How to implement ruler?

v6
1

Hi. Is there a possibility to add ruler (vertical line) in v6? In v5 it’s possible. Didn’t find anything similar in the documentation.

2

That’s not part of the library for 6.x, but the way you’d implement it would be something like this:

  • Create a view plugin that adds an absolutely positioned element to view.scrollDOM. Give it some kind of left border so that it’s visible

  • On init, and whenever the plugin gets an update that has the heightChanged flag set, set the element’s left to X * view.defaultCharacterWidth, and its height to view.contentHeight.

3

Would it be possible to give a few more details?

I assume such a plugin would be passed to the extensions list when initializing CodeMirror. Is there a function that should be used to create the plugin? Also, could you point me to the documentation about how to check the events that trigger a plugin? Thanks!

4

To help those interested in this feature, here is my basic implementation in TypeScript:

import { EditorView, ViewPlugin, ViewUpdate } from "@codemirror/view";

function generateRulerPlugin() {
    let width: number = 0;
    let dom: HTMLDivElement;
    let defaultCharacterWidth: number = 1;

    class RulerPlugin {
        containerDom: HTMLDivElement;
        constructor(view: EditorView) {
            this.containerDom = view.dom.appendChild(document.createElement("div"));
            this.containerDom.style.cssText=`
                position: absolute;
                left: 0;
                top: 0;
                width: 100%;
                height: 100%;
                pointer-events: none;
                overflow: hidden;
            `
            dom = this.containerDom.appendChild(document.createElement("div"));
            dom.style.cssText = `
                position: absolute; 
                border-right: 1px dotted gray; 
                height: 100%; 
                opacity: 0.7;
            `;
            // XXX: This should be equal to the amount of padding on a line.
            // This value should be extracted from CodeMirror rather than hardcoded.
            dom.style.width = "4px";
            defaultCharacterWidth = view.defaultCharacterWidth;
            updateRulerWidth(80);
        }

        update(update: ViewUpdate) {
            defaultCharacterWidth = update.view.defaultCharacterWidth;
            if (update.viewportChanged) {
                updateRulerWidth(width, true);
            }
        }

        destroy() {
            dom.remove();
        }
    }
    
    function updateRulerWidth(newWidth: number, force = false) {
        if ((newWidth !== width || force) && dom) {
            width = newWidth;
            dom.style.left = `${width * defaultCharacterWidth}px`;
        }
    }

    return { rulerPlugin: ViewPlugin.fromClass(RulerPlugin), updateRulerWidth };
}

const { rulerPlugin, updateRulerWidth } = generateRulerPlugin();

Add rulerPlugin to the list of plugins and then call updateRulerWidth when needed to change the ruler width. The only issue I see is that I don’t know how to get the line padding from CodeMirror, so I have hardcoded 4px, which appears to be the default line padding… @marijn is there a way to extract the line padding so that we can compute the exact pixel location of a character?

5

Not really—the editor doesn’t track horizontal padding. But you should be able to read it from the DOM.

6

Will there always be an element with the correct amount of padding (even if there are 0 lines of code displayed)? Any suggestions for a reliable query-selector?

7

It is possible to replace the entire document with a block widget, but that would be very exceptional, and I guess falling back to a default value in that case won’t cause any trouble. Lines have a cm-line class and are children of view.contentDOM.

8

Thanks for sharing, this allows us to add the equivalent feature in JupyterLab 4; see

It supports multiple rulers.