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.).