Package 

Interface IMqttAsyncClient

  • 
public interface IMqttAsyncClient

    Enables an application to communicate with an MQTT server using non-blocking methods.

    It provides applications a simple programming interface to all features of the MQTT version 3.1 specification including:

    • connect
    • publish
    • subscribe
    • unsubscribe
    • disconnect

    There are two styles of MQTT client, this one and IMqttClient.

    • IMqttAsyncClient provides a set of non-blocking methods that return control to the invoking application after initial validation of parameters and state. The main processing is performed in the background so as not to block the application program's thread. This non- blocking approach is handy when the application needs to carry on processing while the MQTT action takes place. For instance connecting to an MQTT server can take time, using the non-blocking connect method allows an application to display a busy indicator while the connect action takes place in the background. Non blocking methods are particularly useful in event oriented programs and graphical programs where invoking methods that take time to complete on the the main or GUI thread can cause problems. The non-blocking interface can also be used in blocking form.
    • IMqttClient provides a set of methods that block and return control to the application program once the MQTT action has completed. It is a thin layer that sits on top of the IMqttAsyncClient implementation and is provided mainly for compatibility with earlier versions of the MQTT client. In most circumstances it is recommended to use IMqttAsyncClient based clients which allow an application to mix both non-blocking and blocking calls.

    An application is not restricted to using one style if an IMqttAsyncClient based client is used as both blocking and non-blocking methods can be used in the same application. If an IMqttClient based client is used then only blocking methods are available to the application. For more details on the blocking client see IMqttClient

    There are two forms of non-blocking method: 

    •     IMqttToken token = asyncClient.method(parms)

      In this form the method returns a token that can be used to track the progress of the action (method). The method provides a waitForCompletion() method that once invoked will block until the action completes. Once completed there are method on the token that can be used to check if the action completed successfully or not. For example to wait until a connect completes: 

       IMqttToken conToken; conToken = asyncClient.client.connect(conToken); ... do some work... conToken.waitForCompletion();

      To turn a method into a blocking invocation the following form can be used: 

       IMqttToken token; token = asyncClient.method(parms).waitForCompletion(); 
    •     IMqttToken token method(parms, Object userContext, IMqttActionListener callback)

      In this form a callback is registered with the method. The callback will be notified when the action succeeds or fails. The callback is invoked on the thread managed by the MQTT client so it is important that processing is minimised in the callback. If not the operation of the MQTT client will be inhibited. For example to be notified (called back) when a connect completes: 

       IMqttToken conToken; conToken = asyncClient.connect("some context",new new MqttAsyncActionListener() { public void onSuccess(IMqttToken asyncActionToken) { log("Connected"); } public void onFailure(IMqttToken asyncActionToken, Throwable exception) { log ("connect failed" +exception); } });
      An optional context object can be passed into the method which will then be made available in the callback. The context is stored by the MQTT client) in the token which is then returned to the invoker. The token is provided to the callback methods where the context can then be accessed.

    To understand when the delivery of a message is complete either of the two methods above can be used to either wait on or be notified when the publish completes. An alternative is to use the deliveryComplete method which will also be notified when a message has been delivered to the requested quality of service.

    • Method Detail

      • connect

         abstract IMqttToken connect(MqttConnectOptions options)

        Connects to an MQTT server using the provided connect options.

        The connection will be established using the options specified in the MqttConnectOptions parameter.

        Parameters:
        options - a set of connection parameters that override the defaults.

      • connect

         abstract IMqttToken connect(Object userContext, IMqttActionListener callback)

        Connects to an MQTT server using the default options.

        The default options are specified in MqttConnectOptions class.

        Parameters:
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when the connect completes.

      • connect

         abstract IMqttToken connect(MqttConnectOptions options, Object userContext, IMqttActionListener callback)

        Connects to an MQTT server using the specified options.

        The server to connect to is specified on the constructor. It is recommended to call setCallback prior to connecting in order that messages destined for the client can be accepted as soon as the client is connected.

        The method returns control before the connect completes. Completion can be tracked by:

        Parameters:
        options - a set of connection parameters that override the defaults.
        userContext - optional object for used to pass context to the callback.
        callback - optional listener that will be notified when the connect completes.

      • disconnect

         abstract IMqttToken disconnect()

        Disconnects from the server.

        An attempt is made to quiesce the client allowing outstanding work to complete before disconnecting. It will wait for a maximum of 30 seconds for work to quiesce before disconnecting. This method must not be called from inside MqttCallback methods.

      • disconnect

         abstract IMqttToken disconnect(long quiesceTimeout)

        Disconnects from the server.

        An attempt is made to quiesce the client allowing outstanding work to complete before disconnecting. It will wait for a maximum of the specified quiesce time for work to complete before disconnecting. This method must not be called from inside MqttCallback methods.

        Parameters:
        quiesceTimeout - the amount of time in milliseconds to allow for existing work to finish before disconnecting.

      • disconnect

         abstract IMqttToken disconnect(Object userContext, IMqttActionListener callback)

        Disconnects from the server.

        An attempt is made to quiesce the client allowing outstanding work to complete before disconnecting. It will wait for a maximum of 30 seconds for work to quiesce before disconnecting. This method must not be called from inside MqttCallback methods.

        Parameters:
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when the disconnect completes.

      • disconnect

         abstract IMqttToken disconnect(long quiesceTimeout, Object userContext, IMqttActionListener callback)

        Disconnects from the server.

        The client will wait for MqttCallback methods to complete. It will then wait for up to the quiesce timeout to allow for work which has already been initiated to complete. For instance when a QoS 2 message has started flowing to the server but the QoS 2 flow has not completed.It prevents new messages being accepted and does not send any messages that have been accepted but not yet started delivery across the network to the server. When work has completed or after the quiesce timeout, the client will disconnect from the server. If the cleanSession flag was set to false and is set to false the next time a connection is made QoS 1 and 2 messages that were not previously delivered will be delivered.

        This method must not be called from inside MqttCallback methods.

        The method returns control before the disconnect completes. Completion can be tracked by:

        Parameters:
        quiesceTimeout - the amount of time in milliseconds to allow for existing work to finish before disconnecting.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when the disconnect completes.

      • disconnectForcibly

         abstract void disconnectForcibly()

        Disconnects from the server forcibly to reset all the states. Could be useful when disconnect attempt failed.

        Because the client is able to establish the TCP/IP connection to a none MQTT server and it will certainly fail to send the disconnect packet. It will wait for a maximum of 30 seconds for work to quiesce before disconnecting and wait for a maximum of 10 seconds for sending the disconnect packet to server.

      • disconnectForcibly

         abstract void disconnectForcibly(long disconnectTimeout)

        Disconnects from the server forcibly to reset all the states. Could be useful when disconnect attempt failed.

        Because the client is able to establish the TCP/IP connection to a none MQTT server and it will certainly fail to send the disconnect packet. It will wait for a maximum of 30 seconds for work to quiesce before disconnecting.

        Parameters:
        disconnectTimeout - the amount of time in milliseconds to allow send disconnect packet to server.

      • disconnectForcibly

         abstract void disconnectForcibly(long quiesceTimeout, long disconnectTimeout)

        Disconnects from the server forcibly to reset all the states. Could be useful when disconnect attempt failed.

        Because the client is able to establish the TCP/IP connection to a none MQTT server and it will certainly fail to send the disconnect packet.

        Parameters:
        quiesceTimeout - the amount of time in milliseconds to allow for existing work to finish before disconnecting.
        disconnectTimeout - the amount of time in milliseconds to allow send disconnect packet to server.

      • isConnected

         abstract boolean isConnected()

        Determines if this client is currently connected to the server.

      • getClientId

         abstract String getClientId()

        Returns the client ID used by this client.

        All clients connected to the same server or server farm must have a unique ID.

      • getServerURI

         abstract String getServerURI()

        Returns the address of the server used by this client.

        The format of the returned String is the same as that used on the constructor.

      • publish

         abstract IMqttDeliveryToken publish(String topic, Array<byte> payload, int qos, boolean retained)

        Publishes a message to a topic on the server.

        A convenience method, which will create a new MqttMessage object with a byte array payload and the specified QoS, and then publish it.

        Parameters:
        topic - to deliver the message to, for example "finance/stock/ibm".
        payload - the byte array to use as the payload
        qos - the Quality of Service to deliver the message at.
        retained - whether or not this message should be retained by the server.

      • publish

         abstract IMqttDeliveryToken publish(String topic, Array<byte> payload, int qos, boolean retained, Object userContext, IMqttActionListener callback)

        Publishes a message to a topic on the server.

        A convenience method, which will create a new MqttMessage object with a byte array payload and the specified QoS, and then publish it.

        Parameters:
        topic - to deliver the message to, for example "finance/stock/ibm".
        payload - the byte array to use as the payload
        qos - the Quality of Service to deliver the message at.
        retained - whether or not this message should be retained by the server.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when message delivery hsa completed to the requested quality of service

      • publish

         abstract IMqttDeliveryToken publish(String topic, MqttMessage message)

        Publishes a message to a topic on the server. Takes an MqttMessage message and delivers it to the server at the requested quality of service.

        Parameters:
        topic - to deliver the message to, for example "finance/stock/ibm".
        message - to deliver to the server

      • publish

         abstract IMqttDeliveryToken publish(String topic, MqttMessage message, Object userContext, IMqttActionListener callback)

        Publishes a message to a topic on the server.

        Once this method has returned cleanly, the message has been accepted for publication by the client and will be delivered on a background thread. In the event the connection fails or the client stops. Messages will be delivered to the requested quality of service once the connection is re-established to the server on condition that:

        • The connection is re-established with the same clientID
        • The original connection was made with (@link MqttConnectOptions#setCleanSession(boolean)} set to false
        • The connection is re-established with (@link MqttConnectOptions#setCleanSession(boolean)} set to false
        • Depending when the failure occurs QoS 0 messages may not be delivered.

        When building an application, the design of the topic tree should take into account the following principles of topic name syntax and semantics:

        • A topic must be at least one character long.
        • Topic names are case sensitive. For example, ACCOUNTS and Accounts are two different topics.
        • Topic names can include the space character. For example, Accounts payable is a valid topic.
        • A leading "/" creates a distinct topic. For example, /finance is different from finance. /finance matches "+/+" and "/+", but not "+".
        • Do not include the null character (Unicode \x0000) in any topic.

        The following principles apply to the construction and content of a topic tree:

        • The length is limited to 64k but within that there are no limits to the number of levels in a topic tree.
        • There can be any number of root nodes; that is, there can be any number of topic trees.

        The method returns control before the publish completes. Completion can be tracked by:

        Parameters:
        topic - to deliver the message to, for example "finance/stock/ibm".
        message - to deliver to the server
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when message delivery has completed to the requested quality of service

      • subscribe

         abstract IMqttToken subscribe(String topicFilter, int qos)

        Subscribe to a topic, which may include wildcards.

        Parameters:
        topicFilter - the topic to subscribe to, which can include wildcards.
        qos - the maximum quality of service at which to subscribe.

      • subscribe

         abstract IMqttToken subscribe(String topicFilter, int qos, Object userContext, IMqttActionListener callback)

        Subscribe to a topic, which may include wildcards.

        Parameters:
        topicFilter - the topic to subscribe to, which can include wildcards.
        qos - the maximum quality of service at which to subscribe.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when subscribe has completed

      • subscribe

         abstract IMqttToken subscribe(Array<String> topicFilters, Array<int> qos)

        Subscribe to multiple topics, each of which may include wildcards.

        Provides an optimized way to subscribe to multiple topics compared to subscribing to each one individually.

        Parameters:
        topicFilters - one or more topics to subscribe to, which can include wildcards
        qos - the maximum quality of service at which to subscribe.

      • subscribe

         abstract IMqttToken subscribe(Array<String> topicFilters, Array<int> qos, Object userContext, IMqttActionListener callback)

        Subscribes to multiple topics, each of which may include wildcards.

        Provides an optimized way to subscribe to multiple topics compared to subscribing to each one individually.

        The setCallback method should be called before this method, otherwise any received messages will be discarded.

        If (@link MqttConnectOptions#setCleanSession(boolean)} was set to true when when connecting to the server then the subscription remains in place until either:

        • The client disconnects
        • An unsubscribe method is called to un-subscribe the topic

        • If (@link MqttConnectOptions#setCleanSession(boolean)} was set to false when connecting to the server then the subscription remains in place until either:

        • The "topic filter" string used when subscribing may contain special characters, which allow you to subscribe to multiple topics at once.

        • The topic level separator is used to introduce structure into the topic, and can therefore be specified within the topic for that purpose. The multi-level wildcard and single-level wildcard can be used for subscriptions, but they cannot be used within a topic by the publisher of a message.

        • The method returns control before the subscribe completes. Completion can be tracked by:

        Parameters:
        topicFilters - one or more topics to subscribe to, which can include wildcards
        qos - the maximum quality of service to subscribe each topic at.Messages published at a lower quality of service will be received at the published QoS.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when subscribe has completed

      • unsubscribe

         abstract IMqttToken unsubscribe(String topicFilter)

        Requests the server unsubscribe the client from a topic.

        Parameters:
        topicFilter - the topic to unsubscribe from.

      • unsubscribe

         abstract IMqttToken unsubscribe(Array<String> topicFilters)

        Requests the server unsubscribe the client from one or more topics.

        Parameters:
        topicFilters - one or more topics to unsubscribe from.

      • unsubscribe

         abstract IMqttToken unsubscribe(String topicFilter, Object userContext, IMqttActionListener callback)

        Requests the server unsubscribe the client from a topics.

        Parameters:
        topicFilter - the topic to unsubscribe from.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when unsubscribe has completed

      • unsubscribe

         abstract IMqttToken unsubscribe(Array<String> topicFilters, Object userContext, IMqttActionListener callback)

        Requests the server unsubscribe the client from one or more topics.

        Unsubcribing is the opposite of subscribing. When the server receives the unsubscribe request it looks to see if it can find a matching subscription for the client and then removes it. After this point the server will send no more messages to the client for this subscription.

        The topic(s) specified on the unsubscribe must match the topic(s) specified in the original subscribe request for the unsubscribe to succeed

        The method returns control before the unsubscribe completes. Completion can be tracked by:

        Parameters:
        topicFilters - one or more topics to unsubscribe from.
        userContext - optional object used to pass context to the callback.
        callback - optional listener that will be notified when unsubscribe has completed

      • setCallback

         abstract void setCallback(MqttCallback callback)

        Sets a callback listener to use for events that happen asynchronously.

        There are a number of events that the listener will be notified about. These include:

        • A new message has arrived and is ready to be processed
        • The connection to the server has been lost
        • Delivery of a message to the server has completed

        Other events that track the progress of an individual operation such as connect and subscribe can be tracked using the MqttToken returned from each non-blocking method or using setting a IMqttActionListener on the non-blocking method.

        Parameters:
        callback - which will be invoked for certain asynchronous events

      • getPendingDeliveryTokens

         abstract Array<IMqttDeliveryToken> getPendingDeliveryTokens()

        Returns the delivery tokens for any outstanding publish operations.

        If a client has been restarted and there are messages that were in the process of being delivered when the client stopped this method returns a token for each in-flight message enabling the delivery to be tracked Alternately the deliveryComplete callback can be used to track the delivery of outstanding messages.

        If a client connects with cleanSession true then there will be no delivery tokens as the cleanSession option deletes all earlier state. For state to be remembered the client must connect with cleanSession set to false

      • close

         abstract void close()

        Close the client Releases all resource associated with the client. After the client has been closed it cannot be reused. For instance attempts to connect will fail.