第 9 章 Unit Tests 单元测试

Our profession has come a long way in the last ten years. In 1997 no one had heard of Test Driven Development. For the vast majority of us, unit tests were short bits of throw-away code that we wrote to make sure our programs “worked.” We would painstakingly write our classes and methods, and then we would concoct some ad hoc code to test them. Typically this would involve some kind of simple driver program that would allow us to manually interact with the program we had written.

I remember writing a C++ program for an embedded real-time system back in the mid-90s. The program was a simple timer with the following signature:

我记得在 20 世纪 90 年代曾为一套嵌入式实时系统编写过 C++程序。该程序是个简单的计时器，有如下签名：

The idea was simple; the execute method of the Command would be executed in a new thread after the specified number of milliseconds. The problem was, how to test it.

I cobbled together a simple driver program that listened to the keyboard. Every time a character was typed, it would schedule a command that would type the same character five seconds later. Then I tapped out a rhythmic melody on the keyboard and waited for that melody to replay on the screen five seconds later.

“I … want-a-girl … just … like-the-girl-who-marr … ied … dear … old … dad.”

I actually sang that melody while typing the “.” key, and then I sang it again as the dots appeared on the screen.

在按下那些“.”键时，我真的在哼着那段旋律，当那些句点出现在屏幕上时，我又再哼了一次。

那就是我的测试！我看到这法子可行，演示给同事们看，然后就把代码扔掉了。

As I said, our profession has come a long way. Nowadays I would write a test that made sure that every nook and cranny of that code worked as I expected it to. I would isolate my code from the operating system rather than just calling the standard timing functions. I would mock out those timing functions so that I had absolute control over the time. I would schedule commands that set boolean flags, and then I would step the time forward, watching those flags and ensuring that they went from false to true just as I changed the time to the right value.

Once I got a suite of tests to pass, I would make sure that those tests were convenient to run for anyone else who needed to work with the code. I would ensure that the tests and the code were checked in together into the same source package.

有了一套运行通过的测试，我会确保任何需要用到代码的人都能方便地使用这些测试。我会确保测试和代码一起签入同一个代码包。

对，我们进步甚多；但还有很长的路要走。敏捷和 TDD 运动鼓舞了许多程序员编写自动化单元测试，每天还有更多人加入这个行列。但是，在争先恐后将测试加入规程中时，许多程序员遗漏了一些关于编写好测试的更细微但却重要的要点。