experimental/canvaskit/htmlcanvas/radialgradient.js

// Note, Skia has a different notion of a "radial" gradient.
// Skia has a twoPointConical gradient that is the same as the
// canvas's RadialGradient.

function RadialCanvasGradient(x1, y1, r1, x2, y2, r2) {
  this._shader = null;
  this._colors = [];
  this._pos = [];

  this.addColorStop = function(offset, color) {
    if (offset < 0 || offset > 1 || !isFinite(offset)) {
      throw 'offset must be between 0 and 1 inclusively';
    }

    color = parseColor(color);
    // From the spec: If multiple stops are added at the same offset on a
    // gradient, then they must be placed in the order added, with the first
    // one closest to the start of the gradient, and each subsequent one
    // infinitesimally further along towards the end point (in effect
    // causing all but the first and last stop added at each point to be
    // ignored).
    // To implement that, if an offset is already in the list,
    // we just overwrite its color (since the user can't remove Color stops
    // after the fact).
    var idx = this._pos.indexOf(offset);
    if (idx !== -1) {
      this._colors[idx] = color;
    } else {
      // insert it in sorted order
      for (idx = 0; idx < this._pos.length; idx++) {
        if (this._pos[idx] > offset) {
          break;
        }
      }
      this._pos   .splice(idx, 0, offset);
      this._colors.splice(idx, 0, color);
    }
  }

  this._copy = function() {
    var rcg = new RadialCanvasGradient(x1, y1, r1, x2, y2, r2);
    rcg._colors = this._colors.slice();
    rcg._pos    = this._pos.slice();
    return rcg;
  }

  this._dispose = function() {
    if (this._shader) {
      this._shader.delete();
      this._shader = null;
    }
  }

  this._getShader = function(currentTransform) {
    // From the spec: "The points in the linear gradient must be transformed
    // as described by the current transformation matrix when rendering."
    var pts = [x1, y1, x2, y2];
    CanvasKit.SkMatrix.mapPoints(currentTransform, pts);
    var sx1 = pts[0];
    var sy1 = pts[1];
    var sx2 = pts[2];
    var sy2 = pts[3];

    var sx = currentTransform[0];
    var sy = currentTransform[4];
    var scaleFactor = (Math.abs(sx) + Math.abs(sy))/2;

    var sr1 = r1 * scaleFactor;
    var sr2 = r2 * scaleFactor;

    this._dispose();
    this._shader = CanvasKit.MakeTwoPointConicalGradientShader(
        [sx1, sy1], sr1, [sx2, sy2], sr2, this._colors, this._pos,
        CanvasKit.TileMode.Clamp);
    return this._shader;
  }
}