js如何获取键盘高度

在移动端或混合app开发中，js如何获取键盘高度，直接贴上代码吧

input是一个html input 标签

 var timer = {
     id:null,
     run:function (callback,time) {
         this.id = window.setInterval(callback,time);
     },
     clean:function () {
         var that = this;
         this.id = window.clearInterval(that.id);
     }
 };
 var keyboardHeight = 0,screenHeight = window.innerHeight;
 input.addEventListener('focus',function (evt) {
     if(!keyboardHeight){
         timer.run(function () {
             if (screenHeight !== window.innerHeight) {
                 keyboardHeight = screenHeight-window.innerHeight;
                 timer.clean()
             }
         }, 50)
     }
 });
 input.focus();