The camera_platform_interface implementation is located at lib/src/android_camera_camerax.dart, and it is implemented using Dart classes that are wrapped versions of native Android Java classes.
In this approach, each native Android library used in the plugin implementation is represented by an equivalent Dart class. Instances of these classes are considered paired and represent each other in Java and Dart, respectively. An InstanceManager, which is essentially a map between long identifiers and objects that also provides notifications when an object has become unreachable by memory. There is both a Dart and Java InstanceManager implementation, so when a Dart instance is created that represens an Android native instance, both are stored in the InstanceManager of their respective language with a shared long identifier. These InstanceManagers take callbacks that run when objects become unrechable or removed, allowing the Dart library to easily handle removing references to native resources automatically. To ensure all created instances are properly managed and to more easily allow for testing, each wrapped Android native class in Dart takes an InstanceManager and has a detached constructor, a constructor that allows for the creation of instances not attached to the InstanceManager and unlinked to a paired Android native instance.
In lib/src/, you will find all of the Dart-wrapped native Android classes that the plugin currently uses in its implementation. As aforementioned, each of these classes uses an InstanceManager (implementation in instance_manager.dart) to manage objects that are created by the plugin implementation that map to objects of the same type created in the native Android code. This plugin uses pigeon to generate the communication layer between Flutter and native Android code, so each of these Dart-wrapped classes may also have Host API and Flutter API implementations that handle communication to the host native Android platform and from the host native Android platform, respectively. The communication interface is defined in the pigeons/camerax_library.dart file. After editing the communication interface, regenerate the communication layer by running dart run pigeon --input pigeons/camerax_library.dart from the plugin root.
In the native Java Android code in android/src/main/java/io/flutter/plugins/camerax/, you'll find the Host API and Flutter API implementations of the same classes wrapped with Dart in lib/src/ that handle communication from that Dart code and to that Dart code, respectively. The Host API implementations should directly delegate calls to the CameraX or other wrapped Android libraries and should not have any additional logic or abstraction; any exceptions should be thoroughly documented in the code. As aforementioned, the objects created in the native Android code map to objects created on the Dart side and are also managed by an InstanceManager (implementation in InstanceManager.java).
If CameraX or other Android classes that you need to access do not have a duplicately named implementation in lib/src/, then follow the same structure described above to add them. Please note that any Dart-wrapped native Android classes that you add should extend JavaObject. Additionally, they should be annotated as @immutable to avoid lint errors with mock objects that are generated for them that you may use for testing.
For more information, please see the design document or feel free to ask any questions on the #hackers-ecosystem channel on Discord. For more information on contributing packages in general, check out our contribution guide.
While none of the generated pigeon files are tested, all plugin impelementation and wrapped native Android classes (Java & Dart) are tested. You can find the Java tests under android/src/test/java/io/flutter/plugins/camerax/ and the Dart tests under test/. To run these tests, please see the instructions in the running plugin tests guide.
Besides pigeon, this plugin also uses mockito to generate mock objects for testing purposes. To generate the mock objects, run dart run build_runner build --delete-conflicting-outputs.