问题:WebDriver click()与JavaScript click()

故事:

在StackOverflow上,我看到用户报告他们无法通过selenium WebDriver“单击”命令单击元素,并且可以通过执行脚本来解决JavaScript单击问题。

Python中的示例:

element = driver.find_element_by_id("myid")
driver.execute_script("arguments[0].click();", element)

WebDriverJS /量角器中的示例:

var elm = $("#myid");
browser.executeScript("arguments[0].click();", elm.getWebElement());

问题:

为什么在常规WebDriver单击不起作用时单击“通过JavaScript”有效?这到底是什么时候发生的,这种解决方法(如果有)的缺点是什么?

我个人使用此变通办法时并未完全理解为什么必须这样做以及它可能导致什么问题。

The Story:

Here on StackOverflow, I’ve seen users reporting that they cannot click an element via selenium WebDriver “click” command and can work around it with a JavaScript click by executing a script.

Example in Python:

element = driver.find_element_by_id("myid")
driver.execute_script("arguments[0].click();", element)

Example in WebDriverJS/Protractor:

var elm = $("#myid");
browser.executeScript("arguments[0].click();", elm.getWebElement());

The Question:

Why is clicking “via JavaScript” works when a regular WebDriver click does not? When exactly is this happening and what is the downside of this workaround (if any)?

I personally used this workaround without fully understanding why I have to do it and what problems it can lead to.


回答 0

当前接受的答案所暗示的相反,关于PhantomJS并没有特定于WebDriver单击和使用JavaScript的区别。

区别

两种方法之间的本质区别是所有浏览器都共有的,可以很简单地解释一下:

  • WebDriver:当WebDriver进行单击时,它会尽可能地尝试模拟真实用户使用浏览器时发生的情况。假设您有一个元素A,它是一个表示“单击我”的按钮,一个元素B是一个div透明的元素,但具有其尺寸和zIndex设置,使其完全覆盖A。然后,您告诉WebDriver单击A。WebDriver将模拟点击,使B 首先获得点击。为什么?因为B涵盖了A,并且如果用户尝试单击A,则B将首先获得该事件。A是否最终会获得click事件取决于B如何处理该事件。无论如何,在这种情况下,WebDriver的行为与真实用户尝试单击A时的行为相同。

  • JavaScript:现在,假设您使用JavaScript来做A.click()这种单击方法不能重现用户尝试单击A时实际发生的情况。JavaScript将click事件直接发送给A,而B将不会获得任何事件。

为什么在WebDriver单击不起作用时JavaScript单击有效?

正如我上面提到的,WebDriver会尽可能地模拟真实用户使用浏览器时发生的情况。事实是,DOM可以包含用户无法与之交互的元素,并且WebDriver不允许您单击这些元素。除了我提到的重叠情况之外,这还意味着无法单击不可见元素。我在“堆栈溢出”问题中看到的一个常见案例是某人试图与DOM中已经存在的GUI元素进行交互,但只有在处理了其他某些元素后才可见。有时在下拉菜单中会发生这种情况:您必须先单击按钮,然后弹出菜单,然后才能选择菜单项。如果有人尝试在菜单可见之前单击菜单项,如果此人随后尝试使用JavaScript进行操作,那么它将起作用,因为事件是直接传递给元素的,而与可见性无关。

什么时候应该使用JavaScript进行点击?

如果您使用Selenium来测试应用程序,那么我对这个问题的回答是“几乎不会”。总的来说,您的Selenium测试应该重现用户对浏览器的操作。以下拉菜单为例:测试应单击首先显示下拉菜单的按钮,然后单击菜单项。如果由于按钮不可见而导致GUI出现问题,或者按钮无法显示菜单项或类似内容,则测试将失败并且您将检测到该错误。如果使用JavaScript单击鼠标,则将无法通过自动测试检测到这些错误。

我说“几乎从不”是因为使用JavaScript可能会有exceptions。但是,它们应该很少见。

如果您使用Selenium 抓取站点,则尝试重现用户行为并不那么重要。因此,使用JavaScript绕过GUI并不是什么大问题。

Contrarily to what the currently accepted answer suggests, there’s nothing specific to PhantomJS when it comes to the difference between having WebDriver do a click and doing it in JavaScript.

The Difference

The essential difference between the two methods is common to all browsers and can be explained pretty simply:

  • WebDriver: When WebDriver does the click, it attempts as best as it can to simulate what happens when a real user uses the browser. Suppose you have an element A which is a button that says “Click me” and an element B which is a div element which is transparent but has its dimensions and zIndex set so that it completely covers A. Then you tell WebDriver to click A. WebDriver will simulate the click so that B receives the click first. Why? Because B covers A, and if a user were to try to click on A, then B would get the event first. Whether or not A would eventually get the click event depends on how B handles the event. At any rate, the behavior with WebDriver in this case is the same as when a real user tries to click on A.

  • JavaScript: Now, suppose you use JavaScript to do A.click(). This method of clicking does not reproduce what really happens when the user tries to click A. JavaScript sends the click event directly to A, and B will not get any event.

Why a JavaScript Click Works When a WebDriver Click Does Not?

As I mentioned above WebDriver will try to simulate as best it can what happens when a real user is using a browser. The fact of the matter is that the DOM can contain elements that a user cannot interact with, and WebDriver won’t allow you to click on these element. Besides the overlapping case I mentioned, this also entails that invisible elements cannot be clicked. A common case I see in Stack Overflow questions is someone who is trying to interact with a GUI element that already exists in the DOM but becomes visible only when some other element has been manipulated. This sometimes happens with dropdown menus: you have to first click on the button the brings up the dropdown before a menu item can be selected. If someone tries to click the menu item before the menu is visible, WebDriver will balk and say that the element cannot be manipulated. If the person then tries to do it with JavaScript, it will work because the event is delivered directly to the element, irrespective of visibility.

When Should You Use JavaScript for Clicking?

If you are using Selenium for testing an application, my answer to this question is “almost never”. By and large, your Selenium test should reproduce what a user would do with the browser. Taking the example of the drop down menu: a test should click on the button that brings up the drop down first, and then click on the menu item. If there is a problem with the GUI because the button is invisible, or the button fails to show the menu items, or something similar, then your test will fail and you’ll have detected the bug. If you use JavaScript to click around, you won’t be able to detect these bugs through automated testing.

I say “almost never” because there may be exceptions where it makes sense to use JavaScript. They should be very rare, though.

If you are using Selenium for scraping sites, then it is not as critical to attempt to reproduce user behavior. So using JavaScript to bypass the GUI is less of an issue.


回答 1

驱动程序执行的单击尝试在JavaScript HTMLElement.click()click事件执行默认操作时,即使元素不可交互,也尽可能模拟真实用户的行为。

不同之处在于:

  • 驱动程序通过将元素滚动到视图中来确保该元素可见,并检查该元素是否可交互

    驱动程序将引发错误:

    • 当点击坐标顶部的元素不是目标元素或后代时
    • 当元素没有正尺寸或完全透明时
    • 当元素是禁用的输入或按​​钮(属性/属性disabledtrue)时
    • 当元素的鼠标指针被禁用(CSS pointer-eventsnone)时


    HTMLElement.click()如果元素被禁用, JavaScript 将始终执行默认操作,或者至多只会静默失败。

  • 如果元素是可聚焦的,则希望驾驶员将其聚焦。

    JavaScript HTMLElement.click()不会。

  • 希望驱动程序像真实用户一样发出所有事件(mousemove,mousedown,mouseup,click等)。

    JavaScript HTMLElement.click()仅发出click事件。该页面可能依赖于这些额外的事件,并且如果未发出这些事件,它们的行为可能会有所不同。

    这些是驱动程序发出的单击Chrome的事件:

    mouseover {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mousemove {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mousedown {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mouseup {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    click {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }

    这是JavaScript注入发出的事件:

    click {target:#topic, clientX:0, clientY:0, isTrusted:false, ... }
  • JavaScript发出的事件.click() 不受信任,并且默认操作不能被调用:

    https://developer.mozilla.org/zh/docs/Web/API/Event/isTrusted
    https://googlechrome.github.io/samples/event-istrusted/index.html

    请注意,某些驱动程序仍在生成不受信任的事件。从2.1版开始,PhantomJS就是这种情况。

  • JavaScript发出的事件.click() 没有click的坐标

    属性clientX, clientY, screenX, screenY, layerX, layerY设置为0。该页面可能依赖于它们,并且可能会有所不同。


可以使用JavaScript .click()抓取一些数据,但这不是在测试环境中。由于它无法模拟用户的行为,因此无法达到测试的目的。因此,如果来自驱动程序的单击失败,则实际用户很可能也将在相同条件下无法执行相同的单击。


是什么使驱动程序在我们期望成功时无法单击元素?

  • 由于延迟或过渡效果,目标元素尚不可见/不可交互。

    一些例子 :

    https://developer.mozilla.org/fr/docs/Web(下拉导航菜单) http://materializecss.com/side-nav.html(下拉侧栏)

    工作环境:

    添加服务员以等待可见性,最小尺寸或稳定位置:

    // wait visible
    browser.wait(ExpectedConditions.visibilityOf(elem), 5000);
    
    // wait visible and not disabled
    browser.wait(ExpectedConditions.elementToBeClickable(elem), 5000);
    
    // wait for minimum width
    browser.wait(function minimumWidth() {
        return elem.getSize().then(size => size.width > 50);
    }, 5000);

    重试单击直到成功:

    browser.wait(function clickSuccessful() {
        return elem.click().then(() => true, (ex) => false);
    }, 5000);

    添加与动画/过渡的持续时间匹配的延迟:

    browser.sleep(250);


  • 目标元素最终被浮动元素覆盖一旦滚动到视图中:

    驱动程序自动将元素滚动到视图中以使其可见。如果页面包含浮动/粘滞元素(菜单,广告,页脚,通知,Cookie策略..),则该元素可能最终被覆盖,将不再可见/不可交互。

    示例:https//twitter.com/?lang = zh-CN

    解决方法:

    将窗口的大小设置为较大的大小,以避免滚动或浮动元素。

    Y将鼠标移到具有负偏移量的元素上,然后单击它:

      browser.actions()
         .mouseMove(elem, {x: 0, y: -250})
         .click()
         .perform();

    单击之前,将元素滚动到窗口的中心:

    browser.executeScript(function scrollCenter(elem) {
      var win = elem.ownerDocument.defaultView || window,
        box = elem.getBoundingClientRect(),
        dy = box.top - (win.innerHeight - box.height) / 2;
      win.scrollTo(win.pageXOffset, win.pageYOffset + dy);
    }, element);
    
    element.click();

    如果无法避免,请隐藏浮动元素:

    browser.executeScript(function scrollCenter(elem) {
      elem.style.display = 'none';
    }, element);

The click executed by the driver tries to simulate the behavior of a real user as close as possible while the JavaScript HTMLElement.click() performs the default action for the click event, even if the element is not interactable.

The differences are:

  • The driver ensures that the element is visible by scrolling it into the view and checks that the element is interactable.

    The driver will raise an error:

    • when the element on top at the coordinates of the click is not the targeted element or a descendant
    • when the element doesn’t have a positive size or if it is fully transparent
    • when the element is a disabled input or button (attribute/property disabled is true)
    • when the element has the mouse pointer disabled (CSS pointer-events is none)


    A JavaScript HTMLElement.click() will always perform the default action or will at best silently fail if the element is a disabled.

  • The driver is expected to bring the element into focus if it is focusable.

    A JavaScript HTMLElement.click() won’t.

  • The driver is expected to emit all the events (mousemove, mousedown, mouseup, click, …) just like like a real user.

    A JavaScript HTMLElement.click() emits only the click event. The page might rely on these extra events and might behave differently if they are not emitted.

    These are the events emitted by the driver for a click with Chrome:

    mouseover {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mousemove {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mousedown {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    mouseup {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    click {target:#topic, clientX:222, clientY:343, isTrusted:true, ... }
    

    And this is the event emitted with a JavaScript injection:

    click {target:#topic, clientX:0, clientY:0, isTrusted:false, ... }
    
  • The event emitted by a JavaScript .click() is not trusted and the default action may not be invoked:

    https://developer.mozilla.org/en/docs/Web/API/Event/isTrusted
    https://googlechrome.github.io/samples/event-istrusted/index.html

    Note that some of the drivers are still generating untrusted events. This is the case with PhantomJS as of version 2.1.

  • The event emitted by a JavaScript .click() doesn’t have the coordinates of the click.

    The properties clientX, clientY, screenX, screenY, layerX, layerY are set to 0. The page might rely on them and might behave differently.


It may be ok to use a JavaScript .click() to scrap some data, but it is not in a testing context. It defeats the purpose of the test since it doesn’t simulate the behavior of a user. So, if the click from the driver fails, then a real user will most likely also fail to perform the same click in the same conditions.


What makes the driver fail to click an element when we expect it to succeed?

  • The targeted element is not yet visible/interactable due to a delay or a transition effect.

    Some examples :

    https://developer.mozilla.org/fr/docs/Web (dropdown navigation menu) http://materializecss.com/side-nav.html (dropdown side bar)

    Workarrounds:

    Add a waiter to wait for the visibility, a minimum size or a steady position :

    // wait visible
    browser.wait(ExpectedConditions.visibilityOf(elem), 5000);
    
    // wait visible and not disabled
    browser.wait(ExpectedConditions.elementToBeClickable(elem), 5000);
    
    // wait for minimum width
    browser.wait(function minimumWidth() {
        return elem.getSize().then(size => size.width > 50);
    }, 5000);
    

    Retry to click until it succeeds :

    browser.wait(function clickSuccessful() {
        return elem.click().then(() => true, (ex) => false);
    }, 5000);
    

    Add a delay matching the duration of the animation/transition :

    browser.sleep(250);
    


  • The targeted element ends-up covered by a floating element once scrolled into the view:

    The driver automatically scrolls the element into the view to make it visible. If the page contains a floating/sticky element (menu, ads, footer, notification, cookie policy..), the element may end-up covered and will no longer be visible/interactable.

    Example: https://twitter.com/?lang=en

    Workarounds:

    Set the size of the window to a larger one to avoid the scrolling or the floating element.

    Mover over the element with a negative Y offset and then click it:

      browser.actions()
         .mouseMove(elem, {x: 0, y: -250})
         .click()
         .perform();
    

    Scroll the element to the center of the window before the click:

    browser.executeScript(function scrollCenter(elem) {
      var win = elem.ownerDocument.defaultView || window,
        box = elem.getBoundingClientRect(),
        dy = box.top - (win.innerHeight - box.height) / 2;
      win.scrollTo(win.pageXOffset, win.pageYOffset + dy);
    }, element);
    
    element.click();
    

    Hide the floating element if it can’t be avoided:

    browser.executeScript(function scrollCenter(elem) {
      elem.style.display = 'none';
    }, element);
    

回答 2

注意:我们将“点击”称为最终用户点击。“ js click”是通过JS点击的

为什么在常规WebDriver单击不起作用时单击“通过JavaScript”有效?

有两种情况会发生这种情况:

I. 如果您正在使用PhamtomJS

这是的最常见的已知行为PhantomJS。例如,某些元素有时不可单击<div>。这是因为PhantomJS最初是用来模拟浏览器引擎的(例如初始HTML + CSS->计算CSS->呈现)。但这并不意味着要以最终用户的方式进行交互(查看,单击,拖动)。因此PhamtomJS,最终用户交互仅部分支持。

JS为什么点击工作?至于任一点击,它们都是平均点击。它就像带有1个枪管2个扳机的枪。视口中的一个,JS中的一个。由于可以PhamtomJS很好地模拟浏览器的引擎,因此JS单击应该可以很好地工作。

二。“ click”的事件处理程序必须在糟糕的时间内绑定。

例如,我们得到了一个 <div>

  • ->我们做一些计算

  • ->然后将click事件绑定到<div>

  • ->加上一些错误的角度编码(例如,不能正确处理示波器的周期)

我们可能最终得到相同的结果。单击将不起作用,因为WebdriverJS在没有单击事件处理程序的情况下会尝试单击该元素。

JS为什么点击工作?JS单击就像将JS直接注入浏览器。可能有2种方式,

拳头是通过devtools控制台(是的,WebdriverJS确实与devtools的控制台进行通信)。

其次是将<script>标记直接注入HTML。

对于每个浏览器,其行为将有所不同。但是无论如何,这些方法比单击按钮更复杂。Click正在使用已经存在的内容(最终用户单击),js click正在通过后门。

对于js,单击将显示为异步任务。这与一个复杂的主题“ 浏览器异步任务和CPU任务调度 ”相关(一会儿再读也找不到文章)。简而言之,这主要是因为js click需要等待CPU任务调度周期,并且在click事件绑定后运行速度会变慢。 (当您发现元素有时可单击,有时却不可单击时,您可能会知道这种情况。)

这到底是什么时候发生的,这种解决方法(如果有)的缺点是什么?

=>如上所述,两者都意味着一个目的,但是关于使用哪个入口:

  • 点击:使用默认情况下提供的浏览器。
  • JS click:正在通过后门。

=>对于性能,很难说,因为它依赖于浏览器。但通常:

  • 单击:并不意味着更快,而只是在CPU执行任务的计划列表中签名更高的位置。
  • JS单击:并不意味着速度较慢,而仅是它登录到CPU任务计划列表的最后位置。

=>缺点:

  • 点击:除了您使用PhamtomJS之外,似乎没有其他缺点。
  • JS单击:对健康非常不利。您可能不小心单击了视图中没有的内容。使用此功能时,请确保该元素在那里并且可供查看,然后单击以作为最终用户的观点。

PS,如果您正在寻找解决方案。

  • 使用PhantomJS?我建议改用无头的Chrome。是的,您可以在Ubuntu上无头设置Chrome。事物的运行方式与Chrome一样,但它没有视图,并且像PhantomJS一样没有虫子。
  • 不使用PhamtomJS但仍然有问题吗?我建议使用带有browser.wait()(分度器的ExpectedCondition)(检查此以获得更多信息

(我想简短一点,但结局很糟。与理论有关的任何事情都难以解释…)

NOTE: let’s call ‘click’ is end-user click. ‘js click’ is click via JS

Why is clicking “via JavaScript” works when a regular WebDriver click does not?

There are 2 cases for this to happen:

I. If you are using PhamtomJS

Then this is the most common known behavior of PhantomJS . Some elements are sometimes not clickable, for example <div>. This is because PhantomJS was original made for simulating the engine of browsers (like initial HTML + CSS -> computing CSS -> rendering). But it does not mean to be interacted with as an end user’s way (viewing, clicking, dragging). Therefore PhamtomJS is only partially supported with end-users interaction.

WHY DOES JS CLICK WORK? As for either click, they are all mean click. It is like a gun with 1 barrel and 2 triggers. One from the viewport, one from JS. Since PhamtomJS great in simulating browser’s engine, a JS click should work perfectly.

II. The event handler of “click” got to bind in the bad period of time.

For example, we got a <div>

  • -> We do some calculation

  • -> then we bind event of click to the <div>.

  • -> Plus with some bad coding of angular (e.g. not handling scope’s cycle properly)

We may end up with the same result. Click won’t work, because WebdriverJS trying to click on the element when it has no click event handler.

WHY DOES JS CLICK WORK? Js click is like injecting js directly into the browser. Possible with 2 ways,

Fist is through devtools console (yes, WebdriverJS does communicate with devtools’ console).

Second is inject a <script> tag directly into HTML.

For each browser, the behavior will be different. But regardless, these methods are more complicating than clicking on the button. Click is using what already there (end-users click), js click is going through backdoor.

And for js click will appear to be an asynchronous task. This is related a with a kinda complex topic of ‘browser asynchronous task and CPU task scheduling‘ (read it a while back can’t find the article again). For short this will mostly result as js click will need to wait for a cycle of task scheduling of CPU and it will be ran a bit slower after the binding of the click event. (You could know this case when you found the element sometimes clickable, sometimes not. )

When exactly is this happening and what is the downside of this workaround (if any)?

=> As mention above, both mean for one purpose, but about using which entrance:

  • Click: is using what providing by default of browser.
  • JS click: is going through backdoor.

=> For performance, it is hard to say because it relies on browsers. But generally:

  • Click: doesn’t mean faster but only signed higher position in schedule list of CPU execution task.
  • JS click: doesn’t mean slower but only it signed into the last position of schedule list of CPU task.

=> Downsides:

  • Click: doesn’t seem to have any downside except you are using PhamtomJS.
  • JS click: very bad for health. You may accidentally click on something that doesn’t there on the view. When you use this, make sure the element is there and available to view and click as the point of view of end-user.

P.S. if you are looking for a solution.

  • Using PhantomJS? I will suggest using Chrome headless instead. Yes, you can set up Chrome headless on Ubuntu. Thing runs just like Chrome but it only does not have a view and less buggy like PhantomJS.
  • Not using PhamtomJS but still having problems? I will suggest using ExpectedCondition of Protractor with browser.wait() (check this for more information)

(I want to make it short, but ended up badly. Anything related with theory is complicated to explain…)


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