Timer

Source code.

This tutorial does not use colcon to build. We use only cargo, which is a Rust's standard build system.

Don't forget loading ROS2's environment as follows. If you already done so, you do not need this.

$ . /opt/ros/iron/setup.bash

Wall-timer

A wall-timer is a timer which periodically invoked. This section describes how to use a wall-timer.

Let's create a package by using a cargo as follows.

$ cargo new wall_timer
$ cd wall_timer

Then, add safe_drive to the dependencies of Cargo.toml.

[dependencies]
safe_drive = "0.3"

The following code is an example using a wall-timer. The important method is Selector::add_wall_timer() which takes a name, a duration, and a callback function.

use safe_drive::{
    context::Context, error::DynError, logger::Logger, msg::common_interfaces::std_msgs, pr_info,
};
use std::{rc::Rc, time::Duration};

fn main() -> Result<(), DynError> {
    // Create a context, a node, a subscriber, a publisher, and a selector.
    let ctx = Context::new()?;
    let node = ctx.create_node("my_node", None, Default::default())?;
    let subscriber = node.create_subscriber::<std_msgs::msg::UInt64>("my_topic", None)?;
    let publisher = node.create_publisher::<std_msgs::msg::UInt64>("my_topic", None)?;
    let mut selector = ctx.create_selector()?;

    // Create a logger.
    // To share this by multiple callback functions, use Rc.
    let logger = Rc::new(Logger::new("wall timer example"));

    // Add a wall timer to publish periodically.
    let mut cnt = Box::new(0);
    let mut msg = std_msgs::msg::UInt64::new().unwrap();
    let logger1 = logger.clone();

    selector.add_wall_timer(
        "publisher", // the name of timer
        Duration::from_secs(1),
        Box::new(move || {
            msg.data = *cnt;
            *cnt += 1;
            publisher.send(&msg).unwrap();
            pr_info!(logger1, "send: msg.data = {}", msg.data);
        }),
    );

    // Add a subscriber.
    selector.add_subscriber(
        subscriber,
        Box::new(move |msg| {
            pr_info!(logger, "received: msg.data = {}", msg.data);
        }),
    );

    // Spin.
    loop {
        selector.wait()?;
    }
}

Timers can be set by a method of selector as follows, and the timers will be invoked when calling the Selector::wait() methods.

#![allow(unused)]
fn main() {
selector.add_wall_timer(
    "publisher", // the name of timer
    Duration::from_secs(1),
    Box::new(move || {
        msg.data = *cnt;
        *cnt += 1;
        publisher.send(&msg).unwrap();
        pr_info!(logger1, "send: msg.data = {}", msg.data);
    }),
);
}

• "publisher" is the name of this timer. The name is used for statistics. You can use any name.
• Duration::from_secs(1) is the duration for periodic invoking. This argument means the callback function is invoked every 1 second.
• Box::new(move || ...) is the callback function.

There is a publisher invoked by a timer, and a subscriber in this code. When executing this, transmission and reception will be confirmed as follows.

$ cargo run
[INFO] [1656557242.842509800] [wall timer example]: send: msg.data = 0
[INFO] [1656557242.842953300] [wall timer example]: received: msg.data = 0
[INFO] [1656557243.843103800] [wall timer example]: send: msg.data = 1
[INFO] [1656557243.843272900] [wall timer example]: received: msg.data = 1
[INFO] [1656557244.843574600] [wall timer example]: send: msg.data = 2
[INFO] [1656557244.844021200] [wall timer example]: received: msg.data = 2
[INFO] [1656557245.844349800] [wall timer example]: send: msg.data = 3
[INFO] [1656557245.844702900] [wall timer example]: received: msg.data = 3

One-shot Timer

A wall-timer is invoked periodically, but one-shot timer is invoked only once. A one-shot can be set by the Selector::add_timer() method as follows.

use safe_drive::{context::Context, error::DynError, logger::Logger, pr_info};
use std::{cell::RefCell, collections::VecDeque, rc::Rc, time::Duration};

pub fn main() -> Result<(), DynError> {
    // Create a context, a publisher, and a logger.
    let ctx = Context::new()?;
    let mut selector = ctx.create_selector()?;
    let logger = Rc::new(Logger::new("one-shot timer example"));

    let queue = Rc::new(RefCell::new(VecDeque::new()));

    // Add a one-shot timer.
    let queue1 = queue.clone();
    selector.add_timer(
        Duration::from_secs(2),
        Box::new(move || {
            pr_info!(logger, "fired!");

            // Insert a timer to the queue.
            let mut q = queue1.borrow_mut();
            let logger1 = logger.clone();
            q.push_back((
                Duration::from_secs(2),
                (Box::new(move || pr_info!(logger1, "fired! again!"))),
            ));
        }),
    );

    // Spin.
    loop {
        {
            // Set timers.
            let mut q = queue.borrow_mut();
            while let Some((dur, f)) = q.pop_front() {
                selector.add_timer(dur, f);
            }
        }

        selector.wait()?;
    }
}

Selector::add_timer() does not take the name, but other arguments are same as Selector::add_wall_timer().

#![allow(unused)]
fn main() {
selector.add_timer(
    Duration::from_secs(2),
    Box::new(move || ...),
);
}

• Duration::from_secs(2) is a duration indicating when the timer will be invoked.
• Box::new(move || ...) is the callback function.

This code reenables a timer in the callback function. To reenable, the callback takes a queue and timers in the queue is reenabled in the spin as follows.

#![allow(unused)]
fn main() {
// Spin.
loop {
    {
        // Set timers.
        let mut q = queue.borrow_mut();
        while let Some((dur, f)) = q.pop_front() {
            selector.add_timer(dur, f);
        }
    }

    selector.wait()?;
}
}

The important thing is that the borrowed resources must be released. To release definitely, the code fraction borrowing the queue is surrounded by braces.

The following is a execution result of this code.

$ cargo run
[INFO] [1657070943.324438900] [one-shot timer example]: fired!
[INFO] [1657070945.324675600] [one-shot timer example]: fired! again!