Protractor: How do promises work?

Question

I have a decent idea of how the $q library in angular works but I'm not sure how protractor or web-driver-js uses them. (especially since the utilizations are slightly different between protractor/web-driver/angular I think)

For example, the sample code I've seen in protractor tutorials seem to be written line by line and assumes the previous one completes before the next. (e.g. browser.get(url) followed by browser.getTitle() ) Is there some sort of implicit promising going on? (I don't have to manually defer resolve/fulfill)

I've read through https://github.com/angular/protractor/blob/master/docs/getting-started.md and skimmed the links to the webdriver documentation and protractor api but my understanding is still a bit fuzzy. Some functions seem to return values and some return promises like a weird mix of synchronous and asynchronous code.

http://markdalgleish.com/2013/06/using-promises-in-angularjs-views/ — zloctb, Aug 22 '16 at 13:56

score 13 · Accepted Answer · answered Mar 27 '14 at 19:30

Is there some sort of implicit promising going on?

Looking at https://github.com/angular/protractor/blob/master/docs/control-flow.md , it looks like the answer is yes, there is, by using a queue of promises, called the control flow. So to expand on your example:

browser.get(url);
var title = browser.getTitle();
expect(title).toEqual('My Title');

Each of the lines above adds to the queue. The variable title is actually a promise, which, at the appropriate point in the control flow, expect unwraps.

Cool, thanks for the link! Good to know that the control flow is doing something. Thought I had misunderstood promises! — user2483724, Mar 27 '14 at 20:35

score 12 · Answer 2 · answered Mar 20 '15 at 02:57

Here is a good article about Protractor control flow, itself inspired by the WebDriver API.

In short (quoting the article), here is how the ControlFlow of WebDriver works under the hood.

When you write:

driver.get('http://www.example.com');
driver.findElement(elem).sendKeys('hello');

What actually happens is:

driver.get('http://www.example.com')
  .then(function() {
    return driver.findElement(elem);
  })
  .then(function(q) {
    return q.sendKeys('hello');
  });

Note: I do not intend to be redundant with the accepted answer, but I believe seeing how promises are actually chained internally can help understand.

Protractor: How do promises work?

2 Answers2

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