When a connection is made between two YARP ports, that connection lasts until it is explicitly disconnected by user action, or until one of the ports is closed.
It is also possible to specify connections that should be made whenever possible. These are called "persistent connections".
Normally, to connect two ports from the command line, you do:
yarp connect <source-port> <target-port> [optional-carrier]
yarp connect /src /dest mcast
If both the /src and /dest ports exist, this will create a connection between them that will last until it is disconnected by a matching call to "yarp disconnect", or until one of the ports closes.
To create a persistent connection, you instead do:
yarp connect --persist <source-port> <target-port> [optional-carrier]
yarp --persist connect /src /dest mcast
This command will succeed even if /src or /dest do not exist at the moment. Whenever a port called /src or /dest is created, YARP will check if it is possible to create this connection, and do so if it can.
For example, suppose we do:
yarp connect --persist /write /read
And then in two terminals we do:
yarp write /write yarp read /read
The "yarp write" and "yarp read" programs will be connected automatically, and something typed on the "yarp write" console will appear on the "yarp read" console.
If either program is killed and restarted, the connection will reappear.
To view all persistent connections, type:
yarp connect --persist
To view all persistent connections involving a particular port, type:
yarp connect --persist /port
To stop the automatic creation of connections between two ports /src and /dest, do:
yarp disconnect --persist /src /dest
It is occasionally useful to view a collection of ports as a single "topic" to read from or write to. A topic can be created in YARP by doing:
yarp topic <topic-name>
yarp topic /topic
A topic can be thought of as a repeater port; anything it receives from any of its inputs, it resends to all its outputs. This bus-like behavior is often useful, but a bit inefficient. So in fact YARP topics are entirely virtual, with their effect achieved via direct connections between the ports connected to the virtual topic port.
To be concrete, here's how topics work. Persistent connections can be made between ports and a topic:
yarp connect --persist /src1 /topic yarp connect --persist /src2 /topic yarp connect --persist /src3 /topic yarp connect --persist /topic /dest1 yarp connect --persist /topic /dest2 yarp connect --persist /topic /dest3
Every input to the topic will be connected to every output from the topic. For example, if all of the ports /src1, /src2, /dest1, and /dest2 exist, then the following connections will be made and maintained:
yarp connect /src1 /dest1 yarp connect /src1 /dest2 yarp connect /src1 /dest3 yarp connect /src2 /dest1 yarp connect /src2 /dest2 yarp connect /src2 /dest3 yarp connect /src3 /dest1 yarp connect /src3 /dest2 yarp connect /src3 /dest3
(if carriers are specified in the persistent connections, the carriers for outgoing connections from the topic are respected and those for incoming connections to the topic are ignored).
To get a list of all topics, do:
yarp topic --list
To remove a topic, do:
yarp topic --remove /topic
Normally, to connect two ports directly in code, you do:
It is also possible to request connections via topics. If, in one program, we do:
and in another, we do:
Then the /camera1 port will end up connecting to the /viewer port whenever both are available.
The "topic://" prefix is not needed if the "/vision" topic has been explicitly created as described in Persistence via topics.
One note of caution: when connecting to a topic, the "connect" command may return before all existing ports connected with that topic becoming connected to each other. If you rely on a connection existing, test for it using either yarp::os::Network::isConnected or yarp::os::Port::getInputCount or yarp::os::Port::getOutputCount.
To connect two ports persistently, without any intermediate topic (only possible with native YARP name service), use code like this: