added Mqtt client Interface definition

Author: andreagilardoni

src/MqttInterface.h

@@ -0,0 +1,128 @@
+#pragma once
+#include <Client.h>
+
+// The Idea for this section of the library is to allow the usage of different implementation for Mqtt Clients
+// while preserving the possibility of having an Arduino standardized interface for Mqtt protocol
+// One should implement MqttClientInterface and provide a way to instantiate the implementation
+
+// namespace arduino { // namespace net { namespace mqtt {
+typedef int error_t; // TODO move this to be generally available
+
+using Topic = const char* const;
+
+#if defined(__has_include)
+  // Check if string_view is available and usable
+    #if __has_include(<functional>)
+        #include <functional>
+
+        // for incoming published messages
+        using MqttReceiveCallback = std::function<void(Topic)>;
+    #endif
+#else
+    typedef void(*MqttReceiveCallback)(Topic)>;
+#endif
+
+// TODO MQTT 5.0 stuff
+
+// TODO define callback for mqtt events. one should be the default, but the user can always change it
+
+// copied from zephyr
+// enum mqtt_conn_return_code: error_t {
+// 	/** Connection accepted. */
+// 	MQTT_CONNECTION_ACCEPTED                = 0x00,
+
+// 	/** The Server does not support the level of the MQTT protocol
+// 	 * requested by the Client.
+// 	 */
+// 	MQTT_UNACCEPTABLE_PROTOCOL_VERSION      = 0x01,
+
+// 	/** The Client identifier is correct UTF-8 but not allowed by the
+// 	 *  Server.
+// 	 */
+// 	MQTT_IDENTIFIER_REJECTED                = 0x02,
+
+// 	/** The Network Connection has been made but the MQTT service is
+// 	 *  unavailable.
+// 	 */
+// 	MQTT_SERVER_UNAVAILABLE                 = 0x03,
+
+// 	/** The data in the user name or password is malformed. */
+// 	MQTT_BAD_USER_NAME_OR_PASSWORD          = 0x04,
+
+// 	/** The Client is not authorized to connect. */
+// 	MQTT_NOT_AUTHORIZED                     = 0x05
+// };
+
+constexpr error_t NotImplementedError= -0x100; // TODO define a proper value
+
+enum MqttQos: uint8_t {
+    MqttQos0 = 0, // At Most once
+    MqttQos1 = 1, // At least once
+    MqttQos2 = 2, // Exactly once
+};
+
+typedef uint8_t MqttPublishFlag;
+enum MqttPublishFlags: MqttPublishFlag {
+    None            = 0,
+    RetainEnabled   = 1,
+    DupEnabled      = 2,
+};
+
+// TODO define mqtt version
+
+constexpr MqttQos QosDefault = MqttQos0;
+// constexpr size_t MqttClientIdMaxLength = 256;
+constexpr size_t MqttClientIdMaxLength = 40;
+
+// TODO it shouldn't be called client, since it is not an arduino client
+class MqttClientInterface {
+public:
+    virtual ~MqttClientInterface() = default;
+
+    virtual error_t connect(IPAddress ip, uint16_t port) = 0;
+    virtual error_t connect(const char *host, uint16_t port) = 0; // TODO should host be string instead of c-string?
+    virtual void disconnect() = 0;
+
+    virtual error_t subscribe(Topic t, MqttQos qos = QosDefault) = 0;
+
+    virtual error_t publish(
+        Topic t, uint8_t payload[],
+        size_t size, MqttQos qos = QosDefault,
+        MqttPublishFlag flags = MqttPublishFlags::None) = 0;
+
+    virtual error_t unsubscribe(Topic t) = 0;
+    virtual void poll() = 0;
+    virtual error_t ping() = 0;
+
+    virtual void setReceiveCallback(MqttReceiveCallback cbk) = 0;
+
+    // nullptr means generate it randomly
+    virtual void setId(const char* client_id = nullptr) = 0;
+
+    // password may be null, if username is null password won't be used
+    virtual void setUsernamePassword(const char* username, const char* password=nullptr) = 0;
+
+    virtual void setWill(
+        Topic willTopic, const uint8_t* will_message,
+        size_t will_size, MqttQos qos=QosDefault,
+        MqttPublishFlag flags = MqttPublishFlags::None) = 0;
+
+    virtual void setClient(arduino::Client*) = 0;
+
+    // FIXME the following definition may cause errors since one can easily pass a context dependent object
+    // virtual void setClient(Client&) = 0;
+    // Could this be a better solution?
+    // virtual void setClient(Client&&) = 0;
+
+    virtual int read() = 0;
+    virtual int read(uint8_t payload[], size_t size) = 0;
+    virtual int available() = 0;
+
+    // The following methods should return the current rx message parameters.
+    // FIXME what if none?
+    virtual String messageTopic() const = 0;
+    virtual int messageDup()      const = 0;
+    virtual uint16_t messageId()  const = 0;
+    virtual MqttQos messageQoS()  const = 0;
+    virtual int messageRetain()   const = 0;
+};