H5 如何解决iOS下Audio不能自动播放(Autoplay)音乐

  var audio ;
        window.onload = function(){
            initAudio();
        }

        document.addEventListener('touchstart', function(){
            initAudio();
        }, false);

        var initAudio = function(){
                  audio = document.getElementById('audio');
        }


<audio src="take_you_fly.mp3" id="audio" controls="controls"  autoplay="autoplay" ></audio>