I have subscribed to a tree of topics and am using that to update values in an HTML table . Sometimes new child topics are added and old child topics are removed. I need to keep my UI in sync, new topics are detected when values arrive for an unknown topic, but I cannot discover when a subscribed topic has been removed.
How can I detect when a topic has been removed?
If you are subscribing to a topic tree using a topic selector such as ?myTopicTreeRoot//, then you when you place this subscription call, you can also state what would happen in case of a unsubscription, as well as subscription, e.g:
session.subscribe("?myTopicTreeRoot//").on({
open: function(subscription) {
console.log('Opened subscription for: ' + subscription.selector);
},
update : function(update, topic) {
console.log('Update for ' + topic + ' : ' + update);
},
subscribe : function(details, topic) {
//This will notify you of every new subscription to a topic in your subtree
console.log('Subscribed to : ' + topic);
},
unsubscribe : function(reason, topic) {
//This will notify you of every unsubscription from a topic in your subtree
console.log('Unsubscribed from : ' + topic);
}
});
Property functions subscribe and unsubscribe are called for topic myTopicTreeRoot and any descendant, e.g. myTopicTreeRoot/foo/bar/baz.
Importantly: Any subscription is ended when the topic is removed, which in turn calls unsubscribe.
Note that it is better to use the subscribe property function. Using the first call to update to imply existence of a new topic is unwise if subscribing to stateless topics.