怎么在JavaScript中使用Canvas实现一个拖拽绘图功能？针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

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一、实现的功能

1、基于oop思想构建，支持坐标点、线条（由坐标点组成，包含方向）、多边形（由多个坐标点组成）、圆形（包含圆心坐标点和半径）等实体

2、原生JavaScript实现，不依赖任何第三方js库和插件

3、多图形绘制（支持画笔、线条、箭头、三角形、矩形、平行四边形、梯形以及多边形和圆形绘制）

4、拖拽式绘制（鼠标移动过程中不断进行canvas重绘）

5、图片绘制（作为背景图片时重绘会发生闪烁现象，暂时有点问题，后面继续完善）

5、清空绘制功能

6、新版本优化绘制性能（使用共享坐标变量数组，减少了大量的对象创建操作）

7、新版本支持箭头绘制功能

二、完整实现代码

DrawingTools =(function(){
//公共方法
      var getDom=function(id){return document.getElementById(id)};
      var isNull=function(s){return s==undefined||typeof(s)=='undefined'||s==null||s=='null'||s==''||s.length<1};
      var hideDefRM=function(){document.oncontextmenu=function(){return false}};//屏蔽浏览器默认鼠标事件
      /**绘图容器*/
      var cbtCanvas;
      /**绘图对象*/
      var cxt;
      /**绘制的图形列表*/
      var shapes=new Array();
      var graphkind={'cursor':0,'pen':1,'line':2,'trian':3,'rect':4,'poly':5,'circle':6,'arrow':21,'parallel':41,'trapezoid':42};
      //背景图片绘制配置
      var bgPictureConfig={
        pic:null,//背景图片地址或路径
        repaint:true,//是否作为永久背景图，每次清除时会进行重绘
      };
      //加载并绘制图片(src:图片路径或地址),默认重绘背景图
      var loadPicture=function(src){
        if(isNull(bgPictureConfig.repaint)||bgPictureConfig.repaint){bgPictureConfig.pic=src}
        var img = new Image();
        img.onload = function(){cxt.drawImage(img,0,0)}
        img.src =src;
      }
      //绘图基础配置
      var paintConfig={lineWidth:1,//线条宽度，默认1
        strokeStyle:'red',//画笔颜色，默认红色
        fillStyle:'red',//填充色
        lineJoin:"round",//线条交角样式，默认圆角
        lineCap:"round",//线条结束样式，默认圆角
      };
      //重新载入绘制样式
      var resetStyle=function(){
        cxt.strokeStyle=paintConfig.strokeStyle;
        cxt.lineWidth=paintConfig.lineWidth;
        cxt.lineJoin=paintConfig.lineJoin;
        cxt.lineCap=paintConfig.lineCap;
        cxt.fillStyle=paintConfig.fillStyle;
      }
      //鼠标图形
      var cursors=['crosshair','pointer'];
      /** 切换鼠标样式*/
      var switchCorser=function(b){
        cbtCanvas.style.cursor=((isNull(b)?isDrawing():b)?cursors[0]:cursors[1]);
      }
      //操作控制变量组
      var ctrlConfig={
        kind:0,//当前绘画分类
        isPainting:false,//是否开始绘制
        startPoint:null,//起始点
        cuGraph:null,//当前绘制的图像
        cuPoint:null,//当前临时坐标点，确定一个坐标点后重新构建
        cuAngle:null,//当前箭头角度
        vertex:[],//坐标点
      }
      /**获取当前坐标点*/
      var getCuPoint=function(i){
        return ctrlConfig.cuPoint[i];
      }
      /**设置当前坐标点*/
      var setCuPoint=function(p,i){
        return ctrlConfig.cuPoint[i]=p;
      }
      /**设置当前临时坐标点值*/
      var setCuPointXY=function(x,y,i){
        if(isNull(ctrlConfig.cuPoint)){
          var arr=new Array();
          arr[i]=new Point(x,y);
          ctrlConfig.cuPoint=arr;
        }else if(isNull(ctrlConfig.cuPoint[i])){
          setCuPoint(new Point(x,y),i);
        }else{
          ctrlConfig.cuPoint[i].setXY(x,y);
        }
        return getCuPoint(i);
      }
      /**是否正在绘制*/
      var isDrawing=function (){
        return ctrlConfig.isPainting;
      }
      /**开始绘制状态*/
      var beginDrawing=function(){
        ctrlConfig.isPainting=true;
      }
      /**结束绘制状态*/
      var stopDrawing=function(){
        ctrlConfig.isPainting=false;
      }
      /**是否有开始坐标点*/
      var hasStartPoint=function(){
        return !isNull(ctrlConfig.startPoint);
      }
      /**设置当前绘制的图形*/
      var setCuGraph=function(g){
        ctrlConfig.cuGraph=g;
      }
      /**获取当前绘制的图形*/
      var getCuGraph=function(){
        return ctrlConfig.cuGraph;
      }
      /**设置开始坐标点（线条的起始点，三角形的顶点，圆形的圆心，四边形的左上角或右下角，多边形的起始点）*/
      var setStartPoint=function(p){
        ctrlConfig.startPoint=p;
      }
      /**获取开始坐标点*/
      var getStartPoint=function(){
        return ctrlConfig.startPoint;
      }
      /**清空全部*/
      var clearAll=function(){
        cxt.clearRect(0,0,cbtCanvas.width,cbtCanvas.height);
      }
      /**重绘*/
      var repaint=function(){
        clearAll();
        /*
        if(bgPictureConfig.repaint){
          loadPicture(bgPictureConfig.pic);
        }*/
      }
      /**点（坐标,绘图的基本要素,包含x,y坐标）*/
      var Point=(function(x1,y1){
        var x=x1,y=y1;
        return{
          set:function(p){
            x=p.x,y=p.y;
          },
          setXY:function(x2,y2){
            x=x2;y=y2;
          },
          setX:function(x3){
            x=x3;
          },
          setY:function(y3){
            y=y3;
          },
          getX:function(){
            return x;
          },
          getY:function(){
            return y;
          }
        }
      });
      /**多角形（三角形、矩形、多边形），由多个点组成*/
      var Poly=(function(ps1){
        var ps=isNull(ps1)?new Array():ps1;
        var size=ps.length;
        return{
          set:function(ps2){
            ps=ps2;
          },
          getSize:function(){
            return size;
          },
          setPoint:function(p,i){
            if(isNull(p)&&isNaN(i)){
              return;
            }
            ps[i]=p;
          },
          setStart:function(p1){
            if(isNull(ps)){
              ps=new Array();
              return ps.push(p1);
            }else{
              ps[0]=p1;
            }
          },
          add:function(p){
            if(isNull(ps)){
              ps=new Array();
            }
            return ps.push(p);
          },
          pop:function(){
            if(isNull(ps)){
              return;
            }
            return ps.pop();
          },
          shift:function(){
            if(isNull(ps)){
              return;
            }
            return ps.shift;
          },
          get:function(){
            if(isNull(ps)){
              return null;
            }
            return ps;
          },
          draw:function(){
            cxt.beginPath();
            for(i in ps){
              if(i==0){
                cxt.moveTo(ps[i].getX(),ps[i].getY());
              }else{
                cxt.lineTo(ps[i].getX(),ps[i].getY());
              }
            }
            cxt.closePath();
            cxt.stroke();
          }
        }
      });
      /*线条(由两个点组成，包含方向)*/
      var Line=(function(p1,p2,al){
        var start=p1,end=p2,angle=al;
        var drawLine=function(){
          cxt.beginPath();
          cxt.moveTo(p1.getX(),p1.getY());
          cxt.lineTo(p2.getX(),p2.getY());
          cxt.stroke();
        }
        //画箭头
        var drawArrow=function() {
          var vertex =ctrlConfig.vertex;
          var x1=p1.getX(),y1=p1.getY(),x2=p2.getX(),y2=p2.getY();
          var el=50,al=15;
          //计算箭头底边两个点（开始点，结束点，两边角度，箭头角度）
          vertex[0] = x1,vertex[1] = y1, vertex[6] = x2,vertex[7] = y2;
          //计算起点坐标与X轴之间的夹角角度值
          var angle = Math.atan2(y2 - y1, x2 - x1) / Math.PI * 180;
          var x = x2 - x1,y = y2 - y1,length = Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
          if (length < 250) {
            el/=2,al/2;
          }else if(length<500){
            el*=length/500,al*=length/500;
          }
          vertex[8] = x2 - el * Math.cos(Math.PI / 180 * (angle + al));
          vertex[9] = y2- el * Math.sin(Math.PI / 180 * (angle + al));
          vertex[4] = x2- el* Math.cos(Math.PI / 180 * (angle - al));
          vertex[5] = y2 - el * Math.sin(Math.PI / 180 * (angle - al));
          //获取另外两个顶点坐标
          x=(vertex[4]+vertex[8])/2,y=(vertex[5]+vertex[9])/2;
          vertex[2] = (vertex[4] + x) / 2;
          vertex[3] = (vertex[5] + y) / 2;
          vertex[10] = (vertex[8] +x) / 2;
          vertex[11] = (vertex[9] +y) / 2;
          //计算完成,开始绘制
          cxt.beginPath();
          cxt.moveTo(vertex[0], vertex[1]);
          cxt.lineTo(vertex[2], vertex[3]);
          cxt.lineTo(vertex[4], vertex[5]);
          cxt.lineTo(vertex[6], vertex[7]);
          cxt.lineTo(vertex[8], vertex[9]);
          cxt.lineTo(vertex[10], vertex[11]);
          cxt.closePath();
          cxt.fill();
          cxt.stroke();
        }
        return{
          setStart:function(s){
            start=s;
          },
          setEnd:function(e){
            end=e;
          },
          getStart:function(){
            return start;
          },
          getEnd:function(){
            return end;
          },
          draw:function(){
            if(angle){
              drawArrow();
            }else{
              drawLine();
            }
          }
        }
      });
      /**圆形(包含圆心点和半径)*/
      var Circle=(function(arr){
        //包含起始点（圆心）和结束点,以及圆半径
        var startPoint=arr.start,endPoint=arr.end,radius=arr.radius;
        /*绘制圆*/
        var drawCircle=function(){
          cxt.beginPath();
          var x=startPoint.getX();
          var y=startPoint.getY();
          if(isNull(radius)){
            radius=calculateRadius(startPoint,endPoint);
          }
          //x,y,半径,开始点,结束点,顺时针/逆时针
          cxt.arc(x,y,radius,0,Math.PI*2,false); // 绘制圆
          cxt.stroke();
        }
        //计算圆半径
        var calculateRadius=function(p1,p2){
          var width=p2.getX()-p1.getX();
          var height=p2.getY()-p1.getY();
          //如果是负数
          if(width<0||height<0){
            width=Math.abs(width);
          }
          //计算两点距离=平方根(width^2+height^2)
          c=Math.sqrt(Math.pow(width,2)+Math.pow(height,2));
          return c;
        }
        return{
          set:function(params){
            startPoint=params.start;
            endPoint=params.end;
            radius=params.radius;
          },
          setPoint:function(p1){
            p=p1;
          },
          getPoint:function(){
            return p;
          },
          setRadius:function(r1){
            radius=r1;
          },
          getRadius:function(){
            return radius;
          },
          calcRadius:calculateRadius,
          //绘制
          draw:drawCircle,
        }
      });
      /**绘制线条工具方法*/
      var drawLine=function(p){
        cxt.beginPath();
        cxt.moveTo(startPosition.getX(),startPosition.getY());
        cxt.lineTo(p.getX(),p.getY());
        cxt.stroke();
      }
      /**绘制三角形工具方法*/
      var drawTrian=function(ps){
        cxt.beginPath();
        var a=ps.get();
        cxt.moveTo(a[0].getX(),a[0].getY());
        cxt.lineTo(a[1].getX(),a[1].getY());
        cxt.lineTo(a[2].getX(),a[2].getY());
        cxt.closePath();
        cxt.stroke();
      }
      /**绘制矩形工具方法*/
      var drawRect=function(p2){
        var p=getStartPoint();
        var width=p.getX()-p2.getX();
        var height=p.getY()-p2.getY();
        cxt.beginPath();
        cxt.strokeRect(x,y,width,height);//绘制矩形
      }
      /*绘制多边形工具方法*/
      var drawpolygon=function(ps){
        if(ps.length>1){//保证只有两个坐标点才是矩形
          cxt.beginPath();
          var p=ctrlConfig.startPoint;
          var x=p.getX();
          var y=p.getY();
          cxt.moveTo(x,y);
          for(p1 in ps){
            cxt.lineTo(p1.getX(),p1.getY());
          }
          cxt.stroke();
        }
      }
      /*绘制圆角矩形工具方法*/
      var drawRoundedRect=function(x,y,width,height,radius){
        cxt.beginPath();
        cxt.moveTo(x,y+radius);
        cxt.lineTo(x,y+height-radius);
        cxt.quadraticCurveTo(x,y+height,x+radius,y+height);
        cxt.lineTo(x+width-radius,y+height);
        cxt.quadraticCurveTo(x+width,y+height,x+width,y+height-radius);
        cxt.lineTo(x+width,y+radius);
        cxt.quadraticCurveTo(x+width,y,x+width-radius,y);
        cxt.lineTo(x+radius,y);
        cxt.quadraticCurveTo(x,y,x,y+radius);
        cxt.stroke();
      }
      /*绘制圆工具方法*/
      var drawCircle=function(c){
        var p=c.getPoint();//坐标点
        var x=p.getX();
        var y=p.getY();
        var r=c.getRadius();
        cxt.beginPath();
        //x,y,半径,开始点,结束点,顺时针/逆时针
        cxt.arc(x,y,r,0,Math.PI*2,false); // 绘制圆
        cxt.stroke();
      }
      //计算圆半径工具方法
      var calculateRadius=function(p1,p2){
        var width=p2.getX()-p1.getX();
        var height=p2.getY()-p1.getY();
        //如果是负数
        if(width<0||height<0){
          width=Math.abs(width);
        }
        //计算两点距离=平方根(width^2+height^2)
        c=Math.sqrt(Math.pow(width,2)+Math.pow(height,2));
        return c;
      }
      //鼠标按键点击（首次点击确定开始坐标点，拖动鼠标不断进行图形重绘）
      var mouseDown = function(e){
        var btnNum = e.button;
        if(btnNum==0){
          console.log("选择："+ctrlConfig.kind);
          //设置起始点
          switch(ctrlConfig.kind){
            case graphkind.pen://画笔(不松开鼠标按键一直画)
              beginDrawing();//开始绘制
              cxt.beginPath();
              cxt.moveTo(e.offsetX,e.offsetY);
              break;
            case graphkind.poly://多边形
              var p=new Point(e.offsetX,e.offsetY);
              if(isDrawing()){
                getCuGraph().add(p);//添加到
              }else{//第一次确定开始坐标
                beginDrawing();//开始绘制
                setStartPoint(p);
                var poly=new Poly();
                poly.add(p);
                setCuGraph(poly);//设置当前绘制图形
              }
              break;
            case graphkind.line://线条
            case graphkind.arrow://方向
            case graphkind.trian://三角形
            case graphkind.rect://矩形
            case graphkind.parallel://平行四边形
            case graphkind.trapezoid://梯形
              beginDrawing();//开始绘制
              var p=new Point(e.offsetX,e.offsetY);
              setStartPoint(p);
              var poly=new Poly();
              poly.add(p);
              setCuGraph(poly);//设置当前绘制图形
              break;
            case graphkind.circle://圆
              console.log("确定图形绘制开始坐标点："+e.offsetX+","+e.offsetY);//点击确定图形的开始坐标点
              beginDrawing();//开始绘制
              var p=new Point(e.offsetX,e.offsetY);
              setStartPoint(p);
              var circle= new Circle({'start':p});
              setCuGraph(circle);
              break;
            case ctrlConfig.cursor: //手型鼠标
            default://默认是手型鼠标，不允许绘制
          }
        }else if(btnNum==2){
          console.log("右键由于结束多边形绘制");
          if(isDrawing()){
            if(ctrlConfig.kind==graphkind.poly){
              repaint();
              getCuGraph().draw();
              stopDrawing();//结束绘制
            }
          }
        }
        hideDefRM();//屏蔽浏览器默认事件
      }
      //鼠标移动（拖动，根据鼠标移动的位置不断重绘图形）
      var mouseMove = function(e){
        if(isDrawing()&&hasStartPoint()){//检查是否开始绘制，检查是否有开始坐标点
          //画笔不需要重绘
          if(ctrlConfig.kind>1){
            repaint();//重绘
          }
          var p=setCuPointXY(e.offsetX,e.offsetY,0);//设置共享的临时坐标点，用于防止重复创建对象
          switch(ctrlConfig.kind){
            case graphkind.pen://画笔(一直画)
              cxt.lineTo(e.offsetX,e.offsetY);
              cxt.stroke();
              break;
            case graphkind.poly://多边形
              var poly=getCuGraph(poly);
              var size=poly.getSize();
              poly.setPoint(p,(size-1));
              poly.draw();
              break;
            case graphkind.line://线条
              var line=new Line(getStartPoint(),p,false);
              ctrlConfig.cuGraph=line;
              line.draw();
              break;
            case graphkind.arrow://方向
              var line=new Line(getStartPoint(),p,true);
              ctrlConfig.cuGraph=line;
              line.draw();
              break;
            case graphkind.trian://三角形
              var lu=getStartPoint();
              var x2=p.getX();
              var x1=lu.getX();
              //三角形左边的点坐标计算方法:(x1-(x2-x1),y2)
              var x3=x1-(x2-x1);
              var l=setCuPointXY(x3,p.getY(),1);//设置共享的临时坐标点，用于防止重复创建对象
              var poly=getCuGraph();//获取当前图形
              poly.set([lu,p,l]);
              poly.draw();//即时绘制
              break;
            case graphkind.parallel://平行四边形
              var lu=getStartPoint();
              var x3=p.getX();
              var x1=lu.getX();
              //平行四边形两个未知坐标点计算方法:(x1-(x3-x1),y3),(x1+(x3-x1),y1)
              var x2=x3+(x3-x1);
              var x4=x1-(x3-x1);
              var ld=setCuPointXY(x2,lu.getY(),1);//设置共享的临时坐标点，用于防止重复创建对象
              var ru=setCuPointXY(x4,p.getY(),2);//设置共享的临时坐标点，用于防止重复创建对象
              var poly=getCuGraph();//获取当前图形
              poly.set([lu,ru,p,ld]);
              poly.draw();//即时绘制
              break;
            case graphkind.trapezoid://梯形
              var lu=getStartPoint();
              var x3=p.getX();
              var x1=lu.getX();
              //梯形两个未知坐标点计算方法:(x3-(x3-x1)/2,y1),(x1-(x3-x1)/2,y3)
              var x2=x3-(x3-x1)/2;
              var x4=x1-(x3-x1)/2;
              var ld=setCuPointXY(x2,lu.getY(),1);
              var ru=setCuPointXY(x4,p.getY(),2);
              var poly=getCuGraph();
              poly.set([lu,ru,p,ld]);
              poly.draw();
              break;
            case graphkind.rect://矩形
              var lu=getStartPoint();
              //矩形右上角和左上角坐标计算方法
              var ld=setCuPointXY(lu.getX(),p.getY(),1);
              var ru=setCuPointXY(p.getX(),lu.getY(),2);
              var poly=getCuGraph();
              poly.set([lu,ru,p,ld]);
              poly.draw();
              break;
            case graphkind.circle://圆
              var circle=getCuGraph();//获取当前图形
              circle.set({'start':getStartPoint(),'end':p});
              circle.draw();//即时绘制
              break;
          }
        }
      }
      //鼠标按键松开
      var mouseUp = function(e){
        if(isDrawing()){
          //console.log("松开鼠标按键:"+e.offsetX+","+e.offsetY);
          //画笔不需要重绘
          if(ctrlConfig.kind>1){
            repaint();
            getCuGraph().draw();
          }
          if(ctrlConfig.kind!=graphkind.poly){//多边形绘制鼠标按键松开不结束绘制，多边形只有右键点击才能结束绘制
            stopDrawing();//结束绘制
          }
        }
      }
      //鼠标移出
      var mouseOut = function(e){
        console.log("鼠标移出绘制区域"+e.offsetX+","+e.offsetY);
        if(isDrawing()){
          console.log("停止绘制");
          if(ctrlConfig.kind>1){
            repaint();
            getCuGraph().draw();
          }
          stopDrawing();//停止绘制
        }
      }
      return{
        isNull:isNull,
        getDom:getDom,
        clear:function(){
          stopDrawing();//停止绘制
          repaint();
        },
        /**初始化*/
        init:function(params){
          cbtCanvas=getDom(params.id);
          //浏览器是否支持Canvas
          if (cbtCanvas.getContext){
            /**绘图对象*/
            cxt=cbtCanvas.getContext("2d");
            cbtCanvas.onmousedown = mouseDown;
            cbtCanvas.onmouseup = mouseUp;
            cbtCanvas.onmousemove = mouseMove;
            cbtCanvas.onmouseout = mouseOut;
            resetStyle();//载入样式
            return true;
          }else{
            return false;
          }
        },
        /**设置背景图片*/
        setBgPic:loadPicture,
        /**选择图形类型*/
        begin:function(k){
          console.log("选择绘制图形："+k);
          if(isNaN(k)){//如果不是数字，先转换为对应字符
            ctrlConfig.kind=kind[k];
          }else{
            ctrlConfig.kind=k;
          }
          switchCorser(true);//切换鼠标样式
        },
        /*手型，并停止绘图*/
        hand:function(){
          ctrlConfig.kind=0;
          stopDrawing();//停止绘制
          switchCorser(false);//切换鼠标样式
        }
      }
    })

三、使用方式

1、图形类型

0:鼠标，1:画笔，2:线条，3:三角形，4:矩形，5:多边形，6:圆形，21:箭头，41:平行四边形，42:梯形

var graphkind={'cursor':0,'pen':1,'line':2,'trian':3,'rect':4,'poly':5,'circle':6,'arrow':21,'parallel':41,'trapezoid':42};

2、初始化以及使用背景图片和画笔选择

var drawUtil=new DrawingTools();
//初始化，(如果浏览器不支持H5，会初始化失败，返回false)
if(drawUtil.init({'id':'calibrationCanvas'})){
//加载图片
var imgsrc='图片地址';
if(!drawUtil.isNull(imgsrc)){
  drawUtil.setBgPic(imgsrc,true);//设置背景图片（异步加载图片）
}
}
drawUtil.begin(1);//选择画笔

2、绘制箭头

drawUtil.begin(21);

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