软件项目技术点(7)——在canvas上绘制自定义图形

AxeSlide软件项目梳理   canvas绘图系列知识点整理

图形种类

目前我们软件可以绘制出来的形状有如下这几种，作为开发者我们一直想支持用户可以拖拽的类似word里面图形库，但目前还没有找到比较合适的库。

下图中所有的图形都是我们自己写代码在canvas上绘制方法出来的，可以改变实心/空心，改变颜色，大小宽高，线条弯曲度，透明度等。

父类shape类

实现的代码如下：所有的图形均继承自shape类，而shape类继承自CommonElement，shape中的所有图形添加到画布上都是一个元素。

这个类里面有很多功能函数：

prepareStyle() { //绘制准备context绘制属性

setPro() { }//绘制前准备参数

public thiner() {//变细

public thicker() {//加粗

changeColor(color: string, callBack: Function) {//改变绘制颜色

changeAlpha(alpha: number, callBack: Function) {//改变透明度

changeFill(isFill: boolean) {//切换实心空心

drawPath() { }//绘制图形路径，所有图形都会重写该方法

draw() {//绘制调用入口

  export class Shape extends CommonElement {
         shapeParameter: ShapeParameter;
         tempFill: boolean;
         tempStroke: boolean;
         constructor(id: string, config: Config, context: CanvasRenderingContext2D, shapeParameter: ShapeParameter, typeName) {
             super(id, config, context, typeName);
             this.shapeParameter = shapeParameter;
         }
         prepareStyle() { //绘制准备context绘制属性
             if (this.shapeParameter.computeLineWidth && this.shapeParameter.isStroke) {
                 this.shapeParameter.lineWidth = Math.max(2, Math.min(this.config.width * editor.canvas.canvasImp.scale, this.config.height * editor.canvas.canvasImp.scale) / this.shapeParameter.lineWidthScale);

             }
             var ctx = this.context;
             ctx.fillStyle = Common.Util.makeRGBA(this.shapeParameter.fillStyle, this.shapeParameter.alpha);
             ctx.strokeStyle = Common.Util.makeRGBA(this.shapeParameter.strokeStyle, this.shapeParameter.alpha);
             ctx.lineWidth = this.shapeParameter.lineWidth / editor.canvas.canvasImp.scale;

         }
         public setPro() { }//绘制前准备参数
         public drawPath() { }//绘制图形路径，所有图形都会重写该方法
         public draw() {//绘制调用入口
             //保存context状态
             this.context.save();
             this.rotate();//将画布旋转到位
             this.setPro();
             this.prepareStyle();
             this.drawPath();
            //恢复context状态
             this.context.restore();
         }
         

所有图形类都有的一个属性ShapeParameter 具体内容如下，每个属性值都有各自的作用

   export class ShapeParameter {//图形属性参数
         lineWidth: number;
         isFill: boolean;
         isStroke: boolean
         fillStyle: string;
         strokeStyle: string
         lineWidthScale: number;
         isAutiClockwise: boolean;
         computeLineWidth: boolean;
         alpha: number;
         lineDash: any;
         constructor() {
             this.lineWidth = 1;
             this.isStroke = true;
             this.isFill = false;
             this.fillStyle = editor.currentShapeColor || editor.canvas.canvasImp.theme.shapeColor;
             this.strokeStyle = editor.currentShapeColor || editor.canvas.canvasImp.theme.shapeColor;
             this.lineWidthScale = lineWidthScale;
             this.isAutiClockwise = false;
             this.computeLineWidth = true;
             this.alpha = editor.currentShapAlpha || 1;
             this.lineDash = [10, 15];
         }
     }

特殊图形绘制计算方法

下面介绍几种绘制各种图形用到的特殊计算方法

如下图中的一个线条，我们可以拖拽中间控制点形成一个弧，这个弧形是某个圆的一部分。

我们定义左侧的第一个点为startPoint，中间的点为controlPoint，右侧的点为endPoint。

我们在绘制出这个图形时，需要知道如何求这三个点确定的圆的圆心坐标怎么计算，还有通过两点计算（x1,y1）为圆心,圆上某点（x2,y2）的角度的计算方法

         GetCenter(): Point {//获取三个点，确定的一个圆的中心点坐标
             var p1 = this.startPoint;
             var p2 = this.controlPoint;
             var p3 = this.endPoint;
             var C1 = new Point((p1.x + p2.x) / 2.0, (p1.y + p2.y) / 2.0);
             var C2 = new Point((p2.x + p3.x) / 2.0, (p2.y + p3.y) / 2.0);
             var I = new Point(0, 0);;
             var k1: number, k2: number;
             if ((p2.y - p1.y) == 0.0) {
                 if ((p3.y - p2.y) == 0.0) {
                     return null;
                 } else {
                     k2 = -1 * (p3.x - p2.x) / (p3.y - p2.y);
                     I.x = C1.x; I.y = k2 * (I.x - C2.x) + C2.y;
                 }
             } else {
                 k1 = -1 * (p2.x - p1.x) / (p2.y - p1.y);
                 if ((p3.y - p2.y) == 0.0) {
                     I.x = C2.x;
                     I.y = k1 * (I.x - C1.x) + C1.y;
                 } else {
                     k2 = -1 * (p3.x - p2.x) / (p3.y - p2.y);
                     if (k1 == k2) {
                         return null;
                     } else {
                         I.x = (C2.y - C1.y + k1 * C1.x - k2 * C2.x) / (k1 - k2); I.y = k1 * (I.x - C1.x) + C1.y;
                     }
                 }
             }
             return I;
         }

         //通过两点计算x1,y1为圆心,圆上某点（x2,y2）的角度（按照arc的角度计算）
         static computeAng(x1, y1, x2, y2) {
             var ang = (x1 - x2) / (y1 - y2);
             ang = Math.atan(ang);
             if (x1 == x2 && y2 > y1) {
                 return 0.5 * Math.PI;
             }
             if (x1 == x2 && y2 < y1) {
                 return 1.5 * Math.PI;
             }
             if (y1 == y2 && x2 > x1) {
                 return 0;
             }
             if (y1 == y2 && x2 < x1) {
                 return Math.PI;
             }
             if (x2 > x1 && y2 > y1) {
                 return Math.PI / 2 - ang;
             }
             else if (x2 < x1 && y2 > y1) {
                 return Math.PI / 2 - ang;
             }
             else if (x2 < x1 && y2 < y1) {
                 return 3 * Math.PI / 2 - ang;
             }
             else if (x2 > x1 && y2 < y1) {
                 return 3 * Math.PI / 2 - ang;
             }
         }

另外再介绍一下我们的这个箭头是如何实现的

         drawIsosceles(context: CanvasRenderingContext2D) {//画等腰箭头
             var triangleSide = this.triangleSide;
             //First the center of the triangle base (where we start to draw the triangle)
             var centerBaseArrowX = this.basePoint.x ;
             var centerBaseArrowY = this.basePoint.y;

             //Let's calculate the first point, easy!
             var ax = centerBaseArrowX + (triangleSide / 2) * Math.cos(this.centerAngle);
             var ay = centerBaseArrowY + (triangleSide / 2) * Math.sin(this.centerAngle);

             //Now time to get mad: the farest triangle point from the arrow body
             var bx = centerBaseArrowX + (1/ 2/Math.sqrt(3) ) * triangleSide * (Math.sin(-this.centerAngle));
             var by = centerBaseArrowY + ( 1/ 2/Math.sqrt(3)) * triangleSide * (Math.cos(-this.centerAngle));

             //Easy , like the a point
             var cx = centerBaseArrowX - (triangleSide / 2) * Math.cos(this.centerAngle);
             var cy = centerBaseArrowY - (triangleSide / 2) * Math.sin(this.centerAngle);

             context.beginPath();
             //We move to the center of the base
             context.moveTo(centerBaseArrowX, centerBaseArrowY);
             context.lineTo(ax, ay);
             context.lineTo(bx, by);
             context.lineTo(cx, cy);
             context.lineTo(centerBaseArrowX, centerBaseArrowY);
             context.fill();
         }

在canvas上绘制多边形的方法

        drawPath(context: CanvasRenderingContext2D) {
            for (var ixVertex = 0; ixVertex <= this.nPoints; ++ixVertex) {
                var angle = ixVertex * 2 * Math.PI / this.nPoints - this.startAngle;
                var point = new Point(this.centerPoint.x + this.radius * Math.cos(angle), this.centerPoint.y + this.radius * Math.sin(angle));
                context.lineTo(point.x, point.y);
            }
        }

在canvas上绘制五角星等多角形的方法

         drawPath(context:CanvasRenderingContext2D) {
             for (var ixVertex = 0; ixVertex <= 2 * this.nPoints; ++ixVertex) {
                 var angle = ixVertex * Math.PI / this.nPoints - Math.PI / 2;
                 var radius = ixVertex % 2 == 0 ? this.outerRadius : this.innerRadius;
                 context.lineTo(this.centerPoint.x + radius * Math.cos(angle), this.centerPoint.y + radius * Math.sin(angle));
             }
         }