Creating Images on the Server

It may be useful for many applications to create images of Diagrams with GoJS, and this page details some of the options for such a task.

Puppeteer

Puppeteer is a Node library which provides a high-level API to control headless Chrome. We can use it to create images server-side. If you have Node and npm installed you can install it with npm install puppeteer.

The following code is a small example using Puppeteer. If you saved the JavaScript as puppet.js and run it with node (node createImage.js) it demonstrate creating two images: One from the Diagram called gojs-screenshot.png and one of the HTML page called page-screenshot.png. The Diagram code in the sample is the same as that in the Minimal sample.

// This example loads the GoJS library then adds HTML from scratch and evaluates some JavaScript,
// then creates a screenshot of Diagram with makeImageData, plus a screenshot of the page.

const puppeteer = require('puppeteer');
const fs = require('fs');

const parseDataUrl = (dataUrl) => {
  const matches = dataUrl.match(/^data:(.+);base64,(.+)$/);
  if (matches.length !== 3) {
    throw new Error('Could not parse data URL.');
  }
  return { mime: matches[1], buffer: Buffer.from(matches[2], 'base64') };
};

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();

  // Point to a version of go.js, either a local file or one on the web at a CDN
  await page.addScriptTag({
    url: 'https://cdn.jsdelivr.net/npm/gojs'
    // path: '../../release/go.js'
  });

  // Create HTML for the page:
  page.setContent('<div id="myDiagramDiv" style="border: solid 1px black; width:400px; height:400px"></div>');

  // Set up a Diagram, and return the result of makeImageData:
  const imageData = await page.evaluate(() => {
    const myDiagram = new go.Diagram("myDiagramDiv",
      {
        "animationManager.isEnabled": false,
        "undoManager.isEnabled": true  // enable undo & redo
      });

    // define a simple Node template
    myDiagram.nodeTemplate =
      new go.Node("Auto")
        .add(  // the Shape will go around the TextBlock
          new go.Shape("RoundedRectangle", { strokeWidth: 0 })
            .bind("fill", "color"),
          new go.TextBlock({ margin: 8 })
            .bind("text", "key")
        );

    myDiagram.model = new go.GraphLinksModel(
      [
        { key: "Alpha", color: "lightblue" },
        { key: "Beta", color: "orange" },
        { key: "Gamma", color: "lightgreen" },
        { key: "Delta", color: "pink" }
      ],
      [
        { from: "Alpha", to: "Beta" },
        { from: "Alpha", to: "Gamma" },
        { from: "Beta", to: "Beta" },
        { from: "Gamma", to: "Delta" },
        { from: "Delta", to: "Alpha" }
      ]);

    return myDiagram.makeImageData();
  });

  // Output the GoJS makeImageData as a .png:
  const { buffer } = parseDataUrl(imageData);
  fs.writeFileSync('diagram-screenshot.png', buffer, 'base64');

  // Output a page screenshot
  await page.screenshot({ path: 'page-screenshot.png' });
  await browser.close();
})();

You can also use Puppeteer to fetch live HTML pages and do the same operations:

// This example loads a web page with a GoJS diagram,
// then creates a screenshot of the Diagram with makeImageData, plus a screenshot of the page.

const puppeteer = require('puppeteer');
const fs = require('fs');

const parseDataUrl = (dataUrl) => {
  const matches = dataUrl.match(/^data:(.+);base64,(.+)$/);
  if (matches.length !== 3) {
    throw new Error('Could not parse data URL.');
  }
  return { mime: matches[1], buffer: Buffer.from(matches[2], 'base64') };
};

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  // This does not have to be a page on the web, it can be a localhost page, or file://
  await page.goto('https://gojs.net/samples/orgChartEditor.html', {
    waitUntil: 'networkidle2' // ensures images are loaded
  });


  const imageData = await page.evaluate(() => {
    window.myDiagram.animationManager.stopAnimation();
    return window.myDiagram.makeImageData({
      background: window.myDiagram.div.style.backgroundColor
    });
  });

  // Output the GoJS makeImageData as a .png:
  const { buffer } = parseDataUrl(imageData);
  fs.writeFileSync('diagram-screenshot.png', buffer, 'base64');

  // Output a page screenshot
  await page.screenshot({ path: 'page-screenshot.png' });
  await browser.close();
})();