了解量角器对 Promise 的使用

Understanding protractor#39;s use of promises(了解量角器对 Promise 的使用)

问题描述

基本上，我正在玩弄一个用于设置先决条件的库(想想用户创建).由于量角器是基于 Promise 的，并且 神奇地进行了所有包装以保留内容为了，我想到了为此目的使用承诺.我写了一些示例代码，但我无法解释观察到的行为，所以也许有人可以帮助我理解发生了什么.

Essentially I am playing around with getting a library for preconditions set up (think user creation). As protractor is promise-based and magically does all the wrapping to keep things in order, I thought of using promises for that purpose. I wrote a little sample code but I cannot explain the observed behaviour so maybe someone can help me understand what's going on.

function timeout(ms) {
    var promise = protractor.promise.defer();
    setTimeout(function() {
        console.log('qwer');
        promise.fulfill(true);
    }, ms);
    return promise.promise;
}

所以现在函数返回一个承诺，我假设它可以像这样内联使用(因为所有代码都应该被包装在一个控制流中)

So now the function returns a promise, which I would assume to be usable inline just like so (as all the code supposedly gets wrapped in a control flow)

describe('test', function() {
  it('bla', function() {
    browser.get('/');
    timeout(5000);
    $('some-element').click();
  });
});

但是这并没有按预期工作(即 console.log 永远不会发生).即使 expect 从该承诺中得到结果，它也不会改变结果.如果实际上通过执行 timeout(5000).then(done) 显式等待完成，则超时完成，但下一个操作不会在该操作之后排队(即打开站点，单击元素，然后5 秒后发生超时日志).

however this does not work as expected (i.e. the console.log never happens). Even when expecting the result from that promise, it does not change the results. If actually explicitly waiting for done by doing timeout(5000).then(done), the timeout completes, but the next action is not queued to after that action (i.e. site gets opened, element clicked, then after 5 seconds the log from timeout happens).

所以我真的很困惑使用控制流来确保一切都按顺序执行以及观察到的行为如何结合在一起的说法.

So I am really confused how the claims of using a control flow to make sure everything gets executed in order and the observed behaviour fit together.

推荐答案

仅仅创建一个 protractor.promise 延迟对象是不够的——你需要告诉控制流.这通常使用 protractor.promise.controlFlow().execute() 来完成.

It's not enough just to create a protractor.promise deferred object - you need to tell the control flow about it. This is usually done with protractor.promise.controlFlow().execute().

function timeout(ms) {
  protractor.promise.controlFlow().execute(function() {
    var deferred = protractor.promise.defer();
    setTimeout(function() {
      console.log('qwer');
      deferred.fulfill(true);
    }, ms);
    return deferred.promise;
  });
}

通读 https://code.google.com/p/selenium/source/browse/javascript/webdriver/promise.js 了解所有幕后的魔法.

Read through https://code.google.com/p/selenium/source/browse/javascript/webdriver/promise.js for all the magic behind the scenes here.

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