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 = .02
    var fingers
   
    var threshold = {'dbl_tap':400, 'tap':250}
    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 (2 == fingers.length && 2 == end_fingers.length)
            return pinch(fingers , end_fingers)
        else if (1 == fingers.length)
            return drag(fingers ,end_fingers)

        }
    function drag(fg1 , fg2){
        var d = {x: fg2[0].x - fg1[0].x , y:fg2[0].y - fg1[0].y }
        #drag 事件

        }
    function pinch(fg1 , fg2){

        var d = []
            d.push( getDistance(fg1[0] , fg1[1]) )
            d.push( getDistance(fg2[0] , fg2[1]) )
        var scale = 1 - (d[0] - d[1])/ d[0] * coefficient
        #缩放 scale>1 pinchout ;scale<1 pinchin

        }
    function getDistance(A ,B){
        if (!A || !B) return 0
        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 = 0; 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