Lesson 2. Simple publisher (EN)

Simple publisher

This example makes only one Publisher object that provide ability to publish message to ROS topic.

Solidity contract

See the example contract:

import 'ROS';

Importing the basic ROS integration contract library. ROS library defines some structures as Publisher, Subscriber, Message and etc.

contract StdString is Message {
    string public data;
    function StdString(string _data) {
        data = _data;
    }
}

Message definition contract contains only one member data with simple type string. ROS messages will be converted to Message contract and submitted by AIRA ROS Bridge, or Message contracts will be converted to ROS message and published to associated topics.

contract SimplePub is ROSCompatible {

The next is a main contract definition, this contract should be child of ROSCompatible contract. The ROSCompatible contract contrains service members that used by AIRA ROS Bridge.

    Publisher      myPub;
    function SampleContract() {
        // Create a new publisher by topic name and type
        myPub = mkPublisher('/hello', 'std_msgs/String');
    }

In this code contract constructor creates Publisher by ROSCompatible public method mkPublisher and arguments topic name and message type. Publisher instance saved in private member for the future.

    function echo(string str) {
        // Publish new message by received string
        myPub.publish(new StdString(str));
    }
}

The last method accepts string argument, packs this into StdString message container, and call publish method of Publisher instance that publish ROS message to associated topic.

ROS interaction

The first, create Web3 deploy instructions for the example contract. And put into the geth console window. After some time you see:

Contract mined! address: 0xc52db6edc294ecf434a322b93e09d87d1aae80ae transactionHash: 0x4882579adef61f44fcef5847e93a5c998c81c30c9f7345f5c48e0f613a6e4bac

With contract address 0xc52db6edc294ecf434a322b93e09d87d1aae80ae you can start AIRA ROS Bridge now, full start instructions is there:

$ node start.js 0xc52db6edc294ecf434a322b93e09d87d1aae80ae
util.debug: Use console.error instead
DEBUG: ROSLib uses utf8 encoding by default.It would be more efficent to use ascii (if possible)
Contract: 0xc52db6edc294ecf434a322b93e09d87d1aae80ae
Publishers:
/hello :: std_msgs/String
Subscribers:

Described on the top contract have a simple ROS interface:

Publishers:

• /hello :: std_msgs/String

Using rostopic tools try to subscribe to /hello topic:

$ rostopic echo /hello

This tool should print received strings to screen. Try to publish string by sendTransaction method in geth console:

> var contract = eth.contract(simple_publisher_abi).at("0xc52db6edc294ecf434a322b93e09d87d1aae80ae")
> contract.echo.sendTransaction("Hello world!", {from: eth.accounts[0], gas: 500000})

After some time rostopic console show:

WARNING: topic [/hello] does not appear to be published yet
data: Hello world!
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