Building a Drawing Pad Widget using p5.js and an Iframe

1. Add a new JS Object

   Paste the following code into the new JS Object:

   export default {
    canvas: `
   <script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.4.0/p5.min.js"></script>
   <script>
    let windowWidth = 500;
    let windowHeight = 500;
    let canvas;
    let drawing = [];
    let currentPath = [];
    let isDrawing = false;
    let colorPicker;
    let clearButton;
    let undoButton;
    let currentColor = '#000000';
    function setup() {
      canvas = createCanvas(windowWidth, windowHeight);
      canvas.position(0, 40);
      canvas.mousePressed(startPath);
      canvas.mouseReleased(endPath);
      colorPicker = document.getElementById('color-picker');
      colorPicker.addEventListener('change', onColorChange);
      clearButton = document.getElementById('clear-button');
      clearButton.addEventListener('click', clearDrawing);
      undoButton = document.getElementById('undo-button');
      undoButton.addEventListener('click', undoDrawing);
    }
    function draw() {
      background(255);
      drawing.forEach(path => {
        stroke(path.color);
        strokeWeight(10);
        noFill();
        beginShape();
        path.forEach(({ x, y }) => vertex(x, y));
        endShape();
      });
      if (isDrawing) {
        stroke(currentColor);
        strokeWeight(10);
        noFill();
        currentPath.push({ x: mouseX, y: mouseY });
        beginShape();
        currentPath.forEach(({ x, y }) => vertex(x, y));
        endShape();
      }
    }
    function startPath() {
      isDrawing = true;
      currentPath = [];
      currentPath.color = currentColor;
      stroke(currentColor);
      drawing.push(currentPath);
    }
    function endPath() {
      currentPath.color = currentColor;
      isDrawing = false;
      autoSaveDrawing();
    }
    function onColorChange() {
      currentColor = colorPicker.value;
      currentColor = color(currentColor);
    }
    function clearDrawing() {
      drawing = [];
      autoSaveDrawing();
    }
    function undoDrawing() {
      drawing.pop();
      autoSaveDrawing();
    }
    function autoSaveDrawing() {
      const svg = \`<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 \${width} \${height}">
        <rect x="0" y="0" width="\${width}" height="\${height}" fill="white"/>
        \${drawing
          .map(
            path => \`<path d="\${path
              .map(({ x, y }) => (x === path[0].x && y === path[0].y ? \`M\${x} \${y}\` : \`L\${x} \${y}\`))
              .join(' ')}" stroke="\${path.color}" stroke-width="10" fill="none"/>\`
          )
          .join('')}
      </svg>\`;
      parent.postMessage({ svg }, '*');
    }
    // Initial canvas setup
    setup();
   </script>
   <style>
    #buttonbar {
      display: flex;
      justify-content: flex-end;
    }
    #color-picker,
    #undo-button,
    #clear-button {
      margin-left: 10px;
    }
   </style>
   <div id="buttonbar">
    <input type="color" id="color-picker" value="#000000" />
    <button id="undo-button">Undo</button>
    <button id="clear-button">Clear</button>
   </div>
   <canvas id="canvas"></canvas>
   `
   }
    

   NOTE: 

   • The windowWidth and windowHeight properties at the top of the script can be changed to set the canvas (and output SVG) size. 

2. Add a new Iframe widget to the canvas

   1. Add the new Iframe widget to your app
   2. Set the srcDoc property to: {{JSObject1.canvas}}

3. Accessing the SVG drawing for saving

   The script posts a message from the Iframe to the parent window (your Appsmith app), which contains an object with an svg property. Each new line drawn will post a new message, updating the drawing which can be accessed at Iframe1.message.svg.

   

4. Modifying the Code (optional)

   You may want to adjust the JSObject code to insert variables from the rest of your app, for values like the color, image size, etc. This code has been written as a template literal (string), making it easy to insert ${variables}. However, the code also contains some template literals nested in the autoSaveDrawing() function that had to be \escaped. The extra backslashes tell the JS interpreter to ignore the next character so that it continues to be read as text instead of code. 

   

Conclusion

Additional Resources

• Building a signature capture widget with an Iframe and SignaturePad.js
• Iframe Experiments: Extending Appsmith with custom iframe code
• p5js.org
• Docs: Iframe Widget