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();