constants support in messages

[koonpeng]

is there a way to reference the constant value like in rclpy? Something like

const marker = {...
  type: ImageMarker.CIRCLE,
...}

I can see that constants are generated in the js messages like so

// Define constants (7 in total)
Object.defineProperty(ImageMarkerWrapper, "CIRCLE", {value: 0, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "LINE_STRIP", {value: 1, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "LINE_LIST", {value: 2, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "POLYGON", {value: 3, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "POINTS", {value: 4, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "ADD", {value: 0, writable: false, enumerable: true, configurable: true});
Object.defineProperty(ImageMarkerWrapper, "REMOVE", {value: 1, writable: false, enumerable: true, configurable: true});

But it needs to be imported like so

const ImageMarker = require('rclnodejs/generated/visualization_msgs/visualization_msgs__msg__ImageMarker');

which doesn't seem like the intended usage.

Also, the typescript interfaces are generated like this

      export type ImageMarker = {
        CIRCLE: 0,
        LINE_STRIP: 1,
        LINE_LIST: 2,
        POLYGON: 3,
        POINTS: 4,
        ADD: 0,
        REMOVE: 1,
        header: std_msgs.msg.Header,
        ns: string,
        id: number,
        type: number,
        action: number,
        position: geometry_msgs.msg.Point,
        scale: number,
        outline_color: std_msgs.msg.ColorRGBA,
        filled: undefined,
        fill_color: std_msgs.msg.ColorRGBA,
        lifetime: builtin_interfaces.msg.Duration,
        points: geometry_msgs.msg.Point[],
        outline_colors: std_msgs.msg.ColorRGBA[]
      };

The constants have to be defined in objects that implements the type.

const marler: ImageMarker = {
  // All the constants have to be defined
  CIRCLE: 0,
  LINE_STRIP: 1,
...
};

Is there an easier way to reference the constants in rclnodejs?

[minggangw]

Thanks for your question, I checked the usage of const variable defined in .msg files, and I think the code probably looks like:

const ImageMarker = rclnodejs.require('visualization_msgs/msg/ImageMarker');
const LINE_LIST = ImageMarker.LINE_LIST;  // you get 2 here.

Not sure it's what you want or you could suggest a better usage, thanks!

[minggangw]

Maybe I should introduce a little bit of the message objects used in rclnodejs to make it clear:

• You can use so called plain objects to pass a message to publisher/client, e.g.

 const node = rclnodejs.createNode('publisher_message_example_node');
 const publisher = node.createPublisher('sensor_msgs/msg/JointState', 'JointState');
 publisher.publish({
      header: {
        stamp: {
          sec: 123456,
          nanosec: 789,
        },
        frame_id: 'main frame',
      },
      name: ['Tom', 'Jerry'],
      position: [1, 2],
      velocity: [2, 3],
      effort: [4, 5, 6],
    });

see https://github.com/RobotWebTools/rclnodejs/blob/develop/example/publisher-message-example.js

• You can also require a JS object which is defined by the generated classes (usually under /generated), e.g.

let JointState = rclnodejs.require('sensor_msgs/msg/JointState');  // rclnodejs.require('sensor_msgs').msg.JointState;
let state = new JointState();
state.position = [1, 2];
state.velocity = [2, 3];
state.effort = [4, 5, 6];

Currently, most the usage is shown by the first way, because it's easier and more obvious and you can use console.log(state) to get the detailed values of it directly.

[koonpeng]

Thanks! I wasn't aware of the require function. For the typescript types, would it be good to make the constants optionals? So there is no need to define them in an object, I am also looking at the possibility of generating the require type declaration so it returns a strongly typed interface instead of a generic object.

[minggangw]

@wayneparrott added the TS support recently, I know he is busy implementing another important feature of rclnodejs, so Wayne would you please have a look if you have time? or @koonpeng you could help to submit a PR based on your requirement, thanks a lot!

[koonpeng]

I have been using a tool I made to generate typescript interfaces, now that rclnodejs has support for typescript, it would be better to use the native support. I will try to see if I can make a PR for this, thanks!

[wayneparrott]

@koonpeng thanks for identifying my goof up on the type-alias msg constants. Definitely not what I envisioned. After reviewing, I'm not sure what I was thinking at the time I impl'ed this. I was experimented with a couple of approaches. Please share suggestions/PR for improvement. In the meantime I plan to carve out some time to look into resolving this asap as well.

[wayneparrott]

A quick thought is to represent the IDL constants for a type_class/msg in interfaces.d.ts as an object literal similar to the snippet below. In this case a dev would need to know that in addition to importing ImageMarker he would also need to import ImageMarkerConstants.

export const ImageMarkerConstants = {
        CIRCLE: 0,
        LINE_STRIP: 1,
        LINE_LIST: 2,
        POLYGON: 3,
        POINTS: 4,
        ADD: 0,
        REMOVE: 1
}
export type ImageMarker = {
        header: std_msgs.msg.Header,
        ns: string,
        id: number,
        type: number,
        action: number,
...

Thoughts?

[koonpeng]

normally this wouldnt work because ImageMarkerConstants doesn't exist in the javascript code. But I just found out there is a special case in 'const enum' where typescript will inline the value in the generated js code. I will see if I can add it to my PR.

[koonpeng]

#551 This PR improve typing information and generate constants as const enum