几乎每篇文档都给你来一个on the fly。知乎搜了几次也没记住，干脆把维基百科的解释翻译了一下：
On the fly is a phrase used to describe something that is being changed while the process that the change affects is ongoing.
On the fly 是一个短语用于表示一种现象：当进程正在运行时这个改变就生效了。
It is used in the automotive, computer, and culinary industries.
它被用于汽车、计算机和烹饪行业。
In cars, on the fly can be used to describe the changing of the cars configuration while it is still driving.
在汽车领域里，on the fly可以用来表述 汽车在行驶时，车子的配置就被改变了。
Processes that can occur while the car is still driving include switching between two wheel drive and four wheel drive on some cars
and opening and closing the roof on some convertible cars.
汽车在行驶时某些车可以进行两驱-四驱变换，某些敞篷车可以做到关闭天窗。 （译者：理解为飞机空中加油）
In computing, on the fly CD writers can read from one CD and write the data to another without saving it on a computer’s memory.
计算机领域里，on the fly的CD烧写器可以直接从一个CD盘里读数据到另一盘里，不需要把数据先保存到电脑的内存里。 （译者：理解为directly）
Switching computer parts on the fly means computer parts are replaced while the computer is still running.
切换计算机部件on the fly表示当计算机正在运行时部件就被替换了。
It can also be used in programming to describe changing a program while it is still running.
在编程里可以表示在程序正在运行时改变程序 （译者：理解为在线升级软件，还不用重启）
In restaurants and other places involved in the preparation of food, the term is used to indicate that an order needs to be made right away。
在餐厅和其他涉及到准备食物的场景，这个术语用来表明一份订单需要立即敲定。 （译者：理解为 right away）
Colloquial usage:
通俗用法：
1.something that was not planned ahead
没有提前约好
2.changes that are made during the execution of same activity: extempore, impromptu.
活动中做出的改变：当场的，即兴的
总结就五个意思：
- 正在运行时改变 （另一个相似的概念：Hot swapping / hot plugging）
- 直接地
- 立即，马上
- 没有提前约好
- 即兴发挥
本来想把即兴发挥也归为没有提前约好，后来一想不对，提前约好是两个人的事，即兴发挥属于一个人的事。一个人喝高了，飘飘然了（on the fly），然后即兴来一首《老司机带带我》，屎尿未及的事。
