moble 设备多指手势识别 （tap , double_tap , pinch）

 function(){
     elem.addEventListener('touchstart', start , false)
     elem.addEventListener('touchend', end , false)
     elem.addEventListener('touchmove', move , false)
     var coefficient = .
     var fingers

     var threshold = {'dbl_tap':, 'tap':}
     var start = null
         ,last_tap = null
     function start(evt){
         fingers = getFingers(evt)
         start = new Date

         }

     function end(act ,evt){
         var now =  new Date
         if (now - start <  threshold.tap){
             //tap
             if (last_tap && (now - last_tap) < threshold.dbl_tap) {
                 #double_tap 事件
                 last_tap = null
             }else{
                 #tap 事件

             }
             last_tap = now
             }
         fingers = null

         }

     function move(evt){
         evt.preventDefault()
         var end_fingers = getFingers(evt)
         if ( == fingers.length &&  == end_fingers.length)
             return pinch(fingers , end_fingers)
         else if ( == fingers.length)
             return drag(fingers ,end_fingers)

         }
     function drag(fg1 , fg2){
         var d = {x: fg2[].x - fg1[].x , y:fg2[].y - fg1[].y }
         #drag 事件

         }
     function pinch(fg1 , fg2){

         var d = []
             d.push( getDistance(fg1[] , fg1[]) )
             d.push( getDistance(fg2[] , fg2[]) )
         var scale =  - (d[] - d[])/ d[] * coefficient
         #缩放 scale> pinchout ;scale< pinchin

         }
     function getDistance(A ,B){
         if (!A || !B) return
         return Math.sqrt((B.x-A.x)*(B.x-A.x)+(B.y-A.y)*(B.y-A.y))

         }
     function getFingers(evt){
      var g = []
         for (var i = ; i < event.touches.length; i++) {
             var t = event.touches[i]
             g.push({x:t.pageX ,y :t.pageY})
             }
         return g
         }
 }

ios 上能监听到 gesturechange事件，但 android机不行  ，从QuoJS（http://quojs.tapquo.com/ ）里扣出来简单支持缩放手势的部分