canvas实现滚动鼠标放大缩小及拖动

1、canvas实现滚动鼠标放大缩小

2、实现鼠标拖动

实现方式canvas转图片

关键代码如下：

<canvas class="canvas"></canvas>

<script type="text/javascript">
　　var canvas = document.getElementsByClassName('canvas');
　　var ctx = canvas.getContext("2d");
　　var img = new Image();


$(function(){
        trackTransforms(ctx);

    var lastX=canvas.width/2, lastY=canvas.height/2;
    
    var dragStart,dragged;
    
    canvas.addEventListener('mousedown',function(evt){
        document.body.style.mozUserSelect = document.body.style.webkitUserSelect = document.body.style.userSelect = 'none';
        lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
        lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
        dragStart = ctx.transformedPoint(lastX,lastY);
        dragged = false;
    },false);
    
    canvas.addEventListener('mousemove',function(evt){
        lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
        lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
        dragged = true;
        if (dragStart){
          var pt = ctx.transformedPoint(lastX,lastY);
          ctx.translate(pt.x-dragStart.x,pt.y-dragStart.y);
          redraw();
              }
    },false);
    
    canvas.addEventListener('mouseup',function(evt){
        dragStart = null;
        if (!dragged) zoom(evt.shiftKey ? -1 : 1 );
    },false);
    
    var scaleFactor = 1.1;
    
    var zoom = function(clicks){
        if(clicks==1) return;
        var pt = ctx.transformedPoint(lastX,lastY);
        ctx.translate(pt.x,pt.y);
        var factor = Math.pow(scaleFactor,clicks);
        ctx.scale(factor,factor);
        ctx.translate(-pt.x,-pt.y);
        redraw();
    }
    
    var handleScroll = function(evt){
        var delta = evt.wheelDelta ? evt.wheelDelta/40 : evt.detail ? -evt.detail : 0;
        if (delta) zoom(delta);
        return evt.preventDefault() && false;
    };
    
    canvas.addEventListener('DOMMouseScroll',handleScroll,false);
    canvas.addEventListener('mousewheel',handleScroll,false);
});
    
img.src = canvas.toDataURL("image/png");

function redraw(){

    // Clear the entire canvas
    var p1 = ctx.transformedPoint(0,0);
    var p2 = ctx.transformedPoint(canvas.width,canvas.height);
    ctx.clearRect(p1.x,p1.y,p2.x-p1.x,p2.y-p1.y);

    ctx.save();
    ctx.setTransform(1,0,0,1,0,0);
    ctx.clearRect(0,0,canvas.width,canvas.height);
    ctx.restore();

    ctx.drawImage(img,0,0);

  }


function trackTransforms(ctx){
    var svg = document.createElementNS("http://www.w3.org/2000/svg",'svg');
    var xform = svg.createSVGMatrix();
    ctx.getTransform = function(){ return xform; };

    var savedTransforms = [];
    var save = ctx.save;
    ctx.save = function(){
        savedTransforms.push(xform.translate(0,0));
        return save.call(ctx);
    };
  
    var restore = ctx.restore;
    ctx.restore = function(){
      xform = savedTransforms.pop();
      return restore.call(ctx);
              };

    var scale = ctx.scale;
    ctx.scale = function(sx,sy){
      xform = xform.scaleNonUniform(sx,sy);
      return scale.call(ctx,sx,sy);
              };
  
    var rotate = ctx.rotate;
    ctx.rotate = function(radians){
        xform = xform.rotate(radians*180/Math.PI);
        return rotate.call(ctx,radians);
    };
  
    var translate = ctx.translate;
    ctx.translate = function(dx,dy){
        xform = xform.translate(dx,dy);
        return translate.call(ctx,dx,dy);
    };
  
    var transform = ctx.transform;
    ctx.transform = function(a,b,c,d,e,f){
        var m2 = svg.createSVGMatrix();
        m2.a=a; m2.b=b; m2.c=c; m2.d=d; m2.e=e; m2.f=f;
        xform = xform.multiply(m2);
        return transform.call(ctx,a,b,c,d,e,f);
    };
  
    var setTransform = ctx.setTransform;
    ctx.setTransform = function(a,b,c,d,e,f){
        xform.a = a;
        xform.b = b;
        xform.c = c;
        xform.d = d;
        xform.e = e;
        xform.f = f;
        return setTransform.call(ctx,a,b,c,d,e,f);
    };
  
    var pt  = svg.createSVGPoint();
    ctx.transformedPoint = function(x,y){
        pt.x=x; pt.y=y;
        return pt.matrixTransform(xform.inverse());
    }
    }
</script>