代码编织梦想

一、BFC的理解和应用

1.BFC的概念?

Block Format Context 块级格式化上下⽂,⼀块独⽴的渲染区域,内部元素的渲染不会影响边界以外的元素。

 形成BFC的常⻅条件:

  • float的值不等于 none;
  • position值是 absolute或 fixed;
  • overflow不是 visible,( overflow:hidden);
  • display的值为 inline-block、table-cell;
  • body根元素

 如何应用?

  • (BFC可以包含浮动元素)清除浮动;
  • 同⼀个BFC下,垂直外边距会发⽣重叠;

 二、margin外边距重叠及解决⽅法

块的上外边距(margin-top)和下外边距(margin-bottom)有时合并(折叠)为单个边距,其⼤⼩为单个边距的最⼤值 (或如果它们相等,则仅为其中⼀个),这种⾏为称为边距重叠。

1.哪些元素会发⽣外边距重叠问题

(1)情况一:相邻兄弟元素的marin-bottom和margin-top的值发⽣重叠

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.bfcCls {
            width: 300px;
            height: 300px;
            border: 1px solid black;

        }

        div.bfcCls div.first {
            width: 100px;
            height: 100px;
            background-color:aqua;
            margin-bottom: 50px;
        }

        div.bfcCls div.second {
            width: 100px;
            height: 100px;
            background-color:orchid;
            margin-top: 80px;
            float: left;
        }
    </style>
</head>
<body>
    <div class="bfcCls">
        <div class="first">
        </div>
        <div class="second">
        </div>
    </div>
</body>
<script>
</script>
</html>

(2) 情况二:⽗级和第⼀个/最后⼀个⼦元素的margin合并

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.bfcCls {
            width: 300px;
            height: 300px;
            background-color:blue;
            margin-top: 50px;
            padding: 1px;
            box-sizing: border-box;
          
        }

        div.bfcCls div.first {
            width: 100px;
            height: 100px;
            background-color:darkgreen;
            margin-top: 50px;
        }
    </style>
</head>
<body>
    <div class="bfcCls">
        <div class="first">
        </div>
    </div>
</body>
<script>

</script>

</html>

 

(3)情况三:空的块级元素

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.bfcCls {
            width: 500px;
            height: 500px;
            background-color:coral;
        }

        div.bfcCls div.first {
            width: 100px;
            height: 100px;
            background-color:hotpink;
            margin-bottom: 50px;
        }

        div.bfcCls div.second {
            margin-top: 20px;
            margin-bottom: 20px;
            border:1px solid turquoise;
            /* padding: 1px; */
            box-sizing: border-box;
        }

        div.bfcCls div.third {
            width: 100px;
            height: 100px;
            background-color: yellowgreen;
            margin-top: 50px;
        }
    </style>
</head>

<body>

    <div class="bfcCls">
        <div class="first">
        </div>
        <div class="second">
        </div>
        <div class="third">
        </div>
    </div>

</body>
<script>

</script>

</html>

(4)情况四:⾼度为auto的⽗元素的margin-bottom和⼦元素的margin-bottom发⽣重叠 

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.bfcCls {
            width: 500px;
            background-color:aqua;
            margin-bottom: 100px;
            /* height: 200px; */
            border-bottom: 1px solid red;
            /* padding-bottom: 1px; */
        }

        div.bfcCls div.first {
            width: 100px;
            height: 100px;
            background-color: green;
            margin-bottom: 50px;
        }
    </style>
</head>

<body>

    <div class="bfcCls">
        <div class="first">
        </div>
    </div>

</body>
<script>

</script>

</html>

 2.BFC可以阻⽌元素被浮动元素覆盖

float布局:就是所说的浮动,浮动之后会让元素脱离⽂档流。

需求:不能让浮动元素遮挡未浮动的元素;

解决办法:让未浮动元素的左右两侧不能出现浮动元素,因此要清除浮动。

 

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        .boxCls {
            width: 500px;
            height: 500px;
            background-color: red;
        }
        .boxCls div.first{
            width: 100px;
            height: 100px;
            background-color: yellow;
            float: left;
        }
        .boxCls div.second{
            width: 100px;
            height: 100px;
            background-color: black;
            float: left;
        }
        .boxCls div.third{
            width: 100px;
            height: 100px;
            background-color: blue;
            float: right;
        }
        .boxCls div.fourth {
            width: 300px;
            height: 200px;
            background-color: green;
            clear: both;
        }

    </style>
</head>
<body>
    <div class="boxCls">
        <div class="first">

        </div>
        <div class="second">

        </div>
        <div class="third">

        </div>
        <div class="fourth">
            无敌大铁锤 Look Here!
        </div>
    </div>
</body>
</html>

 

二、 用定位进行三栏布局

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        html,
        body {
            width: 100%;
            height: 100%;
        }


        div.boxCls {
            height: 100%;
            width: 100%;
            position: relative;
        }

        div.center {
            background-color: red;
            height: 100%;
            position: absolute;
            left: 200px;
            right: 200px;
        }

        div.left {
            background-color: blue;
            width: 200px;
            height: 100%;
            position: absolute;
            left: 0;
        }

        div.right {
            background-color: green;
            width: 200px;
            height: 100%;
            position: absolute;
            right: 0;
        }
    </style>
</head>

<body>
    <div class="boxCls">
        <div class="column left">
        </div>
        <div class="column center">
            <h1 style="display:flex;justify-content:center;">无敌大铁锤</h1>
        </div>
        <div class="column right">
        </div>
    </div>

</body>

</html>

 三、如何实现圣杯布局和双⻜翼布局?

  •  三栏布局,中间⼀栏最先加载和渲染;
  • 两侧内容固定,中间⾃适应;

 (1)圣杯布局:使⽤float,两侧使⽤ margin的负值,以便和中间内容横向重叠,防⽌中间内容被两侧覆盖;

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>圣杯布局</title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        html,
        body {
            width: 100%;
            height: 100%;
        }

        body {
            padding-left: 200px;
            padding-right: 200px;
            box-sizing: border-box;
            min-width: 700px;
        }

        div.column {
            text-align: center;
            height: 100%;
            float: left;
        }

        div.center {
            background-color: red;
            width: 100%;
        }

        div.left {
            background-color: blue;
            width: 200px;
            margin-left: -100%;
            position: relative;
            left: -200px;
        }

        div.right {
            background-color: green;
            width: 200px;
            margin-right: -200px;
        }
    </style>
</head>

<body>

    <div class="column center">

    </div>
    <div class="column left">

    </div>
    <div class="column right">

    </div>
</body>

</html>

 (2)双⻜翼布局:

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>圣杯布局</title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        html,
        body {
            width: 100%;
            height: 100%;
        }

        div.column {
            text-align: center;
            height: 100%;
            float: left;
        }

        div.mainCls {
            background-color: red;
            width: 100%;
        }

        div.mainCls .center {
            background-color: aqua;
            margin-left: 200px;
            margin-right: 200px;
            height: 100%;
        }

        div.left {
            background-color: blue;
            width: 200px;
            margin-left: -100%;
        }

        div.right {
            background-color: green;
            width: 200px;
            margin-left: -200px;
        }

        div.footer {
            background-color: gainsboro;
            text-align: center;
            
        }
        div.clearfix::before {
            content: "";
            display: block;
            clear: both;
        }
    </style>
</head>

<body>

    <div class="column mainCls">
        <div class="center">

        </div>
    </div>

    <div class="column left">

    </div>
    <div class="column right">

    </div>
    
    <div class='clearfix footer'>
       这是页脚
    </div>
</body>

</html>

 四、flex布局

Flex 是 Flexible Box 的缩写,意为“弹性布局”或者“弹性盒⼦”,是 CSS3 中的⼀种新的布局模式,可以简便、响应式地实现各种⻚⾯布局,当⻚⾯需要适应不同的屏幕⼤⼩以及设备类型时⾮常适⽤。⽬前,⼏乎所有的浏览器都⽀持 Flex 布局。

采⽤ Flex 布局的元素,称为 Flex 容器(flex container),简称“容器”。它的所有⼦元素⾃动成为容器成员,称为 Flex 项⽬(flex item),简称“项⽬”。容器默认存在两根轴,分别为⽔平的主轴(main axis)和垂直的交叉轴 (cross axis)。主轴的开始位置叫做 main start,结束位置叫做 main end;交叉轴的开始位置叫做 cross start,结束位置叫做 cross end。项⽬默认沿主轴排列。单个项⽬占据的主轴空间叫做 main size,占据的交叉轴 空间叫做 cross size。

 

1.对齐方式

(1)flex-direction:指定弹性盒⼦中⼦元素的排列⽅式

 

(2) flex-wrap: 属性⽤来设置当弹性盒⼦的⼦元素(项⽬)超出⽗容器时是否换⾏

 默认值:no wrap,项⽬根据容器⾃动调整,如果想⾃适应,设置该值为wrap;

 

 (3)justify-content:设置弹性盒⼦中项⽬在主轴(横轴)⽅向上的对⻬⽅式

 

 

 (4)align-items:⽤来设置弹性盒⼦中项⽬在侧轴(纵轴)⽅向上的对⻬⽅式

 

注:  默认值:stetch,【不能为项⽬设置⾼度】;

项目属性:

1> order:number ⽤来设置项⽬在容器中出现的顺序,您可以通过具体的数值来定义项⽬在容器中的位置;

 2>align-self:允许您为某个项⽬设置不同于其它项⽬的对⻬⽅式,该属性可以覆盖 align-items 属性的值;

 3>flex:是 flex-grow、flex-shrink 和 flex-basis 三个属性的简写;

  • flex-grow:(必填参数)⼀个数字,⽤来设置项⽬相对于其他项⽬的增⻓量,默认值为 0;
  • flex-shrink:(选填参数)⼀个数字,⽤来设置项⽬相对于其他项⽬的收缩量,默认值为 1;
  • flex-basis:(选填参数)项⽬的⻓度,合法值为 auto(默认值,表示⾃动)、inherit(表示从⽗元素继承该 属性的值) 或者以具体的值加 "%"、"px"、"em" 等单位的形式;

 2.定位

absolute和relative分别依据什么来定位?

(1)相对定位

相对定位:相对于本身进⾏定位,原来的位置仍然存在;

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <meta http-equiv="X-UA-Compatible" content="IE=edge">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>Document</title>
 <style type="text/css">
 .parent{
 height: 200px;
 width: 100%;
 background-color: rebeccapurple;
 }
 div.parent div.first {
 width: 100px;
 height: 100px;
 display: inline-block;
 background-color: yellow;
 /*相对定位*/
 position: relative;
 left: 50px
 }
 div.parent div.second {
 width: 100px;
 height: 100px;
 background-color: gray;
 display: inline-block;
 }
 </style>
</head>
<body>
 <div class="parent">
 <div class="first"></div>
 <div class="second"></div>
 </div>
</body>
</html>

 (2)绝对定位

绝对定位:想对于最近已定位的⽗元素,如果没有,相对于body进⾏定位,脱离⽂档流,形成BFC;

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <meta http-equiv="X-UA-Compatible" content="IE=edge">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>Document</title>
 <style type="text/css">
 *{
 padding: 0;
 margin:0;
 }
 .parent{
 height: 200px;
 width: 100%;
 background-color: rebeccapurple;
 }
 div.parent div.first {
 width: 100px;
 height: 100px;
 display: inline-block;
 background-color: yellow;
 }
 div.parent div.second {
 width: 100px;
 height: 100px;
 background-color: gray;
 display: inline-block;
 /* 绝对定位 */
 position: absolute;
 left:10px;
 top:10px;
 }
因为⽗元素没有定位,因此相对于body进⾏定位
 </style>
</head>
<body>
 <div class="parent">
 <div class="first"></div>
 <div class="second"></div>
 </div>
</body>
</html>

 因为⽗元素没有定位,因此相对于body进⾏定位

 因为⽗元素进⾏了定位,因此⼦元素的绝对定位是相对于定位⽗元素。

注:⽗元素相对定位,⼦元素绝对定位

 3.居中对齐方式

(1) ⽔平居中

  • inline元素:text-align:center;
  • block元素:margin:auto;
  • absolute元素:left:50%+margin-left:负值;
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.boxCls {
            width: 100%;
            height: 80px;
            border: 1px solid black;
        }

        div.first {
            text-align: center;
        }

        div.second p {
            width: 500px;
            background-color:aqua;
            margin: 0 auto;
        }

        div.third {
            position: relative;
        }

        div.third p {
            position: absolute;
            width: 500px;
            background-color:blue;
            left: 50%;
            margin-left: -250px;
        }
    </style>
</head>

<body>
    <div class="boxCls first">
        <span>无敌大铁锤</span>
    </div>
    <div class="boxCls second">
        <p>无敌大铁锤</p>
    </div>
    <div class="boxCls third">
        <p>无敌大铁锤</p>
    </div>
</body>

</html>
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        div.boxCls {
            width: 100%;
            height: 80px;
            border: 1px solid black;
        }

        div.first {
            text-align: center;
            line-height: 80px;
        }

        div.second,div.third,div.fourth {
            position: relative;
        }
        div.second div {
            width: 500px;
            height: 50px;
            background-color:aqua;
            position: absolute;
            top: 50%;
            left:50%;
            margin-top:-25px;
            margin-left:-250px;   
        }
        div.third div{
            width: 500px;
            height: 50px;
            background-color:chocolate;
            /* 转换 */
            position: absolute;
            top:50%;
            left: 50%;
            transform: translate(-50%,-50%);
        }

        div.fourth div{
            width: 500px;
            height: 50px;
            background-color:deeppink;
            position: absolute;
            left: 0;
            top:0;
            right: 0;
            bottom: 0;
            margin: auto;
        }

    </style>
</head>

<body>
    <div class="boxCls first">
        <span>无敌大铁锤</span>
    </div>
    <div class="boxCls second">
        <div>无敌大铁锤</div>
    </div>
    <div class="boxCls third">
        <div>无敌大铁锤</div>
    </div>
    <div class="boxCls fourth">
        <div>无敌大铁锤</div>
    </div>
</body>

</html>

 (2)垂直居中

  • inline元素:line-height:⾏⾼;
  • absolute元素: top:50%+margin-top:负值;
  • absolute元素:(CSS3)transform:translate(-50%,-50%);
  • absolute元素:left, top,bottom.right都为0,margin:auto;

 

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本文链接:https://blog.csdn.net/weixin_57959921/article/details/129828571