CSS 组合器

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Timo Grossenbacher

Instructor

四种组合器

结构：h2#someid {space|>|+|~} .someclass

space：后代组合器

>：子代组合器

+：相邻兄弟选择器

~：通用兄弟选择器

后代与子代组合器

<html>
  <body>
    <div class = 'first'>
      <a>A link.</a>
      <p>The first paragraph with 
        <a>another link</a>.
      </p>
    </div>
    <div>
      Not an actual paragraph, 
      but with a <a href="#">link</a>.
    </div>
  </body>
</html>

html %>% 
    html_elements('div.first a')

{xml_nodeset (2)}
[1] <a>A link.</a>
[2] <a>another link</a>

html %>% 
    html_elements('div.first > a')

{xml_nodeset (1)}
[1] <a>A link.</a>

兄弟组合器

<html>
  <body>
    <div class = 'first'>
      <a>A link.</a>
      <p>The first paragraph with 
        <a>another link</a>.
      </p>
    </div>
    <div>
      Not an actual paragraph, 
      but with a <a href="#">link</a>.
    </div>
    <p>A paragraph.</p>
  </body>
</html>

html %>% html_elements('div.first + div')

{xml_nodeset (1)}
[1] <div>\n Not an actual...

html %>% html_elements('div.first ~ div')

{xml_nodeset (1)}
[1] <div>\n Not an actual...

html %>% html_elements('div.first ~ *')

{xml_nodeset (2)}
[1] <div>\n Not an actual... [2] <p>A paragraph...

组合器的实用场景

...
    <div id = 'start'>
      <h1 class = 'first'>First</h1>
    </div>
    <div id = 'end'>
      <p class = 'text1'>Some text.</p>
      <p class = 'text2'>More text.</p>
    </div>
...

html %>% html_elements('.text2')

{xml_nodeset (1)}
[1] <p class="text2">More text.</p>

...
    <div>
      <h1>First</h1>
    </div>
    <div>
      <p>Some text.</p>
      <p>More text.</p>
    </div>
...

html %>% html_elements('p + p')

{xml_nodeset (1)}
[1] <p>More text.</p>

让我们来练习！

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