waitForSelector 通过，但 assertExists 对同一个选择器-前端问题

waitForSelector passes, but assertExists fails for the same selector(waitForSelector 通过，但 assertExists 对同一个选择器失败)

本文介绍了waitForSelector 通过，但 assertExists 对同一个选择器失败的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

当我调用以下函数时，waitForSelector 为选择器"传递，但 assertExists 对同一选择器失败.怎么可能?


When I call the following function, waitForSelector passes for 'selector', but assertExists fails for the same selector. How is it possible?
casper.waitForSelector(selector, function() {
    casper.test.assertExists(selector, sectionName + " opened up successfully.");
}, function() {
    casper.test.fail(sectionName + " did not load in given time");
}, max_timeout);

这里是使用 :nth-child 重现问题的完整示例 选择器.
推荐答案
这是一个已知的错误(参见 #11632，#11737)在 WebKit 的 Qt4 分支中(来自2010).当使用 :nth-child() 或 :nth-of-type() CSS3 选择器时会发生这种情况.选择器第二次运行时，它返回不同的结果(大部分时间是 null).唯一已知的解决方法是使用 XPath 表达式而不是 CSS3 选择器.此错误已在 PhantomJS 2 中修复，因为它使用 WebKit 的 Qt5 分支(版本 538.1).
This is a known bug (see #11632, #11737) in the Qt4 fork of WebKit (from 2010). It happens when :nth-child() or :nth-of-type() CSS3 selectors are used. The second time the selector is run, it returns a different result (most of the time null). The only known workaround is to use XPath expressions instead of CSS3 selectors. This bug is fixed in PhantomJS 2 as it uses a Qt5 fork of WebKit (version 538.1).
这是在 http://example.com 上重现问题的最小脚本(修改自 这里):
This is a minimal script to reproduce the issue on http://example.com (modified from here):
var casper = require('casper').create(),
    x = require('casper').selectXPath;

casper.start('http://example.com', function() {
    var selector = 'p:nth-child(3) > a',
        xpSelector = '//*[local-name()="p" and position()=3]/a';
    var first = this.exists(selector);
    var second = this.exists(selector);
    if(first !== second) {
        console.error('Expected First selector to equal the Second');
    } else {
        console.log('Passed');
    }
    first = this.exists(x(xpSelector));
    second = this.exists(x(xpSelector));
    if(first !== second) {
        console.error('Expected First selector to equal the Second');
    } else {
        console.log('Passed');
    }
}).run();

输出:
标记是:
Markup is:

<h1>文本</h1><p>文本</p><p><a href="url">文本</a></p></div>

<div> <h1>text</h1> <p>text</p> <p><a href="url">text</a></p> </div>

XPath 表达式看起来有点尴尬，因为它直接再现了 CSS 选择器的预期行为.通常人们会写 //p[2]/a.

The XPath expression looks a little awkward, because it directly reproduces the intended behavior of the CSS selector. Normally one would write //p[2]/a.

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