Writing async tests

I don’t know about you, but I’m always amazed every time I see some clever usage of generator functions (the yield keyword). From things like speeding up your UI, to implementing thread pools, or computing prime numbers and infinite sequences, they’re always interesting. Last week, Shawn showed me a cool async pattern using generators which I really liked so I decided to blog about it.

Let’s start with the problem: I was writing some set of tests which were highly dependent on triggering actions and waiting events propagation to continue. This usually involves a lot of callbacks from short timeouts or event listeners, and you end up with code like this: [semi-pseudocode]

function goOn() {
  doThing3();
  setTimeout(finishUp, 10);
}

function test() {
  doThing1();
  setTimeout(function() {
    addEventListener("interesting-event", goOn, false);
    doThing2();
  }, 10);
}

Which is the modern day equivalent of spreading gotos everywhere on your code (“oh, the horror!”), and makes it very hard to follow the intended logic, specially if you didn’t aptly name your functions with numbers to help you out.

To improve that, we can avoid nesting all of the calls and write a sequential function, and use the yield keyword to exit the function at one point and then continue running it from that point on. It’s like a set of sleep/wakeUp calls, and all the wakeUp control won’t be so mixed with our logic. So we would write as follows:

function wakeUp() {
  testGen.next();
}
function test() {
   doThing1();
   setTimeout(wakeUp, 10); yield;
   doThing2();
   addEventListener("interesting-event", wakeUp, false); yield;
   doThing3();
   setTimeout(wakeUp, 10); yield;
   finishUp();   
}

Cleaner, isn’t it? Oh, and just a note, when you use this make sure you don’t end up subtly changing the logic of your calls, unlike “a friend of mine” who did this and broke the tests he had written. 🙂

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