Pigeon

Pigeon is a code generator tool to make communication between Flutter and the host platform type-safe, easier, and faster.

Pigeon removes the necessity to manage strings across multiple platforms and languages. It also improves efficiency over common method channel patterns. Most importantly though, it removes the need to write custom platform channel code, since pigeon generates it for you.

For usage examples, see the Example README.

Features

Supported Platforms

Currently pigeon supports generating:

• Kotlin and Java code for Android
• Swift and Objective-C code for iOS and macOS
• C++ code for Windows
• GObject code for Linux

Supported Datatypes

Pigeon uses the StandardMessageCodec so it supports any datatype platform channels support.

Custom classes, nested datatypes, and enums are also supported.

Basic inheritance with empty sealed parent classes is allowed only in the Swift, Kotlin, and Dart generators.

Nullable enums in Objective-C generated code will be wrapped in a class to allow for nullability.

By default, custom classes in Swift are defined as structs. Structs don't support some features - recursive data, or Objective-C interop. Use the @SwiftClass annotation when defining the class to generate the data as a Swift class instead.

Synchronous and Asynchronous methods

While all calls across platform channel APIs (such as pigeon methods) are asynchronous, pigeon methods can be written on the native side as synchronous methods, to make it simpler to always reply exactly once.

If asynchronous methods are needed, the @async annotation can be used. This will require results or errors to be returned via a provided callback. Example.

Error Handling

Kotlin, Java and Swift

All Host API exceptions are translated into Flutter PlatformException.

• For synchronous methods, thrown exceptions will be caught and translated.
• For asynchronous methods, there is no default exception handling; errors should be returned via the provided callback.

To pass custom details into PlatformException for error handling, use FlutterError in your Host API. Example.

For swift, use PigeonError instead of FlutterError when throwing an error. See Example#Swift for more details.

Objective-C and C++

Host API errors can be sent using the provided FlutterError class (translated into PlatformException).

For synchronous methods:

• Objective-C - Set the error argument to a FlutterError reference.
• C++ - Return a FlutterError.

For async methods:

• Return a FlutterError through the provided callback.

Task Queue

When targeting a Flutter version that supports the TaskQueue API the threading model for handling HostApi methods can be selected with the TaskQueue annotation.

Multi-Instance Support

Host and Flutter APIs now support the ability to provide a unique message channel suffix string to the api to allow for multiple instances to be created and operate in parallel.

Usage

1. Add pigeon as a dev_dependency.
2. Make a “.dart” file outside of your “lib” directory for defining the communication interface.
3. Run pigeon on your “.dart” file to generate the required Dart and host-language code: flutter pub get then dart run pigeon with suitable arguments. Example.
4. Add the generated Dart code to ./lib for compilation.
5. Implement the host-language code and add it to your build (see below).
6. Call the generated Dart methods.

Rules for defining your communication interface

Example

1. The file should contain no method or function definitions, only declarations.
2. Custom classes used by APIs are defined as classes with fields of the supported datatypes (see the supported Datatypes section).
3. APIs should be defined as an abstract class with either @HostApi() or @FlutterApi() as metadata. @HostApi() being for procedures that are defined on the host platform and the @FlutterApi() for procedures that are defined in Dart.
4. Method declarations on the API classes should have arguments and a return value whose types are defined in the file, are supported datatypes, or are void.
5. Event channels are supported only on the Swift, Kotlin, and Dart generators.
6. Event channel methods should be wrapped in an abstract class with the metadata @EventChannelApi.
7. Event channel definitions should not include the Stream return type, just the type that is being streamed.
8. Objective-C and Swift have special naming conventions that can be utilized with the @ObjCSelector and @SwiftFunction respectively.

Flutter calling into iOS steps

1. Add the generated Objective-C or Swift code to your Xcode project for compilation (e.g. ios/Runner.xcworkspace or .podspec).
2. Implement the generated protocol for handling the calls on iOS, set it up as the handler for the messages.

Flutter calling into Android Steps

1. Add the generated Java or Kotlin code to your ./android/app/src/main/java directory for compilation.
2. Implement the generated Java or Kotlin interface for handling the calls on Android, set it up as the handler for the messages.

Flutter calling into Windows Steps

1. Add the generated C++ code to your ./windows directory for compilation, and to your windows/CMakeLists.txt file.
2. Implement the generated C++ abstract class for handling the calls on Windows, set it up as the handler for the messages.

Flutter calling into macOS steps

1. Add the generated Objective-C or Swift code to your Xcode project for compilation (e.g. macos/Runner.xcworkspace or .podspec).
2. Implement the generated protocol for handling the calls on macOS, set it up as the handler for the messages.

Flutter calling into Linux steps

1. Add the generated GObject code to your ./linux directory for compilation, and to your linux/CMakeLists.txt file.
2. Implement the generated protocol for handling the calls on Linux, set it up as the vtable for the API object.

Calling into Flutter from the host platform

Pigeon also supports calling in the opposite direction. The steps are similar but reversed. For more information look at the annotation @FlutterApi() which denotes APIs that live in Flutter but are invoked from the host platform. Example.

Stability of generated code

Pigeon is intended to replace direct use of method channels in the internal implementation of plugins and applications. Because the expected use of Pigeon is as an internal implementation detail, its development strongly favors improvements to generated code over consistency with previous generated code, so breaking changes in generated code are common.

As a result, using Pigeon-generated code in public APIs is strongy discouraged, as doing so will likely create situations where you are unable to update to a new version of Pigeon without causing breaking changes for your clients.

Inter-version compatibility

The generated message channel code used for Pigeon communication is an internal implementation detail of Pigeon that is subject to change without warning, and changes to the communication are not considered breaking changes. Both sides of the communication (the Dart code and the host-language code) must be generated with the same version of Pigeon. Using code generated with different versions has undefined behavior, including potentially crashing the application.

This means that Pigeon-generated code should not be split across packages. For example, putting the generated Dart code in a platform interface package and the generated host-language code in a platform implementation package is very likely to cause crashes for some plugin clients after updates.

Feedback

File an issue in flutter/flutter with “[pigeon]” at the start of the title.