# Messaging ## How are messages passed between the framework and platform code? * `ServicesBinding.initInstances` sets the global message handler (`Window.onPlatformMessage`) to `ServicesBinding.defaultBinaryMessenger`. This instance processes messages from the platform (via `BinaryMessenger.handlePlatformMessage`) and allows other framework code to register message handlers (via `BinaryMessenger.setMessageHandler`). Handlers subscribe to a channel (an identifier used to multiplex the single engine callback) using an identifier shared by the framework and the platform. ## What are the building blocks of messaging? * `BinaryMessenger` multiplexes the global message handler via channel names, supporting handler registration and bidirectional binary messaging. Sending a message produces a future that resolves to the raw response. * `MessageCodec` defines an interface to encode and decode byte data (`MessageCodec.encodeMessage`, `MessageCodec.decodeMessage`). A cross-platform binary codec is available (`StandardMessageCodec`) as well as a JSON-based codec (`JSONMessageCodec`). The platform must implement a corresponding codec natively. * `MethodCodec` is analogous to `MessageCodec` (but otherwise independent) encoding and decoding `MethodCall` instances that wrap a method name and a dynamic list of arguments. Method-based codecs pack and unpack results into envelopes to distinguish success and error outcomes. * `BasicMessageChannel` provides a thin wrapper around `BinaryMessager` that uses the provided codec to encode and decode messages to and from raw byte data. * `MethodChannel` provides a thin wrapper around `BinaryMessager` that uses the provided method codec to encode and decode method invocations. Responses to incoming invocations are packed into envelopes indicating outcome; similarly, results from outgoing invocations are unpacked from their encoded envelope. These are returned as futures. * Success envelopes are unpacked and the result returned. * Error envelopes throw a `PlatformException`. * Unrecognized methods throw a `MissingPluginException` (except when using an `OptionalMethodChannel`). * `EventChannel` is a helper that exposes a remote stream as a local stream. The initial subscription is handled by invoking a remote method called `listen` (via `MethodChannel.invokeMethod`) which causes the platform to begin emitting a stream of envelope-encoded items. A top-level handler is installed (via `ServicesBinding.defaultBinaryMessenger.setMessageHandler`) to unpack and forward items to an output stream in the framework. If the stream ends (for any reason), a remote method called `cancel` is invoked and the global handler cleared. * `SystemChannels` is a singleton instance that provides references to messaging channels that are essential to the framework (`SystemChannels.system`, `SystemChannels.keyEvent`, etc.). --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://flutter.megathink.com/core/messaging.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.