Messaging

如何在框架和平台代码之间传递消息？

• ServicesBinding.initInstances 设置全局消息控制器 (Window.onPlatformMessage) 给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.).