你可以在 CSS 中使用 if/else 条件吗? [英] Can you use if/else conditions in CSS?

问题描述

我想在我的 CSS 中使用条件.

I would like to use conditions in my CSS.

这个想法是我有一个变量，当网站运行时我会替换它以生成正确的样式表.

The idea is that I have a variable that I replace when the site is run to generate the right style-sheet.

我想要它根据这个变量改变样式表！

I want it so that according to this variable the style-sheet changes!

看起来像:

[if {var} eq 2 ]
    background-position : 150px 8px;
[else]
    background-position : 4px 8px; 

这可以吗?你是怎么做到的?

Can this be done? How do you do this?

推荐答案

不是传统意义上的，但如果您可以访问 HTML，则可以为此使用类.考虑一下:

Not in the traditional sense, but you can use classes for this, if you have access to the HTML. Consider this:

<p class="normal">Text</p>

<p class="active">Text</p>

在您的 CSS 文件中:

and in your CSS file:

p.normal {
  background-position : 150px 8px;
}
p.active {
  background-position : 4px 8px;
}

这就是 CSS 的方法.

还有像 Sass 这样的 CSS 预处理器.您可以在此处使用 条件，如下所示:p>

Then there are CSS preprocessors like Sass. You can use conditionals there, which'd look like this:

$type: monster;
p {
  @if $type == ocean {
    color: blue;
  } @else if $type == matador {
    color: red;
  } @else if $type == monster {
    color: green;
  } @else {
    color: black;
  }
}

缺点是，您必须对样式表进行预处理，并且条件是在编译时而不是运行时评估的.

Disadvantages are, that you're bound to pre-process your stylesheets, and that the condition is evaluated at compile time, not run time.

CSS 的一个新特性是 自定义属性(又名 CSS 变量).它们在运行时进行评估(在支持它们的浏览器中).

A newer feature of CSS proper are custom properties (a.k.a. CSS variables). They are evaluated at run time (in browsers supporting them).

有了他们，你可以做一些事情:

With them you could do something along the line:

:root {
  --main-bg-color: brown;
}

.one {
  background-color: var(--main-bg-color);
}

.two {
  background-color: black;
}

<小时>

最后，您可以使用您最喜欢的服务器端语言预处理您的样式表.如果你使用 PHP，提供一个 style.css.php 文件，看起来像这样:

---

Finally, you can preprocess your stylesheet with your favourite server-side language. If you're using PHP, serve a style.css.php file, that looks something like this:

p {
  background-position: <?php echo (@$_GET['foo'] == 'bar')? "150" : "4"; ?>px 8px;
}

在这种情况下，您会受到性能影响，因为缓存这样的样式表会很困难.

In this case, you will however have a performance impact, since caching such a stylesheet will be difficult.

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