1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
|
---
title: env()
slug: Web/CSS/env()
tags:
- CSS
- env
- env()
- iPhoneX
translation_of: Web/CSS/env()
---
<div>{{CSSRef}}</div>
<div> <span class="seoSummary"><strong><code>env()</code></strong> <a href="/en-US/docs/Web/CSS">CSS</a> </span>函数以类似于 <span class="seoSummary">{{cssxref("var")}} </span>函数和 <span class="seoSummary"><a href="/en-US/docs/Web/CSS/--*">custom properties</a> </span>的方式将用户代理定义的环境变量值插入你的 CSS 中。区别在于，环境变量除了由用户代理定义而不是由用户定义外，还被全局作用在文档中，而自定义属性则限定在声明它们的元素中。</div>
<div><span style="font-size: 1rem; letter-spacing: -0.00278rem;">为了告诉浏览器使用屏幕上所有的可用空间，并以此使用</span><code style="font-size: 1rem; letter-spacing: -0.00278rem;">env()</code><span style="font-size: 1rem; letter-spacing: -0.00278rem;">变量，我们需要添加一个新的视</span><span style="font-size: 1rem; letter-spacing: -0.00278rem;">口元值：</span></div>
<div class="highlight highlight-text-html-basic">
<pre><<span class="pl-ent">meta</span> <span class="pl-e">name</span>=<span class="pl-s"><span class="pl-pds">"</span>viewport<span class="pl-pds">"</span></span> <span class="pl-e">content</span>=<span class="pl-s"><span class="pl-pds">"</span>... <strong>viewport-fit=cover</strong><span class="pl-pds">"</span></span>></pre>
</div>
<pre class="brush: css">body {
padding:
env(safe-area-inset-top, 20px)
env(safe-area-inset-right, 20px)
env(safe-area-inset-bottom, 20px)
env(safe-area-inset-left, 20px);
}</pre>
<p>另外，与自定义属性不同，自定义属性不能在声明之外使用，而<code>env()</code>函数可以代替属性值或描述符的任何部分（例如，在 <a href="/en-US/docs/Web/CSS/@media">媒体查询的规则</a> 中）。 随着规范的发展，它也可能在像是 选择器 等其他地方使用。</p>
<p>最初由iOS浏览器提供，用于允许开发人员将其内容放置在视口的安全区域中，该规范中定义的<code>safe-area-inset-* </code>值可用于确保内容即使在非矩形的视区中也可以完全显示。</p>
<h2 id="语法">语法</h2>
<pre class="brush: css">/* Using the four safe area inset values with no fallback values */
env(safe-area-inset-top);
env(safe-area-inset-right);
env(safe-area-inset-bottom);
env(safe-area-inset-left);
/* Using them with fallback values */
env(safe-area-inset-top, 20px);
env(safe-area-inset-right, 1em);
env(safe-area-inset-bottom, 0.5vh);
env(safe-area-inset-left, 1.4rem);
</pre>
<h3 id="Values">Values</h3>
<dl>
<dt><code id="safe-area-inset-top" style="white-space: nowrap;">safe-area-inset-top</code>, <code id="safe-area-inset-right" style="white-space: nowrap;">safe-area-inset-right</code>, <code id="safe-area-inset-bottom" style="white-space: nowrap;">safe-area-inset-bottom</code>, <code id="safe-area-inset-left" style="white-space: nowrap;">safe-area-inset-left</code></dt>
<dd><code style="white-space: nowrap;">safe-area-inset-*</code>由四个定义了视口边缘内矩形的 top, right, bottom 和 left 的环境变量组成，这样可以安全地放入内容，而不会有被非矩形的显示切断的风险。对于矩形视口，例如普通的笔记本电脑显示器，其值等于零。 对于非矩形显示器（如圆形表盘，iPhoneX屏幕），在用户代理设置的四个值形成的矩形内，所有内容均可见。</dd>
</dl>
<p><strong>注意</strong>: 不同于其他的 CSS 属性，用户代理定义的属性名字对大小写敏感。</p>
<h3 id="Formal_syntax">Formal syntax</h3>
{{CSSSyntax}}
<h2 id="Examples">Examples</h2>
<p>下面的示例使用<code>env()</code>的第二个可选参数，如果环境变量不可用，该参数可让您设置备用值</p>
<pre class="brush: html"><p>
If the <code>env()</code> function is supported in your browser,
this paragraph’s text will have 50px of padding between it and
the left border — but not the top, right and bottom.
This is because the accompanying CSS is the equivalent of
<code>padding: 0 0 0 50px</code>, because, unlike other CSS
properties, user agent property names are case-sensitive.
</p></pre>
<pre class="brush: css">p {
width: 300px;
border: 2px solid red;
padding:
env(safe-area-inset-top, 50px)
env(safe-area-inset-right, 50px)
env(safe-area-inset-bottom, 50px)
env(SAFE-AREA-INSET-LEFT, 50px);
}</pre>
<p>{{EmbedLiveSample("Examples")}}</p>
<h3 id="Example_values">Example values</h3>
<pre class="brush: css">padding: env(safe-area-inset-bottom, 50px); /* zero for all rectangular user agents */
padding: env(Safe-area-inset-bottom, 50px); /* 50px because UA properties are case sensitive */
padding: env(x, 50px 20px); /* as if padding: '50px 20px' were set because x is not a valid environment variable */
padding: env(x, 50px, 20px); /* ignored because '50px, 20px' is not a valid padding value and x is not a valid environment variable */
</pre>
<p>向下兼容的语法和自定义属性一样，允许使用逗号。但是如果属性值不支持逗号，则该值无效。</p>
<p><strong>Note</strong>: User agent properties are not reset by the <a href="/en-US/docs/Web/CSS/all">all</a> property.</p>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<thead>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName("CSS3 Environment Variables", "#env-function", "env()")}}</td>
<td>{{Spec2("CSS3 Environment Variables")}}</td>
<td>Initial definition.</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{Compat("css.properties.custom-property.env")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li>{{CSSxRef("var", "var(…)")}}</li>
<li><a href="/en-US/docs/Web/CSS/CSS_Variables">CSS Custom Properties for Cascading Variables</a></li>
<li><a href="/en-US/docs/Web/CSS/--*">Custom Properties (--*)</a></li>
<li><a href="/en-US/docs/Web/CSS/Using_CSS_variables">Using CSS custom properties (variables)</a></li>
<li>{{CSSxRef("@viewport/viewport-fit", "viewport-fit (@viewport)")}}</li>
</ul>
|
