何时在 Selenium Webdriver 中使用显式等待与隐式等待? [英] When to use explicit wait vs implicit wait in Selenium Webdriver?
问题描述
我正在使用:
driver.manage().timeouts().implicitlyWait(180, TimeUnit.SECONDS);
但是对于下面的元素它仍然不断失败
But it still fails continuously for the below element
driver.findElement(By.id("name")).clear();
driver.findElement(By.id("name")).sendKeys("Create_title_01");
我添加了等待代码:
for (int second = 0;; second++) {
if (second >= 120) fail("timeout");
try { if (isElementPresent(By.id("name"))) break; } catch (Exception e) {}
Thread.sleep(1000);
}
隐式等待不应该处理直到找到元素吗?如果我使用显式等待而不是我添加的具有 Thread.sleep()
的代码会更好吗?
Shouldn't implicit wait take care of waiting till an element is found?
Also would it be better if I use Explicit wait instead of the code I have added that has Thread.sleep()
?
推荐答案
TL;DR:始终使用显式等待.忘记隐式等待的存在.
TL;DR: Always use explicit wait. Forget that implicit wait exists.
以下是显式等待和隐式等待之间差异的简要概述:
Here is a quick rundown on the differences between explicit and implicit wait:
显式等待:
- 记录和定义的行为.
- 在 selenium 的本地部分运行(以您的代码语言).
- 适用于您能想到的任何条件.
- 返回成功或超时错误.
- 可以将缺少元素定义为成功条件.
- 可以自定义重试和要忽略的异常之间的延迟.
隐式等待:
- 未记录且几乎未定义的行为.
- 在 selenium 的远程部分(控制浏览器的部分)中运行.
- 仅适用于查找元素方法.
- 返回找到的元素或(超时后)未找到的元素.
- 如果检查元素的缺失必须一直等到超时.
- 除全局超时外无法自定义.
带有解释的代码示例.第一个隐式等待:
Code examples with explanation. First implicit wait:
WebDriver driver = new FirefoxDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("http://somedomain/url_that_delays_loading");
WebElement myDynamicElement = driver.findElement(By.id("myDynamicElement"));
现在显式等待:
WebDriver driver = new FirefoxDriver();
driver.get("http://somedomain/url_that_delays_loading");
WebDriverWait wait = new WebDriverWait(driver, 10);
WebElement myDynamicElement = wait.until(
ExpectedConditions.presenceOfElementLocated(By.id("myDynamicElement")));
两个代码示例都做同样的事情.找到某个元素并在 10 秒后未找到则放弃.隐式等待只能做到这一点.它只能尝试找到超时的元素.显式等待的优势在于它可以等待各种条件.还可以自定义超时并忽略某些异常.
Both code examples do the same thing. Find a certain element and give up if not found after 10 seconds. The implicit wait can do only this. It can only try to find an element with a timeout. The strength of explicit wait is that it can wait for all kinds of conditions. Also customize timeout and ignore certain exceptions.
可能的条件示例:elementToBeClickable
、numberOfElementsToBeMoreThan
或 invisibilityOf
.以下是内置预期条件的列表:https://seleniumhq.github.io/selenium/docs/api/java/org/openqa/selenium/support/ui/ExpectedConditions.html
Example of possible conditions: elementToBeClickable
, numberOfElementsToBeMoreThan
or invisibilityOf
. Here is a list of the built in expected conditions: https://seleniumhq.github.io/selenium/docs/api/java/org/openqa/selenium/support/ui/ExpectedConditions.html
更多解释:
隐式等待超时仅对 findElement*
方法有效.如果设置,则所有 findElement*
将在声明无法找到元素之前等待"设置的时间.
The implicit wait timeout has effect only on findElement*
methods. If set then all findElement*
will "wait" for the set time before declaring that the element cannot be found.
findElement*
将如何等待尚未定义.这取决于浏览器或操作系统或 selenium 版本.可能的实现是:
How a findElement*
will wait is not defined. It depends on browser or operating system or version of selenium. Possible implementations are:
- 反复尝试查找元素直到超时.找到元素后立即返回.
- 尝试查找元素.等到超时.再试一次.
- 等到超时.尝试查找元素.
这个列表是通过观察和阅读错误报告以及对 selenium 源代码的粗略阅读收集的.
This list is gathered from observations and reading bug reports and cursory reading of selenium source code.
我的结论:隐式等待是不好的.能力有限.该行为未记录在案且依赖于实现.
My conclusion: Implicit wait is bad. The capabilities are limited. The behaviour is undocumented and implementation dependent.
显式等待可以完成隐式等待可以做的一切,甚至更多.显式等待的唯一缺点是代码更冗长.但是这种冗长使代码变得明确.显式优于隐式.对吗?
Explicit wait can do everything implicit wait can and more. The only disadvantage of explicit wait is a bit more verbose code. But that verbosity makes the code explicit. And explicit is better that implicit. Right?
进一步阅读:
- 官方文档(并没有真正解释除了混合隐式和显式等待发出警告之外,还有其他问题).
- 回答 Jim Evans 提出的相关问题.Jim Evans 是 selenium 的维护者.总结:不要混用隐式等待和显式等待.
- 两篇博文详细解释了隐式和显式等待:
- http://vnrtech.blogspot.de/2013/04/selenium-implicit-wait.html
- http://vnrtech.blogspot.de/2013/04/selenium-explicit-wait.html
- Official documentation (does not really explain the problematic other than warning from mixing implicit and explicit wait).
- Answer on a related question from Jim Evans. Jim Evans is a maintainer of selenium. Summary: don't mix implicit and explicit wait.
- Two blog posts explaining implicit and explicit wait in great detail:
- http://vnrtech.blogspot.de/2013/04/selenium-implicit-wait.html
- http://vnrtech.blogspot.de/2013/04/selenium-explicit-wait.html
- http://code.google.com/p/selenium/问题/详细信息?id=2934
- http://code.google.com/p/selenium/问题/详细信息?id=4471
- http://code.google.com/p/selenium/问题/详细信息?id=7972
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