CSSposition:absoluteComprehensive understanding of analysis-CSS Tutorial-php.cn

1. Characteristics of absolute positioning

Absolute positioning has the same characteristics as floating, that is, wrapping and destructiveness.

In terms of destructiveness, floating only destroys the height of the element and retains the width of the element; while the height and width of absolutely positioned elements are gone.

Please look at the following code:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>绝对定位的特征</title>
        <style>
            .box1,.box2,.box3 {   
                background-color: orange;   
                margin-bottom: 30px;   
            }   

            .absolute {   
                position: absolute;   
            }   

            .wraper {   
                display:inline-block;   
                margin-left: 300px;   
            }   

            .float {   
                float: left;   
            }   

            .box3 {   
                position: absolute;   
            }   
        </style>
    </head>
    <body>
        <p class="box1">
            <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            <img src="http://pic1.win4000.com/wallpaper/c/537b28b60619b.jpg" alt="A picture" style="width:240px;height:180px" />
            <p>这是普通流中的两幅图像。</p>
        </p>
        <p class="box2">
            <img class="absolute" src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            <img src="http://pic1.win4000.com/wallpaper/c/537b28b60619b.jpg" alt="A picture" style="width:240px;height:180px" />

            <p class="wraper">
                <img class="float" src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
                <img src="http://pic1.win4000.com/wallpaper/c/537b28b60619b.jpg" alt="A picture" style="width:240px;height:180px" />
            </p>
            <p>左图，将第一幅图像绝对定位，其完全脱离文档流，并且覆盖在第二幅图像之上；由此看出，绝对定位的破坏性不仅让元素没有了高度，甚至没有了宽度。</p>
            <p>右图，将第一幅图像左浮动，其虽然脱离了文档流，但是并没有覆盖在其他元素之上；浮动的破坏性仅仅破坏了元素的高度，而保留了元素的宽度。</p>
        </p>
        <p class="box3">
            <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            <img src="http://pic1.win4000.com/wallpaper/c/537b28b60619b.jpg" alt="A picture" style="width:240px;height:180px" />
            <p>将容器绝对定位，体现其包裹性。</p>
        </p>
    </body>
</html>

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2. General rules for absolute positioning:

When an element is positioned absolutely, it is completely removed from the document flow and positioned relative to its containing block.

The absolutely positioned containing block is its nearest positioned ancestor element.

If the ancestor element is a block-level element, the containing block is the padding boundary of the ancestor element, that is, the border.

If the ancestor element is an inline element, the containing block is the content boundary of the ancestor element, that is, the content area.

If there is no positioned ancestor element, the element's containing block is defined as the initial containing block.

Offset attributes: top, right, bottom, left.

If the absolutely positioned element does not set the offset attribute, although it is out of the document flow, its position will remain in place.

The offset property can be a negative value to position the element outside the containing block.

The code is here:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>绝对定位的一般规则</title>
        <style>
            body {   
                background-color: #ccc;   
            }   
            .container {   
                width:500px;   
                background-color: orange;   
                margin:20px auto 50px auto;   
                padding:20px;   
                border:2px solid red;   
            }   

            .box2 img,.box3 img,   
            .box4 img,.box5 img {   
                position: absolute;   
            }   

            .box3 img,.box4 img {   
                left:0;   
                bottom:0;   
            }   

            .box5 img {   
                left: -30px;   
                bottom: -50px;   
            }   

            .block {   
                position :relative;   
                height: 200px;   
            }   
        </style>
    </head>
    <body>
        <p class="container">
            <p class="box1">
                <p>元素绝对定位时，会完全脱离文档流，并相对于其包含块定位。绝对定位的包含块，是其最近的已定位的祖先元素。</p>
                <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
                <ul>
                    <li>如果这个祖先元素是块级元素，包含块则为该祖先元素的内边距边界，即边框。</li>
                    <li>如果这个祖先元素是行内元素，包含块则为该祖先元素的内容边界，即内容区。</li>
                    <li>如果没有已定位的祖先元素，元素的包含块定义为初始包含块（一个视窗大小的矩形）。</li>
                </ul>
            </p><!--关闭box1-->
        </p><!--关闭container-->
        <p class="container">
            <p class="box2">
            <p>不管有没有已经定位的祖先元素，只要没有偏移量，绝对定位之后，原地不动，脱离文档流。</p>
            <p>下面这个已经绝对定位的图像原地没动，但是已经脱离了文档流。</p>
            <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            </p><!--关闭box2-->
        </p><!--关闭container-->
        <p class="container">
            <p class="box3">
                <p>没有已经定位的祖先元素，有偏移量，绝对定位之后，以初始包含块（一个视窗大小的矩形）为基准进行偏移。</p>
                <p>当偏移量为left:0; buttom:0时，图像水平偏移到了初始包含块左下角（打开网页最开始的那一个视窗的左下角）。</p>
                <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            </p><!--关闭box3-->
        </p><!--关闭container-->
        <p class="container block">
            <p class="box4">
                <p>有已经定位的祖先元素，有偏移量，绝对定位之后，以已经定位的祖先元素为基准进行偏移。</p>
                <p>此处已经定位的祖先元素为这个图像的容器p元素，偏移量为left:0; bottom:0时，图像到了这个容器的左下角（以边框为界）。</p>
                <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            </p><!--关闭box4-->
        </p><!--关闭container-->
        <p class="container block">
            <p class="box5">
                <p>有已经定位的祖先元素，有偏移量，绝对定位之后，以已经定位的祖先元素为基准进行偏移。</p>
                <p>此处已经定位的祖先元素为这个图像的容器p元素，偏移量为left:-30px; bottom:-50px时，图像到了这个容器之外（以边框为界）。</p>
                <img src="http://a.hiphotos.baidu.com/zhidao/pic/item/72f082025aafa40fa38bfc55a964034f79f019ec.jpg" alt="A picture" style="width:175px;height:100px" />
            </p><!--关闭box5-->
        </p><!--关闭container-->
    </body>
</html>

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3. Use margin to adjust the position of absolutely positioned elements

Absolutely positioned elements, in addition to being offset using top, right, bottom, and left, can also be positioned accurately through margin values, and are more adaptable.

Example:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>用margin调整绝对定位元素的位置</title>
        <style>
            .box1,.box2,.box3 {   
                display: inline-block;   
                margin-right: 150px;   
                border:1px solid blue;   
            }   

            span {   
                background-color: red;   
            }   

            .box2 span,.box3 span {   
                position: absolute;   
            }   

            .meng {   
                margin-left: -3em;   
            }   

            .box4 {   
                border:1px solid red;   
                width: 500px;   
                margin: 50px auto 0 auto;   
                padding:20px;   
            }   

            li {   
                margin-bottom: 20px;   
            }   
        </style>
    </head>
    <body>
        <p class="box1">
            <span>卡哇伊</span>
            <img src="http://imgsrc.baidu.com/forum/w%3D580/sign=0c101fe665380cd7e61ea2e59145ad14/f9a3492762d0f7032de1758a08fa513d2797c542.jpg" style="width:200px;height:300px" />
            <span>萌萌哒</span>
        </p>
        <p class="box2">
            <span>卡哇伊</span>
            <img src="http://imgsrc.baidu.com/forum/w%3D580/sign=0c101fe665380cd7e61ea2e59145ad14/f9a3492762d0f7032de1758a08fa513d2797c542.jpg" style="width:200px;height:300px" />
            <span>萌萌哒</span>
            </p>
        <p class="box3">
            <span>卡哇伊</span>
            <img src="http://imgsrc.baidu.com/forum/w%3D580/sign=0c101fe665380cd7e61ea2e59145ad14/f9a3492762d0f7032de1758a08fa513d2797c542.jpg" style="width:200px;height:300px" />
            <span class="meng">萌萌哒</span>
        </p>
        <p class="box4">
            <ol>
                <li>第一幅图，最开始的样子。</li>
                <li>第二幅图，两个标签绝对定位，但是不指定任何偏移量。</li>
                <li>第三幅图，用margin负值调整“萌萌哒”的位置，完成。</li>
            </ol>
        </p>
    </body>
</html>

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Abandon the offset attribute and use margin instead to adjust absolutely positioned elements. The principle is:

Absolutely positioned elements, When the offset is not set, although it is completely out of the document flow, it is still in its original position.

Use the offset attribute for precise positioning. The specific position depends on the containing block of the absolutely positioned element. Different containing blocks will have completely different positioning results.

Using margin for precise positioning does not depend on anything else and is more controllable.

4. Absolute positioning and overall layout

How to use absolute positioning to lay out the overall page?

Simple and crude, it will be a waste if you don’t learn! ! !

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>绝对定位与整体页面布局</title>
        <style>
            * {   
                margin: 0;   
            }/*重置所有margin为0，这一步极其重要，否则布局必乱。*/   

            html,body,.page {   
                width: 100%;   
                height: 100%;   
                overflow: hidden;   
            }   

            .page {   
                position: absolute;   
                top: 0;   
                right: 0;   
                bottom: 0;   
                left: 0;   
                background-color: #ccc;   
            }   

            .header {   
                position: absolute;   
                height: 50px;   
                left: 0;   
                right: 0;   
                background-color: purple;   
                padding: 0 5px;   
                z-index: 1;   
            }   

            .header>h1 {   
                line-height: 50px;   
                text-align: center;   
            }   

            .aside {   
                position: absolute;   
                top: 50px;   
                bottom: 50px;   
                left: 0;   
                width: 100px;   
                background-color: orange;   
            }   

            .footer {   
                position: absolute;   
                right: 0;   
                bottom: 0;   
                left: 0;   
                height: 50px;   
                background-color: purple;   
            }   

            .footer>h1 {   
                text-align: center;   
                line-height: 50px;   
            }   

            .content {   
                position: absolute;   
                top: 50px;   
                right: 0;   
                bottom: 50px;   
                left: 100px;   
                background-color: cyan;   
                overflow: auto;   
            }   

            .content h1 {   
                margin-top: 100px;   
                margin-left: 50px;   
            }   

            .content li {   
                margin-left: 100px;   
                margin-top: 80px;   
                font-size: 24px;   
                line-height: 1.5;   
            }   

            ol {   
                margin-bottom: 80px;   
            }   
        </style>
    </head>
    <body>
        <p class="page">
            <p class="header">
                <h1>Header</h1>
            </p>
            <p class="aside">
                <h1>Aside</h1>
            </p>
            <p class="content">
                <h1>实现原理</h1>
                <ol>
                    <li>创建一个p.page，绝对定位，铺满全屏。</li>
                    <li>创建一个p.header，绝对定位，设定高度。</li>
                    <li>创建一个p.aside，绝对定位，设定宽度。</li>
                    <li>创建一个p.footer，绝对定位，设定高度。</li>
                    <li>创建一个p.content，绝对定位，根据周围p的宽高设定它的宽高；<br />
                    以p.content元素取代原body元素，所有的页面内容都放在这里面。</li>
                </ol>
            </p>
            <p class="footer">
                <h1>Footer</h1>
            </p>
        </p>
    </body>
</html>

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