firebase SDKs in static frameworks and libraries is safe, meaning there will be no symbol collisions, for both vendor and in-app internal libraries.
If the static framework is used from an app and its extensions, then it will be copied to each target, increasing the app download size.
If the static framework is used from an app and its dynamic dependencies, then it will be copied in both the app and its dependencies, which will result in undefined behavior.