在硒文档中澄清隐式和显式等待混合的原因 [英] Clarification of the cause of mixing Implicit and Explicit waits in Selenium doc
问题描述
我正在阅读 SeleniumHQ 文档，跨以下语句.
I was reading the SeleniumHQ documentation and came across the following statements.
警告:请勿混合使用隐式等待和显式等待.这样做可能会导致不可预测的等待时间.例如，将隐式等待设置为10s，将显式等待设置为15秒，则可能导致20秒后发生超时."
出于某种原因，我无法理解这一点.对我来说，总超时时间是20秒.谁能解释我是否缺少某些东西?
For some reason, I cannot get this to make sense. The total timeout 20s is the main confusion point for me. Can anyone explain if I am missing something?
编辑
我的问题不是关于混合这些等待的实现/后果.这完全是关于文档中的语句和超时的计算.
第二次编辑
根据以下测试，该文档看起来是正确的. 我仍然需要解释.
Looks like the doc is correct according to the tests below. I still need the explanation though.
using System;
using System.Diagnostics;
using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;
namespace Test
{
[TestFixture]
public class Test
{
private IWebDriver _webDriver;
[Test]
public void ExplicitVsImplicitWaitTest()
{
_webDriver = new ChromeDriver();
_webDriver.Manage().Timeouts().ImplicitlyWait(TimeSpan.FromSeconds(10));
_webDriver.Navigate().GoToUrl("https://www.google.com/");
_webDriver.Manage().Window.Maximize();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
try
{
//new WebDriverWait(_webDriver, TimeSpan.FromSeconds(15)).Until(
//ExpectedConditions.ElementExists(By.CssSelector("Should Fail")));
_webDriver.FindElement(By.CssSelector("Should Fail"));
}
catch ( NoSuchElementException exception)
//catch ( OpenQA.Selenium.WebDriverTimeoutException)
{
stopwatch.Stop();
Console.WriteLine(stopwatch.Elapsed);
}
_webDriver.Quit();
}
}
}
以秒为单位的时间:00:00:10.0167290
Time in second: 00:00:10.0167290
只需明确等待
using System;
using System.Diagnostics;
using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;
using OpenQA.Selenium.Support.UI;
namespace Test
{
[TestFixture]
public class Test
{
private IWebDriver _webDriver;
[Test]
public void ExplicitVsImplicitWaitTest()
{
_webDriver = new ChromeDriver();
//_webDriver.Manage().Timeouts().ImplicitlyWait(TimeSpan.FromSeconds(10));
_webDriver.Navigate().GoToUrl("https://www.google.com/");
_webDriver.Manage().Window.Maximize();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
try
{
new WebDriverWait(_webDriver, TimeSpan.FromSeconds(15)).Until(
ExpectedConditions.ElementExists(By.CssSelector("Should Fail")));
_webDriver.FindElement(By.CssSelector("Should Fail"));
}
//catch ( NoSuchElementException exception)
catch ( OpenQA.Selenium.WebDriverTimeoutException)
{
stopwatch.Stop();
Console.WriteLine(stopwatch.Elapsed);
}
_webDriver.Quit();
}
}
}
以秒为单位的时间:00:00:15.2463079
Time in second: 00:00:15.2463079
混合，显式和隐式都
using System;
using System.Diagnostics;
using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;
using OpenQA.Selenium.Support.UI;
namespace Test
{
[TestFixture]
public class Test
{
private IWebDriver _webDriver;
[Test]
public void ExplicitVsImplicitWaitTest()
{
_webDriver = new ChromeDriver();
_webDriver.Manage().Timeouts().ImplicitlyWait(TimeSpan.FromSeconds(10));
_webDriver.Navigate().GoToUrl("https://www.google.com/");
_webDriver.Manage().Window.Maximize();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
try
{
new WebDriverWait(_webDriver, TimeSpan.FromSeconds(15)).Until(
ExpectedConditions.ElementExists(By.CssSelector("Should Fail")));
_webDriver.FindElement(By.CssSelector("Should Fail"));
}
//catch ( NoSuchElementException exception)
catch ( OpenQA.Selenium.WebDriverTimeoutException)
{
stopwatch.Stop();
Console.WriteLine(stopwatch.Elapsed);
}
_webDriver.Quit();
}
}
}
以秒为单位的时间:00:00:20.5771817
Time in second: 00:00:20.5771817
推荐答案
我的问题不是关于这些等待的实现.这完全是关于文档中的语句和超时的计算.
My question is not about the implementation of those waits. It's entirely about the statements and calculation of timeout on the doc.
但是您必须知道如何实现它们才能理解正在发生的事情.这是两种类型的等待混合使用时发生的情况.我正在跳过对讨论不重要的那些步骤.
But you have to know how they are implemented to make sense of what is going on. Here is what happens with your mix of the two types of waits. I'm kipping over those steps that are not important for the discussion.
-
您的脚本设置了隐式等待.
Your script sets an implicit wait.
您的脚本开始显式等待，以检查元素是否存在.明确的等待通过轮询进行工作.因此，它将命令发送到浏览器以检查元素的存在.
Your script starts an explicit wait checking whether the element exists. The explicit wait works by polling. So it sends a command to the browser to check for the existence of the element.
由于已经设置了隐式等待，发送给浏览器的命令需要10秒钟才能返回失败.
Because of the implicit wait already set, the command sent to the browser takes 10 seconds to return a failure.
您的显式等待将检查它是否已达到其15s的时间限制.当前等待时间为10秒(加上执行脚本，网络延迟等所需的时间极少)，少于15秒.因此，它没有等待完成，而是重新发出与上面的步骤2中相同的命令.
Your explicit wait checks whether it has reached its time limit which is 15s. It is currently at 10s (+ a tiny amount of time taken to execute the script, network latency, etc.) into the wait, which is less then 15s. So it is not done waiting and reissues the same command as in step 2 above.
由于隐式等待，发送到浏览器的命令需要10秒才能返回失败.
Because of the implicit wait, the command sent to the browser takes 10 seconds to return a failure.
当显式等待再次检查其自身的超时时，已经超过15秒，因此超时.
When the explicit wait checks again for its own timeout, more than 15s have elapsed, so it times out.
因此，显式等待轮询两次，每次花费10秒，这意味着总共需要20秒(加上很少的时间来记账).
So the explicit wait polls twice, and each time takes 10 seconds which mean 20 seconds total (plus a tiny amount of time to account for the bookkeeping).
显式等待不执行任何操作来检测和补偿已设置的隐式等待.而且，它不会继续与发送给浏览器的命令并行运行.在执行浏览器命令时，显式等待不会执行任何簿记操作或无法超时.它必须等待浏览器命令完成后才能检查它是否应该超时.
An explicit wait does not do anything to detect and compensate for an implicit wait that has been set. And it does not continue running in parallel to the commands it sends to the browser. While a browser command executes, the explicit wait is not doing any bookkeeping or able to time out. It has to wait for the browser command to complete before checking whether it should time out.
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