CaveDraw源码06：油漆桶算法的实现

 // 这里包含了一种 newColor==oldColor 的特殊情况。
_floodFillWithQueue(imageData, queue, newColor, oldColor) {
    let color, pixel;
    while ((pixel = queue.pop())) {
        color = this._rgbFromImageDataWithPixel(imageData, pixel.x, pixel.y);
        if (this._compareColor(color, oldColor) && !this._compareColor(color, newColor)) {
            this._setColor4ImageDataWithPixel(imageData, pixel.x, pixel.y, newColor);
            if (pixel.x > 0)
                queue.push({
                    x: pixel.x - 1,
                    y: pixel.y
                });
            if (pixel.x < imageData.width - 1)
                queue.push({
                    x: pixel.x + 1,
                    y: pixel.y
                });
            if (pixel.y > 0)
                queue.push({
                    x: pixel.x,
                    y: pixel.y - 1
                });
            if (pixel.y < imageData.height - 1)
                queue.push({
                    x: pixel.x,
                    y: pixel.y + 1
                });
        }
    }
}

// 由于 window.devicePixelRatio 的存在，所以要转化为 canvasEle 上实际的点，即对应imageData中的点
const startPixel = {
  x: parseInt(this.board.startPoint.x),
  y: parseInt(this.board.startPoint.y)
};

_2rgb(color) {
  this.offscreenContext.fillStyle = color;
  this.offscreenContext.fillRect(0, 0, this.offscreenCanvas.width, this.offscreenCanvas.height);
  const imgDataOffscreenCanvas = this.offscreenContext.getImageData(0, 0, this.offscreenCanvas.width, this.offscreenCanvas.height);
  return this._rgbFromImageData(imgDataOffscreenCanvas, 0);
}