Thrift offers a convenient way to define structures and services. Structures are bundles of data, and services are bundles of methods. Structures fit nicely into streaming communication, while services are intended for RPC communication. But sometimes we want to offer a set of services around a particular structure. One recurring desire is a way to manipulate or read just part of a structure. This is convenient if that structure represents configuration data for a program.
With YARP, every Thrift structure has a corresponding
Editor service that can be used to manipulate or read parts of it. For example, suppose we have the following structure in
When compiled, that will give is a
Settings class that can read or write from a port. That class will read the structure in full. For example, here's a test program that will read the structure from a port and show its value:
(If you want to quickly compile this program, just put it in a file called
<something>.cpp in an empty directory, then add
settings.thrift, then run
yarp cmake && cmake . && make && ./yarpy, specifying
-DYARP_DIR=<path_to_yarp> to cmake if needed.)
Once that is running, from another terminal we can write to the settings:
echo "1 sam (1 2 3)" | yarp write /write /settings
And we see the resulting change of the entire structure:
yarp: Port /settings active at tcp://192.168.1.2:10002 Settings 0 "" () Settings 0 "" () Settings 1 sam (1 2 3) ...
Now, suppose we want to offer a way to change parts of the structure individually. We can make the following two-line modification:
Now, we can send a new kind of message to the port:
echo 'set name "sam"' | yarp write /write /settings
(It is now also possible to use
yarp rpc /settings to send commands and get status feedback). Now we can see that just the part of the structure we care about will change:
yarp: Port /settings active at tcp://192.168.1.2:10002 Settings 0 "" () Settings 0 "" () Settings 0 sam () ...
The editor also lets us capture events generated just before and just after a field is changed. For example, if we want to do something just before or just after the
name field is changed, we could do this:
Now if we send a message to change the
name field, we'll see:
yarp: Port /settings active at tcp://192.168.1.2:10002 Settings 0 "" () Settings 0 "" () About to set the name, it is currently '' Just set the name, it is now 'sam' Settings 0 sam () ...