CSS实现垂直居中

在常规的网页中,宽一般有最大限制,而高没有,因而CSS规范并没有给出块级元素和多行文字垂直居中,建议性的实现方案。换句话说,但凡有垂直居中的要求,必然是设计者对包含块的高做了固定限制。

默认的CSS块级元素的高由内容决定,通常内容有多少,高度就会算多少。固定了高意味着设计者必须确认内容的高度不会超过限制,否则会产生不合预期的结果。

众所周知,限制高仅需要指定height样式属性即可。

确认内容高度需要了解内容的组成,可能会是一张可替换的图片,也可能会是一段行数不确定的文本,甚至也可能是另外一个块级元素,每种内容有各自的模型在控制着自己的展示。

情形1: 垂直居中一张图片

图片的模型最简单,行内元素,限制高,宽会等比例缩放。假设要在高度为200px的块中放置一个高度不超过100px的图片,垂直居中可以这样写:

#middleWrapper {
  height: 200px;
  line-height: 200px;
}
#middleWrapper > img {
  vertical-align: middle;
  max-height: 100px;
}
<div class="middleWrapper">
  <img src="http://mjm1990.com/wp-content/uploads/2015/01/violin.jpg">
</div>

上面的代码中重要之处在于line-height恰好等于height,这样整个包含块就变成了一行,在单行内居中行内元素,只需要指定vertical-align是middle即可。

垂直居中一张图片的应用场景很多,比如一个带有标志图片的按钮,再比如购物网站商品页上商品的图片展示(固定的方框内放各种不同尺寸的图片)等。

情形2: 垂直居中一行

这种情况与情形1类似,不同之处在于文字如果一行放不下会换行,当换出新的一行,文字便超出了包含块,这时通常的处理手法是将超出部分隐藏(overflow: hidden)。更加普遍的做法是设置样式属性white-space为nowrap不允许文字换行,并且将超出部分换用三个点代替(text-overflow: ellipsis)。下面的例子中,将内容超过一行的文本,垂直居中地放在高200px的包含块中,并且只显示一行。

#middleWrapper {
  height: 200px;
  line-height: 200px;
  text-overflow: ellipsis;
  overflow: hidden;
}
#middleWrapper > span {
  vertical-align: middle;
  white-space: nowrap;
}
<div class="middleWrapper">
  <span>It is a truth universally acknowledged that a single man in possession of a good   fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.</span>
</div>
It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.

注意white-space属性可以放在包含块中或是直接放在文本标签中均可,因为这个属性允许继承,并且只能作用在文本上。两个overflow属性却要放在包含块中,用户代理需要根据包含块的高度进行计算隐藏掉那一部分文字。值得一提的是text-overflow属性是CSS3中加入的新内容,主流浏览器都支持,但也有些例外,比如UC浏览器的Android手机版。

垂直居中一行文字的应用场景包括按钮上的文字,水平导航栏中的文字等等。

情形3: 垂直居中几行文字

如果要垂直居中的文字行数是确定的,并且包含块框的高度恰好等于总行框的高度,那么只需要将包含块的高度设置为总行框高度即可。例如有两行,字体13px,每行行高17px,那么包含块的高度为34px。

这种情形最常见的应用场景是移动购物网站,浏览页商品描述的格式。这种场景下往往也要求能够隐藏多出来的行,以及将多出的部分替换为三个点。这种做法,纯粹的CSS无法胜任,只能计算出行宽,估算出展示的字符数用JS脚本控制。说估算是由于英文字体多数不是等宽,如果无法知道每个字母的宽度,难以精确地算出要展示的字符数目。

情形4: 垂直居中一段文字

当一段文字行数不确定,且包含块的高度一定大于总行框高度时,需要将整段文字放在一个不限制高的行内块级元素中,像下面的写法一样:

#middleWrapper {
  height: 200px;
  line-height: 200px;
}
#middleWrapper > div {
  vertical-align: middle;
  display: inline-block;
  line-height: 1.5;
  text-align: justify;
}
<div class="middleWrapper">
  <div>It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.</div>
</div>
It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.

不同于单行元素的居中,多行元素的line-height需要重置成较小的值,上面例子中取了文字的1.5倍,远小于父级元素的200px。将块级元素变成行内块级元素也是关键。如果仍旧只是块级元素,即使充值了line-height也不会生效,因为块级元素的一行会去和父级包含块的一行去居中对齐。

这种情况的应用场景极少遇到,恐怕只有按照列为主题布局时,才可能需要。

另外有一种通行的做法是将要居中的内容放置在一个单元格中,这种方案可以应对以上所有情况。

#middleWrapper {
  height: 200px;
  line-height: 1.5;
  vertical-align: middle;
  display: table-cell;
}
<div class="middleWrapper">
It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.
</div>
It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife. However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.

此外,还有一些方法也可行,但显得不够优雅,比如设定上下的padding为某个固定值可以应对某些情形等。

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