【Flutter】Flutter 混合开发 ( Flutter 与 Native 通信 | Android 端实现 EventChannel 通信 )(一）

public final class EventChannel {
  private static final String TAG = "EventChannel#";
  private final BinaryMessenger messenger;
  private final String name;
  private final MethodCodec codec;
  /**
   * Creates a new channel associated with the specified {@link BinaryMessenger} and with the
   * specified name and the standard {@link MethodCodec}.
   *
   * @param messenger a {@link BinaryMessenger}.
   * @param name a channel name String.
   */
  public EventChannel(BinaryMessenger messenger, String name) {
    this(messenger, name, StandardMethodCodec.INSTANCE);
  }
  /**
   * Creates a new channel associated with the specified {@link BinaryMessenger} and with the
   * specified name and {@link MethodCodec}.
   *
   * @param messenger a {@link BinaryMessenger}.
   * @param name a channel name String.
   * @param codec a {@link MessageCodec}.
   */
  public EventChannel(BinaryMessenger messenger, String name, MethodCodec codec) {
    if (BuildConfig.DEBUG) {
      if (messenger == null) {
        Log.e(TAG, "Parameter messenger must not be null.");
      }
      if (name == null) {
        Log.e(TAG, "Parameter name must not be null.");
      }
      if (codec == null) {
        Log.e(TAG, "Parameter codec must not be null.");
      }
    }
    this.messenger = messenger;
    this.name = name;
    this.codec = codec;
  }
}

/**
   * Registers a stream handler on this channel.
   *
   * <p>Overrides any existing handler registration for (the name of) this channel.
   *
   * <p>If no handler has been registered, any incoming stream setup requests will be handled
   * silently by providing an empty stream.
   *
   * @param handler a {@link StreamHandler}, or null to deregister.
   */
  @UiThread
  public void setStreamHandler(final StreamHandler handler) {
    messenger.setMessageHandler(
        name, handler == null ? null : new IncomingStreamRequestHandler(handler));
  }

/**
   * Handler of stream setup and teardown requests.
   *
   * <p>Implementations must be prepared to accept sequences of alternating calls to {@link
   * #onListen(Object, EventSink)} and {@link #onCancel(Object)}. Implementations should ideally
   * consume no resources when the last such call is not {@code onListen}. In typical situations,
   * this means that the implementation should register itself with platform-specific event sources
   * {@code onListen} and deregister again {@code onCancel}.
   */
  public interface StreamHandler {
    /**
     * Handles a request to set up an event stream.
     *
     * <p>Any uncaught exception thrown by this method will be caught by the channel implementation
     * and logged. An error result message will be sent back to Flutter.
     *
     * @param arguments stream configuration arguments, possibly null.
     * @param events an {@link EventSink} for emitting events to the Flutter receiver.
     */
    void onListen(Object arguments, EventSink events);
    /**
     * Handles a request to tear down the most recently created event stream.
     *
     * <p>Any uncaught exception thrown by this method will be caught by the channel implementation
     * and logged. An error result message will be sent back to Flutter.
     *
     * <p>The channel implementation may call this method with null arguments to separate a pair of
     * two consecutive set up requests. Such request pairs may occur during Flutter hot restart. Any
     * uncaught exception thrown in this situation will be logged without notifying Flutter.
     *
     * @param arguments stream configuration arguments, possibly null.
     */
    void onCancel(Object arguments);
  }

/**
   * Event callback. Supports dual use: Producers of events to be sent to Flutter act as clients of
   * this interface for sending events. Consumers of events sent from Flutter implement this
   * interface for handling received events (the latter facility has not been implemented yet).
   */
  public interface EventSink {
    /**
     * Consumes a successful event.
     *
     * @param event the event, possibly null.
     */
    void success(Object event);
    /**
     * Consumes an error event.
     *
     * @param errorCode an error code String.
     * @param errorMessage a human-readable error message String, possibly null.
     * @param errorDetails error details, possibly null
     */
    void error(String errorCode, String errorMessage, Object errorDetails);
    /**
     * Consumes end of stream. Ensuing calls to {@link #success(Object)} or {@link #error(String,
     * String, Object)}, if any, are ignored.
     */
    void endOfStream();
  }