(function (document) {
  var metas = document.getElementsByTagName('meta'),
    changeViewportContent = function (content) {
      for (var i = 0; i < metas.length; i++) {
        if (metas[i].name == 'viewport') {
          metas[i].content = content
        }
      }
    },
    initialize = function () {
      changeViewportContent('width=device-width, minimum-scale=1.0, maximum-scale=1.0')
    },
    gestureStart = function () {
      changeViewportContent('width=device-width, minimum-scale=0.25, maximum-scale=1.6')
    },
    gestureEnd = function () {
      initialize()
    }

  if (navigator.userAgent.match(/iPhone/i)) {
    initialize()

    document.addEventListener('touchstart', gestureStart, false)
    document.addEventListener('touchend', gestureEnd, false)
  }
})(document)