canves的常用方法

function circlePosition(center,du,r){
    var cx=Math.sin(du*2*Math.PI / 360) * r ;
    var cy=Math.cos(du*2*Math.PI / 360) * r ;
    var ax = center.x+cx;
    var ay = center.y+cy;
    return {x:ax,y:ay};
}

CanvasRenderingContext2D.prototype.roundRect = function (x, y, w, h, r) {
    if (w < 2 * r) r = w / 2;
    if (h < 2 * r) r = h / 2;
    this.beginPath();
    this.moveTo(x+r, y);
    this.arcTo(x+w, y, x+w, y+h, r);
    this.arcTo(x+w, y+h, x, y+h, r);
    this.arcTo(x, y+h, x, y, r);
    this.arcTo(x, y, x+w, y, r);
    this.closePath();
    return this;
}
用法：
context.roundRect(x,y,w,h,r).stroke();

var bbox = canvas.getBoundingClientRect();
    return {
        x:x - bbox.left - (bbox.width - canvas.width) / 2,
        y:y - bbox.top - (bbox.height - canvas.height) / 2
    };
}
 

canvas.onmousedown=function(event){
   var pos=windowToCanvas(canvas,event.clientX,event.clientY);     
 }

 canvas.onmouseup=function(){
     //canvas.onmousemove=null;
     canvas.onmouseup=null;
     canvas.style.cursor="default";
 }

canvas.onmousemove=function(event){
    canvas.style.cursor="move";
    var pos=windowToCanvas(canvas,event.clientX,event.clientY);
}


canvas.onmousewheel=canvas.onwheel=function(event){
   var pos=windowToCanvas(canvas,event.clientX,event.clientY);
   event.wheelDelta=event.wheelDelta?event.wheelDelta:(event.deltaY*(-40));
   if(event.wheelDelta>0){
   }else{
   }
}

function drawLine(x,y,x1,y1){
    context.lineWidth = "0.5";  
    context.beginPath();  
    context.strokeStyle="#569CD6";  
    context.moveTo(x,y);  
    context.lineTo(x1,y1);  
    context.closePath(); 
    context.stroke();  
}

function drawCircle(x,y,r){
    context.beginPath();
    context.lineWidth="0.5";
    context.arc(x,y,r,0,2*Math.PI);
    context.stroke();
    context.closePath();
}

function drawText(x,y,lingH,color,str){
    context.font="12px Verdana";
    context.fillStyle="#F2F2F2";
    var lineWidth = 0;
    var canvasWidth = nodeWith-30;
    var initHeight=20;//绘制字体距离canvas顶部初始的高度
    var lastSubStrIndex= 0; //每次开始截取的字符串的索引
    for(let i=0;i<str.length;i++){ 
        lineWidth+=ctx.measureText(str[i]).width; 
        if(lineWidth>canvasWidth){  
            context.fillText(str.substring(lastSubStrIndex,i),x+5,y+initHeight);//绘制截取部分
            initHeight+=15;//20为字体的高度
            lineWidth=0;
            lastSubStrIndex=i;
        } 
        if(i==str.length-1){//绘制剩余部分
            context.fillText(str.substring(lastSubStrIndex,i+1),x+5,y+initHeight);
        }
    }
}