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CSS

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HTML 

              
                <html>
<body>
  <style>
    example-canvas-component {
      width: 100px;
      height: 100px;
      border: 1px solid black;
    }
  </style>
  <example-canvas-component></example-canvas-component>
  <script type="text/javascript">
    // This is an 'object' that is capable of updating it's internal state based upon
    // provided environmental conditions and is also capable of drawing itself.
    // Notice this is not a web component but rather just a plain old ES6 class.
    class ExampleLine {
      constructor({context, contextWidth, contextHeight, color = 'rgb(0,0,0)', thickness = 2, speed = 2, sensitivity = 0.1, minOpacity = 0.2}) {
        this.context = context;
        this.contextWidth = contextWidth;
        this.contextHeight = contextHeight;
        this.color = color;
        this.thickness = thickness;
        this.speed = speed;
        this.sensitivity = sensitivity;
        this.minOpacity = minOpacity;
        this.alpha = 1;

        // Initialize the internal state of the object.
        // Positions should be stored as scaled values. 0 indicates the minimum width or height
        // and 1 indicates the maximum width or height.
        this.targets = [
          { p1: { x: 0, y: 0 },
            p2: { x: 1, y: 1 } },
          { p1: { x: 0, y: 1 },
            p2: { x: 1, y: 0 } }
        ];

        this.targetIndex = 0;
        this.p1 = this.targets[0].p1;
        this.p2 = this.targets[0].p2;
      }

      get previousTarget() {
        return this.targetIndex === 0 ? this.targets[this.targets.length - 1] : this.targets[this.targetIndex - 1];
      }

      get target() {
        return this.targets[this.targetIndex];
      }

      // Only convert from scaled positions to absolute positions when they are needed to draw.
      // Internal state should be scaled positions so that the client can resize the component.
      get pos() {
        return {
          p1: { x: this.contextWidth  * this.p1.x,
                y: this.contextHeight * this.p1.y },
          p2: { x: this.contextWidth  * this.p2.x,
                y: this.contextHeight * this.p2.y }
        }
      }

      updateTarget() {
        if (this.targetIndex === this.targets.length - 1) {
          this.targetIndex = 0;
        } else {
          this.targetIndex++;
        }
      }

      hasReachedTarget() {
        // Move algorithms into utility methods.
        return distanceBetween(this.p1, this.target.p1) < this.sensitivity &&
               distanceBetween(this.p2, this.target.p2) < this.sensitivity;
      }

      // Update the internal state of this object
      update(mousePosition = { x: 0, y: 0 }) {
        if (this.hasReachedTarget()) this.updateTarget();
        
        // Update internal state based upon environment information
        this.alpha =  ((mousePosition.x / this.contextWidth) * (1 - this.minOpacity)) + this.minOpacity;

        // Move algorithms into utility methods.
        this.p1 = easePoint(this.previousTarget.p1, this.p1, this.target.p1, this.speed);
        this.p2 = easePoint(this.previousTarget.p2, this.p2, this.target.p2, this.speed);
      }

      // Drawing or updating objects might require environment information. In thise case
      // we need a mouse position in order to draw the line.
      draw() {
        this.context.beginPath();
        this.context.lineWidth = this.thickness;
        this.context.strokeStyle = this.color;
        this.context.globalAlpha = this.alpha;
        this.context.lineTo(this.pos.p1.x, this.pos.p1.y);
        this.context.lineTo(this.pos.p2.x, this.pos.p2.y);
        this.context.stroke();
      }
    }

    // A web component that creates a canvas and acts as the 'stage' in which various
    // 'objects' can update draw themselves.
    class ExampleCanvasComponent extends HTMLElement {
      constructor() {
        // Here we simply create a canvas and apply default styling. We will manually
        // Update the size of the canvas to fill the element through JavaScript.
        // By doing this we will allow the client code to determine the size of the component
        // And we will do all of our calculations as scaled values
        super();
        this.shadow = this.attachShadow({mode: 'open'});
        this.shadow.innerHTML = `
          <style media="screen">
            :host {
              cursor: pointer;
              display: block;
            }
          </style>
          <canvas></canvas>
        `;
      }

      connectedCallback() {
        this.canvas = this.shadow.querySelector('canvas');
        this.context = this.canvas.getContext('2d');
        this.mousePosition = {x: 0, y: 0};

        // Update canvas to fill the space of the component.
        this.canvas.width = this.clientWidth;
        this.canvas.height = this.clientHeight;
        this.objects = []
        this.hovering = false;

        // Make sensible default values.
        // These defaults also modify the client provided values so that providing "1" acts as the anchor value.
        // Values above 1 will be more than the default and values below 1 will be less than the default.
        this._defaultSpeed = 0.1;
        this._defaultLineThickness = 2;

        // Provide configuration options through attributes on the element.
        this.lineThickness = (parseFloat(this.getAttribute('line-thickness')) * this._defaultLineThickness) || this._defaultLineThickness;
        this.speed = (parseFloat(this.getAttribute('speed')) * this._defaultSpeed) || this._defaultSpeed;

        // Set the scene by creating objects
        this.objects.push(new ExampleLine({
          context: this.context,
          contextWidth: this.canvas.width,
          contextHeight: this.canvas.height,
          thickness: this.lineThickness,
          speed: this.speed
        }));

        // The component is resopobsible for maintaining environment information.
        this.canvas.addEventListener('mousemove', event => {
          let rect = this.canvas.getBoundingClientRect();
          this.mousePosition = {
            x: event.clientX - rect.left,
            y: event.clientY - rect.top
          };
        }, false);
        this.canvas.addEventListener("mouseenter", event => this.hovering = true);
        this.canvas.addEventListener("mouseleave", event => this.hovering = false);

        // Run the animation loop.
        let animate = () => {
          requestAnimationFrame(animate);
          // Remember to only perform updates if they are needed.
          // In this case we only want to perform the animation if the mouse is hovering.
          if (this.hovering) {
            this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
            this.doAnimate();
          }
        }
        animate();
      }

      doAnimate() {
        this.update();
        this.draw();
      }

      // Update logic for each object in the canvas should be abstracted away by
      // a different class. Current environment information must be passed in.
      update() {
        this.objects.forEach(object => object.update(this.mousePosition));
      }

      // Drawing logic for each object in the canvas should be abstracted away by
      // a different class. Current environment information must be passed in.
      draw() {
        this.objects.forEach(object => object.draw());
      }
    }

    customElements.define('example-canvas-component', ExampleCanvasComponent);

    /** @function distanceBetween
     *  @param p1 An object with an x and a y value.
     *  @param p2 An object with an x and a y value.
     *  The distance between two points. p1 and p2 should each have an x and y property.
     */
    function distanceBetween(p1, p2) {
      return Math.sqrt(Math.pow(p1.x - p2.x, 2) + Math.pow(p1.y - p2.y, 2));
    }

    /** @function easePoint
     *  @param p0 The original starting point. An object with an x and a y value.
     *  @param p1 The current point. An object with an x and a y value.
     *  @param p2 The destination. An object with an x and a y value.
     *  @param speedScale A value to scale the movement by.
     *  @returns a new point moved from p1 towards p2 with an easing function applied
     *  based upon the original starting point p0. The speed of the movement is scalled by speedScale.
     */
    function easePoint(p0, p1, p2, speedScale) {
      var xDiff = p2.x - p1.x;
      var yDiff = p2.y - p1.y;
      var xTotal = Math.abs(p2.x - p0.x);
      var yTotal = Math.abs(p2.y - p0.y);

      var newX;
      if (xTotal > 0) {
        var xSpeedAdjust = Math.pow(Math.abs(Math.sin((Math.abs(xDiff) / xTotal) * Math.PI)), 1/3);
        var xMove = xDiff * speedScale * xSpeedAdjust;
        newX = p1.x + xMove;
      } else {
        newX = p2.x;
      }

      var newY;
      if (yTotal > 0) {
        var ySpeedAdjust = Math.pow(Math.abs(Math.sin((Math.abs(yDiff) / yTotal) * Math.PI)), 1/3);
        var yMove = yDiff * speedScale * ySpeedAdjust;
        newY = p1.y + yMove;
      } else {
        newY = p2.y;
      }

      return {
        x: newX,
        y: newY
      };
    }
  </script>
</body>
</html>
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