Bootstrap弹性盒子

Bootstrap提供了一整套响应式的Flexbox实用程序，方便用户快速管理网格列、导航、组件等的布局、对齐方式和大小调整。这些实用程序充分利用了Flexbox强大的布局功能，使用户能够轻松创建灵活且适应不同屏幕尺寸的布局。

一、启用flex

使用display通用类别来创建一个flexbox容器，并将直属的子元素转换为flex属性。增加额外的flex属性，也能够进一步修改flex容器和属性。

<div class="d-flex p-2 bd-highlight">I'm a flexbox container!</div>

<div class="d-inline-flex p-2 bd-highlight">I'm an inline flexbox container!</div>

.d-flex和.d-inline-flex也包含响应式的变化：

• .d-flex
• .d-inline-flex
• .d-sm-flex
• .d-sm-inline-flex
• .d-md-flex
• .d-md-inline-flex
• .d-lg-flex
• .d-lg-inline-flex
• .d-xl-flex
• .d-xl-inline-flex
• .d-xxl-flex
• .d-xxl-inline-flex

二、方向

透过通用类别来设定flex容器与内层flex的方向，在大多的情况下你可以忽略水平的class，因为浏览器预设值是row。不过有些特定情境下需要特别设定这个值（像是响应式情境）。

使用.flex-row来设定水平的方向（浏览器预设值），或是使用.flex-row-reverse来作水平方向的反转。

<div class="d-flex flex-row bd-highlight mb-3">
<div class="p-2 bd-highlight">Flex item 1</div>
<div class="p-2 bd-highlight">Flex item 2</div>
<div class="p-2 bd-highlight">Flex item 3</div>
</div>
<div class="d-flex flex-row-reverse bd-highlight">
<div class="p-2 bd-highlight">Flex item 1</div>
<div class="p-2 bd-highlight">Flex item 2</div>
<div class="p-2 bd-highlight">Flex item 3</div>
</div>

使用.flex-column设置垂直方向，或者使用.flex-column-reverse作垂直方向的反转。

<div class="d-flex flex-column bd-highlight mb-3">
<div class="p-2 bd-highlight">Flex item 1</div>
<div class="p-2 bd-highlight">Flex item 2</div>
<div class="p-2 bd-highlight">Flex item 3</div>
</div>
<div class="d-flex flex-column-reverse bd-highlight">
<div class="p-2 bd-highlight">Flex item 1</div>
<div class="p-2 bd-highlight">Flex item 2</div>
<div class="p-2 bd-highlight">Flex item 3</div>
</div>

flex-direction的响应式变化：

• .flex-row
• .flex-row-reverse
• .flex-column
• .flex-column-reverse
• .flex-sm-row
• .flex-sm-row-reverse
• .flex-sm-column
• .flex-sm-column-reverse
• .flex-md-row
• .flex-md-row-reverse
• .flex-md-column
• .flex-md-column-reverse
• .flex-lg-row
• .flex-lg-row-reverse
• .flex-lg-column
• .flex-lg-column-reverse
• .flex-xl-row
• .flex-xl-row-reverse
• .flex-xl-column
• .flex-xl-column-reverse
• .flex-xxl-row
• .flex-xxl-row-reverse
• .flex-xxl-column
• .flex-xxl-column-reverse

三、调整内容

使用flexbox容器上的justify-content通用类别改变flex物件在主轴上的对齐（x轴开始，如果flex-direction: column则为y轴）。从start（浏览器预设），end、center、between、around或evenly中做选择。

<div class="d-flex justify-content-start">...</div>
<div class="d-flex justify-content-end">...</div>
<div class="d-flex justify-content-center">...</div>
<div class="d-flex justify-content-between">...</div>
<div class="d-flex justify-content-around">...</div>
<div class="d-flex justify-content-evenly">...</div>

justify-content还存在响应变体：

• .justify-content-start
• .justify-content-end
• .justify-content-center
• .justify-content-between
• .justify-content-around
• .justify-content-evenly
• .justify-content-sm-start
• .justify-content-sm-end
• .justify-content-sm-center
• .justify-content-sm-between
• .justify-content-sm-around
• .justify-content-sm-evenly
• .justify-content-md-start
• .justify-content-md-end
• .justify-content-md-center
• .justify-content-md-between
• .justify-content-md-around
• .justify-content-md-evenly
• .justify-content-lg-start
• .justify-content-lg-end
• .justify-content-lg-center
• .justify-content-lg-between
• .justify-content-lg-around
• .justify-content-lg-evenly
• .justify-content-xl-start
• .justify-content-xl-end
• .justify-content-xl-center
• .justify-content-xl-between
• .justify-content-xl-around
• .justify-content-xl-evenly
• .justify-content-xxl-start
• .justify-content-xxl-end
• .justify-content-xxl-center
• .justify-content-xxl-between
• .justify-content-xxl-around
• .justify-content-xxl-evenly

四、对齐项目

在flexbox容器上使用align-items工具改变横轴上flex物件的对齐（y轴开始，如果设定flex-direction: column则为x轴），从start、end、center、baseline或是stretch（浏览器预设）中做选择。

<div class="d-flex align-items-start">...</div>
<div class="d-flex align-items-end">...</div>
<div class="d-flex align-items-center">...</div>
<div class="d-flex align-items-baseline">...</div>
<div class="d-flex align-items-stretch">...</div>

align-items包含的响应式变：

• .align-items-start
• .align-items-end
• .align-items-center
• .align-items-baseline
• .align-items-stretch
• .align-items-sm-start
• .align-items-sm-end
• .align-items-sm-center
• .align-items-sm-baseline
• .align-items-sm-stretch
• .align-items-md-start
• .align-items-md-end
• .align-items-md-center
• .align-items-md-baseline
• .align-items-md-stretch
• .align-items-lg-start
• .align-items-lg-end
• .align-items-lg-center
• .align-items-lg-baseline
• .align-items-lg-stretch
• .align-items-xl-start
• .align-items-xl-end
• .align-items-xl-center
• .align-items-xl-baseline
• .align-items-xl-stretch
• .align-items-xxl-start
• .align-items-xxl-end
• .align-items-xxl-center
• .align-items-xxl-baseline
• .align-items-xxl-stretch

五、自身对齐

使用flexbox物件上的align-self通用类别单独改变在横轴上的对齐（y轴开始，如果设定flex-direction: column，则为x轴）。从与align-items相同的选项中选择：start、end、center、baseline、或是stretch（溜览器预设）。

<div class="align-self-start">Aligned flex item</div>
<div class="align-self-end">Aligned flex item</div>
<div class="align-self-center">Aligned flex item</div>
<div class="align-self-baseline">Aligned flex item</div>
<div class="align-self-stretch">Aligned flex item</div>

align-self的响应式变化：

• .align-self-start
• .align-self-end
• .align-self-center
• .align-self-baseline
• .align-self-stretch
• .align-self-sm-start
• .align-self-sm-end
• .align-self-sm-center
• .align-self-sm-baseline
• .align-self-sm-stretch
• .align-self-md-start
• .align-self-md-end
• .align-self-md-center
• .align-self-md-baseline
• .align-self-md-stretch
• .align-self-lg-start
• .align-self-lg-end
• .align-self-lg-center
• .align-self-lg-baseline
• .align-self-lg-stretch
• .align-self-xl-start
• .align-self-xl-end
• .align-self-xl-center
• .align-self-xl-baseline
• .align-self-xl-stretch
• .align-self-xxl-start
• .align-self-xxl-end
• .align-self-xxl-center
• .align-self-xxl-baseline
• .align-self-xxl-stretch

六、填充

在一系列的同级元素上使用.flex-fill来强制设定它们的宽度与内容相等（如果它们的内容不超过边框，则各个宽度相等），同时占用所有可用的水平空间。

<div class="d-flex bd-highlight">
<div class="p-2 flex-fill bd-highlight">Flex item with a lot of content</div>
<div class="p-2 flex-fill bd-highlight">Flex item</div>
<div class="p-2 flex-fill bd-highlight">Flex item</div>
</div>

flex-fill同样包含响应式的变化：

• .flex-fill
• .flex-sm-fill
• .flex-md-fill
• .flex-lg-fill
• .flex-xl-fill
• .flex-xxl-fill

七、伸缩值

使用.flex-grow-*通用类别来切换flex项目弹性增长、填充可用空间的能力。在下面的示例中，.flex-grow-1元素使用它可以使用的所有可用空间，同时允许剩馀的两个flex物件保留必要的空间。

<div class="d-flex bd-highlight">
<div class="p-2 flex-grow-1 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Third flex item</div>
</div>

如果需要，使用.flex-shrink-*通用类别来切换项目的缩小能力。在下面的示例中，带有.flex-shrink-1的第二个flex项目的文字内容会被强制换行，”收缩”以允许更多空间用于上一个带有.w-100的flex项目。

<div class="d-flex bd-highlight">
<div class="p-2 w-100 bd-highlight">Flex item</div>
<div class="p-2 flex-shrink-1 bd-highlight">Flex item</div>
</div>

flex-grow和flex-shrink也包含响应式的变化：

• .flex-{grow|shrink}-0
• .flex-{grow|shrink}-1
• .flex-sm-{grow|shrink}-0
• .flex-sm-{grow|shrink}-1
• .flex-md-{grow|shrink}-0
• .flex-md-{grow|shrink}-1
• .flex-lg-{grow|shrink}-0
• .flex-lg-{grow|shrink}-1
• .flex-xl-{grow|shrink}-0
• .flex-xl-{grow|shrink}-1
• .flex-xxl-{grow|shrink}-0
• .flex-xxl-{grow|shrink}-1

八、自动边距

当你混用Flex对齐与auto margin时候，Flexbox可以执行一些令人惊艳的功能。下面的示例是透过自动margin来控制flex项目的三个案例：预设（无自动margin），向右推两个项目（.me-auto），并向左推两个项目（.ms-auto）。

<div class="d-flex bd-highlight mb-3">
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
</div>
<div class="d-flex bd-highlight mb-3">
<div class="me-auto p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
</div>
<div class="d-flex bd-highlight mb-3">
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="ms-auto p-2 bd-highlight">Flex item</div>
</div>

搭配align-items：

混合以下属性align-items、flex-direction: column，和margin-top: auto或margin-bottom: auto，会将一个flex项目移动到容器的顶部或底部。

<div class="d-flex align-items-start flex-column bd-highlight mb-3" style="height: 200px;">
<div class="mb-auto p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
</div>
<div class="d-flex align-items-end flex-column bd-highlight mb-3" style="height: 200px;">
<div class="p-2 bd-highlight">Flex item</div>
<div class="p-2 bd-highlight">Flex item</div>
<div class="mt-auto p-2 bd-highlight">Flex item</div>
</div>

九、换行

改变Flex项目在Flex容器中换行的方式。可以选择使用.flex-nowrap完全不换行（浏览器预设），使用.flex-wrap进行换行，或使用.flex-wrap-reverse进行反向换行。

<div class="d-flex flex-nowrap">
...
</div>

<div class="d-flex flex-wrap">
...
</div>

<div class="d-flex flex-wrap-reverse">
...
</div>

flex-wrap也包含响应式变化：

• .flex-nowrap
• .flex-wrap
• .flex-wrap-reverse
• .flex-sm-nowrap
• .flex-sm-wrap
• .flex-sm-wrap-reverse
• .flex-md-nowrap
• .flex-md-wrap
• .flex-md-wrap-reverse
• .flex-lg-nowrap
• .flex-lg-wrap
• .flex-lg-wrap-reverse
• .flex-xl-nowrap
• .flex-xl-wrap
• .flex-xl-wrap-reverse
• .flex-xxl-nowrap
• .flex-xxl-wrap
• .flex-xxl-wrap-reverse

十、排序

使用order通用类别改变特定flex项目显示的排序。Bootstrap仅提供选项将一个物件排在第一个或最后一个，以及重置以使用DOM排序。由于order只能使用从0到5的整数值，任何额外值则需要自定义的CSS。

<div class="d-flex flex-nowrap bd-highlight">
<div class="order-3 p-2 bd-highlight">First flex item</div>
<div class="order-2 p-2 bd-highlight">Second flex item</div>
<div class="order-1 p-2 bd-highlight">Third flex item</div>
</div>

order也包含的响应式属性：

• .order-0
• .order-1
• .order-2
• .order-3
• .order-4
• .order-5
• .order-sm-0
• .order-sm-1
• .order-sm-2
• .order-sm-3
• .order-sm-4
• .order-sm-5
• .order-md-0
• .order-md-1
• .order-md-2
• .order-md-3
• .order-md-4
• .order-md-5
• .order-lg-0
• .order-lg-1
• .order-lg-2
• .order-lg-3
• .order-lg-4
• .order-lg-5
• .order-xl-0
• .order-xl-1
• .order-xl-2
• .order-xl-3
• .order-xl-4
• .order-xl-5
• .order-xxl-0
• .order-xxl-1
• .order-xxl-2
• .order-xxl-3
• .order-xxl-4
• .order-xxl-5

此外还有响应式的.order-first和.order-last类别，它们分别透过order: -1和order: 6，来更改元素的order顺序。

• .order-first
• .order-last
• .order-sm-first
• .order-sm-last
• .order-md-first
• .order-md-last
• .order-lg-first
• .order-lg-last
• .order-xl-first
• .order-xl-last
• .order-xxl-first
• .order-xxl-last

十一、对齐内容

使用flexbox容器上的align-content通用类别将flex项目一起对齐于横轴上。从start（浏览器预设）、end、center、between、around，或stretch中选择。以下为了呈现这通用类别的效果，我们强制执行flex-wrap: wrap并增加了Flex项目的数量。

注意：此特性对于单行的Flex项目没有作用。

<div class="d-flex align-content-start flex-wrap">
...
</div>

<div class="d-flex align-content-end flex-wrap">...</div>

<div class="d-flex align-content-center flex-wrap">...</div>

<div class="d-flex align-content-between flex-wrap">...</div>

<div class="d-flex align-content-around flex-wrap">...</div>

<div class="d-flex align-content-stretch flex-wrap">...</div>

align-content也包含响应式变化：

• .align-content-start
• .align-content-end
• .align-content-center
• .align-content-around
• .align-content-stretch
• .align-content-sm-start
• .align-content-sm-end
• .align-content-sm-center
• .align-content-sm-around
• .align-content-sm-stretch
• .align-content-md-start
• .align-content-md-end
• .align-content-md-center
• .align-content-md-around
• .align-content-md-stretch
• .align-content-lg-start
• .align-content-lg-end
• .align-content-lg-center
• .align-content-lg-around
• .align-content-lg-stretch
• .align-content-xl-start
• .align-content-xl-end
• .align-content-xl-center
• .align-content-xl-around
• .align-content-xl-stretch
• .align-content-xxl-start
• .align-content-xxl-end
• .align-content-xxl-center
• .align-content-xxl-around
• .align-content-xxl-stretch