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Fetching the current value of a topic

A client can send a fetch request for the state of a topic. If the topic is of a type that maintains its state, the Diffusion™ server provides the current state of that topic to the client.

Required permissions: select_topic and read_topic permissions for the specified topics

A fetch request enables you to get the current value of a topic without subscribing to the topic.

The value fetched from a topic are received through a fetch stream, not through any value streams or topic streams registered against the topic.

Fetch streams are not typed and are used to receive responses to fetch requests for all topics that match the topic selectors used when registering the fetch stream.

Fetch the value of a topic or set of topics by using a topic selector to make a fetch request:

JavaScript 
session.fetch("topic_selector").on('value', function(value, path) {
    // Do something
});
Apple 
 [session.topics fetchWithTopicSelectorExpression:@"topic_selector" delegate:self];
Java and Android 
Topics topics = session.feature(Topics.class);

topics.fetch(topic_selector, new FetchStream.Default);
.NET 
var topics = session.Topics;

topics.Fetch( topic_selector, new FetchStreamDefault );
C 
FETCH_PARAMS_T params = {
       .selector = topic_selector,
       .on_topic_message = on_topic_message,
       .on_fetch = on_fetch,
       .on_status_message = on_fetch_status_message
};

/*
 * Issue the fetch request.
 */
fetch(session, params);

These examples pass in a default or no-op fetch stream.

To make use of the fetched values, implement a fetch stream to handle the values:

JavaScript 
session.fetch("topic_selector").on({
    value : function(value, path) {
                console.log("Value for topic '" + path + "' is: " + value);
            },
    error : function(error) { 
                console.log("Error on fetch: '" + error);
            },
    close : function() { 
                console.log("Fetch stream closed.");
            }
});
Apple 
@implementation FetchExample (PTDiffusionFetchStreamDelegate)

-(void)diffusionStream:(PTDiffusionStream * const)stream
     didFetchTopicPath:(NSString * const)topicPath
               content:(PTDiffusionContent * const)content {
    NSLog(@"Fetch Result: %@ = \"%@\"", topicPath, content);
}

-(void)diffusionDidCloseStream:(PTDiffusionStream * const)stream {
    NSLog(@"Fetch stream finished.");
}

-(void)diffusionStream:(PTDiffusionStream * const)stream didFailWithError:(NSError * const)error {
    NSLog(@"Fetch stream failed error: %@", error);
}

@end
Java and Android 
private final class myFetchStream implements Topics.FetchStream {

	@Override
	public void onClose() {
		LOG.info("Fetch stream closed.");
	}

	@Override
	public void onDiscard() {
		LOG.info("Fetch stream discarded.");
	}

	@Override
	public void onFetchReply(String topicPath, Content content) {
		LOG.info("Fetched value " + content.asString() +" from topic " + topicPath);
	}

}
.NET 
internal sealed class myFetchStream : IFetchStream {

    public void OnClose() {
        Console.WriteLine( "The fetch stream is now closed." );
    }

    public void OnDiscard() {
        Console.WriteLine( "The fetch stream was discarded." );
    }

    public void OnFetchReply( string topicPath, IContent content ) {
        Console.WriteLine( "Fetched value {0} from topic {1}", topicPath, content.AsString() );
    }
}
C 
/*
 * When a fetched message is received, this callback in invoked.
 */
static int
on_topic_message(SESSION_T *session, const TOPIC_MESSAGE_T *msg)
{
        printf("Received message for topic %s\n", msg->name);
        printf("Payload: %.*s\n", (int)msg->payload->len, msg->payload->data);

#ifdef DEBUG
                topic_message_debug(response->payload);
#endif

        return HANDLER_SUCCESS;
}

This page last modified: 2015/02/02