Subscribe Method

Subscribes the component to one or more topic filters.


public func subscribe(topicFilter: String, qos: Int32) throws -> Void


This method subscribes the class to one or more topic filters using one or more QoS levels.

The Subscribed event will fire once for each topic filter given once the server acknowledges the subscription request(s). Keep in mind that the server is allowed to start publishing messages before it sends the acknowledgment.

It is legal to call Subscribe again for the same topic filter at any time, and pass a different QoS value at that time if desired. The Subscribed event will fire as normal when doing this.

Subscribe Examples

// Subscribed event handler.
mqtt1.OnSubscribed += (s, e) => {
  if (e.ResponseCode <= 2)
    Console.WriteLine("Subscribed to " + e.TopicFilter + " at QoS " + e.QOS + ".");
    Console.WriteLine("Failed to subscribe to " + e.TopicFilter + ".");

// Basic, subscribe to some topic filters, all at the same QoS level.
mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 2);

// A bit more advanced, subscribe to the same topic filters, but at different QoS levels.
// The 0 is ignored here since we've specified individual QoS values explicitly.
mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 0);

Topic Filters

The string passed for TopicFilter must contain one or more valid topic filter strings, separated by the delimiter string specified by the TopicDelimiter configuration setting (, by default).

A topic filter is a case-sensitive string between 1 and 65535 characters long (per topic filter), and can include any character other than the null character. Certain characters have special meanings:

  • / - The topic level separator
  • # - The multi-level wildcard (zero or more levels)
  • + - The single-level wildcard (exactly one level)
  • Leading $ - Denotes a "system topic"

Note that both types of wildcards may be used in the same topic filter.

Topic Level Separators

The topic level separator, as its name implies, is used to separate a topic name (or in this case, filter) into "levels". This concept of topic names having levels is what allows topic filters to match multiple topics through the use of wildcards. For the examples in the next sections, assume the following topics exist:

  • home/floor1
  • home/floor1/livingRoom
  • home/floor1/livingRoom/temperature
  • home/floor1/kitchen/temperature
  • home/floor1/kitchen/fridge/temperature
  • home/floor2/bedroom1
  • home/floor2/bedroom1/temperature

Multi-level Wildcards

The multi-level wildcard character is used at the end of a topic filter to make it match an arbitrary number of successive levels. For example, the topic filter home/floor1/# would match the following topics:

  • home/floor1 (because it can match zero levels)
  • home/floor1/livingRoom
  • home/floor1/livingRoom/temperature
  • home/floor1/kitchen/temperature
  • home/floor1/kitchen/fridge/temperature

Here are some things to keep in mind when using a multi-level wildcard:

  • # must always be the last character in the topic filter (e.g., home/floor1/#/livingRoom is not valid)
  • # must always be preceded by a / (e.g., home/floor1# is not valid)
  • # by itself is a valid topic filter, and will match all topics except system topics

Single-level Wildcards

The single-level wildcard character is used between two /s in a topic filter to make it any single level. For example, the topic filter home/floor1/+/temperature would match the following topics:

  • home/floor1/livingRoom/temperature
  • home/floor1/kitchen/temperature

Any number of single-level wildcards are supported in a topic filter. For example, the topic filter home/+/+/temperature would match the following topics:

  • home/floor1/livingRoom/temperature
  • home/floor1/kitchen/temperature
  • home/floor2/bedroom1/temperature

Here are some things to keep in mind when using single-level wildcards:

  • + must always be separated from other levels using /s (e.g., home/floor1+ is invalid, but +/floor1/+ is valid)
  • + by itself is a valid topic filter, and will match all topics with exactly one level in their name except system topics
  • Remember, topic names with a leading / have a zero-length string as their first level. So a topic named /people would be matched by the topic filter +/+, but not by +
  • + must match exactly one level. So for example, the topic filter home/floor1/kitchen/+/temperature would match /home/floor1/kitchen/fridge/temperature, but not home/floor1/kitchen/temperature

System Topics

Topic names which begin with a $ are "system topics". Typically, the server prohibits clients from publishing to such topics, but permits subscribing to them. As described above, wildcards will never match the first level of a topic if it begins with a $.

Note that MQTT 5 defines shared subscriptions, a special type of system topic. See the MQTT 5 notes below for more.

Requested QoS Values

The QoS value passed for QOS will be used to subscribe to all given topic filters, unless the TopicQOSArray configuration setting is used to specify explicit QoS values for each individual topic filter.

QoS values range from 0 to 2 and have the following meanings:

QoS LevelDescription
0 At most once - The published message is sent once, and if it does not arrive it is lost.
1 At least once - Guarantees that the published message arrives, but there may be duplicates.
2 Exactly once - Guarantees that the publish message arrives and that there are no duplicates.

QoS is not guaranteed to be end-to-end in MQTT. The server must downgrade a message's QoS level when delivering it to clients who specified a lower "maximum acceptable" QoS when they subscribed. For example, if Client X subscribes to a topic at QoS 1, and Client Y publishes a message to that topic at QoS 2, the server will downgrade the message to QoS 1 when attempting to deliver it to Client X.

MQTT 5 Notes

MQTT 5 defines several new features that apply to subscriptions, including Shared Subscriptions, Subscription Options and Subscription Identifiers. Some of these features require configuration settings to be set prior to calling Subscribe. In these cases, note that thee configuration settings will continue to be applied to any future SUBSCRIBE packets sent with the method unless they are updated or reset. To prevent a value from being included in packets after it has been set previously, set it to "", or -1 (for integers).

Shared Subscriptions

Shared subscriptions allow a client to subscribe to a topic as a part of a group of clients, where each client in the group is subscribing to the same topic. Each message published to such a group will be delivered to only one client in the group, making shared subscriptions useful when several clients share the processing of the publications in parallel.

Subscribing to a topic as part of a shared subscription is similar to a normal subscription, the only difference being the syntax of the topic filter passed to the Subscribe method.

The format of this filter is $share/{ShareName}/{filter}, where

  • $share is a string literal identifying the subscription as a shared subscriptions.
  • {ShareName} is a string at least one character long that must not include /, + or #.
  • {filter} is a full topic filter formatted just as in a non-shared subscription.

Unsubscribing is done by passing the same value to the Unsubscribe method.

A shared subscription behaves the same way as a non-shared subscription, except that the $share/{ShareName} portion of the topic filter is ignored when filtering publication topics, and that the server will distribute delivery of messages amongst the group, with each message delivered to only one client per group per topic.

Shared subscriptions can be implemented without any additional properties or methods, simply by using the Subscribe and Unsubscribe methods. However, if it is necessary to verify that the feature is supported by the server, check the "SharedSubscriptionAvailable" value of the ConnAckProperties config.

Shared Subscription Example

mqtt1.Subscribe("$share/myShareGroup/myTopicFilter/subTopic", 2);

Subscription Options

MQTT 5 defines three subscription options which can be configured on a per-subscription basis and are useful for message bridge applications:

  • No Local flag - indicates that messages must not be forwarded to a connection with a ClientId equal to the ClientId of the publishing connection (cannot be set on a shared subscription).
  • Retain As Published flag - indicates that messages forwarded using this subscription keep the retain flag they were published with (as opposed to always being set to False).
  • Retain Handling option - specifies whether retained messages are sent when the subscription is established. Possible values are:

    0Send retained messages at the time of the subscribe
    1Send retained messages at subscribe only if the subscription does not currently exist
    2Do not send retained messages at the time of the subscribe

To specify these options in the subscribe method, the class has three corresponding configuration settings (TopicNLArray, TopicRAPArray and TopicRHArray) which each take a string value in the form of a comma-separated list. Each element in the list corresponds to a topic filter passed to the Subscribe method, as with TopicQOSArray.

If the subscription is only for one topic, these lists would have only one value. If the number of elements in the string is less than the number of topic filters in the subscription, the final element will be applied to all remaining topics.

Subscription Options Example

mqtt1.Connected = true;
mqtt1.Config("TopicNLArray=true, false, true"); // TestTopic1 = true, TestTopic2 = false, TestTopic3 = true
mqtt1.Config("TopicRAPArray=false"); // TestTopic1 = false, TestTopic2 = false, TestTopic3 = false
mqtt1.Config("TopicRHArray=1, 0"); // // TestTopic1 = 1, TestTopic2 = 0, TestTopic3 = 0
mqtt1.Subscribe("TestTopic1, TestTopic2, TestTopic3", 1);

Subscription Identifiers

MQTT 5 allows clients to specify a numeric subscription identifier which will be returned with messages delivered for that subscription.

To do so with the class, set the SubscriptionIdentifier configuration setting before calling Subscribe. To update the identifier, call Subscribe with a new SubscriptionIdentifier.

To verify that a server supports subscription identifiers, check the "SubscriptionIdentifiersAvailable" value in the ConnAckProperties configuration setting. See the SubscriptionIdentifiers property of the MQTTMessage type for details on returning the ids associated with messages.

Note that a topic may have multiple identifiers associated with it (due to wildcards), and the same identifier may be associated with multiple topics.

Subscription Options Example

mqtt1.Connected = true;
mqtt1.OnMessageAck += (o, e) => {
  String ids = mqtt1.IncomingMessages[e.Index].SubscriptionIdentifiers; // ids = "123,321"

mqtt1.Subscribe("test_topic/subtopic", 1);
mqtt1.Subscribe("test_topic/+", 1);

mqtt2.Connected = true;
mqtt2.PublishMessage("test_topic/subtopic", 1, "hello world"); 

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