cavans实现清除圆形区域

touchmove: function(e) {
                if (this.complete || this.disabled) {
                    return;
                }
                // ctx.globalCompositeOperation = 'destination-out'; 
                ctx.moveTo(this.startX, this.startY);
                // ctx.beginPath();
                // ctx.arc(this.startX, this.startY, 20, 0, Math.PI * 20);
                // ctx.fill();
                let stepClear = 1; // 剩餘清除的小矩形數
                clearArc(this.startX, this.startY, this.touchSize/2);
                // 清除出一個圓形區域
                function clearArc(x, y, radius) {
                    var calcWidth = radius - stepClear;
                    var calcHeight = Math.sqrt(radius * radius - calcWidth * calcWidth);
                
                    var posX = x - calcWidth;
                    var posY = y - calcHeight;
                
                    var widthX = 2 * calcWidth;
                    var heightY = 2 * calcHeight;
                    if (stepClear <= radius) {
                        ctx.clearRect(posX, posY, widthX, heightY);
                        stepClear += 1;
                        clearArc(x, y, radius);
                    }
                }
                // ctx.clearRect(this.startX, this.startY, this.touchSize, this.touchSize);
                ctx.draw(true);
                //记录移动点位
                this.startX = e.touches[0].x;
                this.startY = e.touches[0].y;
            }