定义类型为函数 func
github.com\eclipse\paho.mqtt.golang@v1.4.3\options.go
// CredentialsProvider allows the username and password to be updated
// before reconnecting. It should return the current username and password.
type CredentialsProvider func() (username string, password string)
// MessageHandler is a callback type which can be set to be
// executed upon the arrival of messages published to topics
// to which the client is subscribed.
type MessageHandler func(Client, Message)
// ConnectionLostHandler is a callback type which can be set to be
// executed upon an unintended disconnection from the MQTT broker.
// Disconnects caused by calling Disconnect or ForceDisconnect will
// not cause an OnConnectionLost callback to execute.
type ConnectionLostHandler func(Client, error)
// OnConnectHandler is a callback that is called when the client
// state changes from unconnected/disconnected to connected. Both
// at initial connection and on reconnection
type OnConnectHandler func(Client)
// ReconnectHandler is invoked prior to reconnecting after
// the initial connection is lost
type ReconnectHandler func(Client, *ClientOptions)
// ConnectionAttemptHandler is invoked prior to making the initial connection.
type ConnectionAttemptHandler func(broker *url.URL, tlsCfg *tls.Config) *tls.Config
// OpenConnectionFunc is invoked to establish the underlying network connection
// Its purpose if for custom network transports.
// Does not carry out any MQTT specific handshakes.
type OpenConnectionFunc func(uri *url.URL, options ClientOptions) (net.Conn, error)
dart
typedef ListMapCallback = Function(List<Map<String, dynamic>> listMap);
typedef ListCallback = Function(List<dynamic> list);
typedef FileCallback = Function(XFile file);
typedef NullCallback = Function();
