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

public final class BasicMessageChannel<T> {
  @NonNull private final BinaryMessenger messenger;
  @NonNull private final String name;
  @NonNull private final MessageCodec<T> codec;
  /**
   * Creates a new channel associated with the specified {@link BinaryMessenger} and with the
   * specified name and {@link MessageCodec}.
   *
   * @param messenger a {@link BinaryMessenger}.
   * @param name a channel name String.
   * @param codec a {@link MessageCodec}.
   */
  public BasicMessageChannel(
      @NonNull BinaryMessenger messenger, @NonNull String name, @NonNull MessageCodec<T> 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 message handler on this channel for receiving messages sent from the Flutter
   * application.
   *
   * <p>Overrides any existing handler registration for (the name of) this channel.
   *
   * <p>If no handler has been registered, any incoming message on this channel will be handled
   * silently by sending a null reply.
   *
   * @param handler a {@link MessageHandler}, or null to deregister.
   */
  @UiThread
  public void setMessageHandler(@Nullable final MessageHandler<T> handler) {
    messenger.setMessageHandler(name, handler == null ? null : new IncomingMessageHandler(handler));
  }

/** A handler of incoming messages. */
  public interface MessageHandler<T> {
    /**
     * Handles the specified message received from Flutter.
     *
     * <p>Handler implementations must reply to all incoming messages, by submitting a single reply
     * message to the given {@link Reply}. Failure to do so will result in lingering Flutter reply
     * handlers. The reply may be submitted asynchronously.
     *
     * <p>Any uncaught exception thrown by this method, or the preceding message decoding, will be
     * caught by the channel implementation and logged, and a null reply message will be sent back
     * to Flutter.
     *
     * <p>Any uncaught exception thrown during encoding a reply message submitted to the {@link
     * Reply} is treated similarly: the exception is logged, and a null reply is sent to Flutter.
     *
     * @param message the message, possibly null.
     * @param reply a {@link Reply} for sending a single message reply back to Flutter.
     */
    void onMessage(@Nullable T message, @NonNull Reply<T> reply);
  }