Flex布局入门指南

那咱们就开始吧！

Flex容器属性
Flex是一组规则，用于在父容器中自动扩展多列和多行。
简单的理解就是Flex有一组样式规则，这些样式规则能够帮助你对内容按行或列来布局。而这些规则都将运用于Flex容器元素上。

display:flex
在Flexbox中与其余CSS属性略有不一样。在Flexbox中有一个Flex容器的概念（主容器），在Flex容器中的直接子元素都是Flex项目。而咱们刚才提到的Flex规则，都只用于Flex容器上。

Flex布局是一个完整的模块而不是一个单独的属性，它包括了完整的一套属性。其中有的属性是设置在容器（container，也能够叫作父元素，称为flex
container）上，有的则是设置在容器的项目上（item，也能够叫作子元素，称为flex items）上。

Properties for the Parent
(flex container)

Properties for the Children
(flex items)

display

This defines a flex container; inline or block depending on the given value. It enables a flex context for all its direct children.

.container {
  display: flex; /* or inline-flex */
}

Note that CSS columns have no effect on a flex container.

flex-direction

the four possible values of flex-direction being shown: top to bottom, bottom to top, right to left, and left to right
This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.

.container {
  flex-direction: row | row-reverse | column | column-reverse;
}

row (default): left to right in ltr; right to left in rtl
row-reverse: right to left in ltr; left to right in rtl
column: same as row but top to bottom
column-reverse: same as row-reverse but bottom to top

flex-wrap

two rows of boxes, the first wrapping down onto the second

By default, flex items will all try to fit onto one line. You can change that and allow the items to wrap as needed with this property.

.container{
  flex-wrap: nowrap | wrap | wrap-reverse;
}

nowrap (default): all flex items will be on one line
wrap: flex items will wrap onto multiple lines, from top to bottom.
wrap-reverse: flex items will wrap onto multiple lines from bottom to top.
There are some visual demos of flex-wrap here.

justify-content

flex items within a flex container demonstrating the different spacing options
This defines the alignment along the main axis. It helps distribute extra free space left over when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.

.container {
  justify-content: flex-start | flex-end | center | space-between | space-around | space-evenly;
}

flex-start (default): items are packed toward the start line
flex-end: items are packed toward the end line
center: items are centered along the line
space-between: items are evenly distributed in the line; first item is on the start line, last item on the end line
space-around: items are evenly distributed in the line with equal space around them. Note that visually the spaces aren't equal, since all the items have equal space on both sides. The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies.
space-evenly: items are distributed so that the spacing between any two items (and the space to the edges) is equal.

align-items

demonstration of differnet alignment options, like all boxes stuck to the top of a flex parent, the bottom, stretched out, or along a baseline
This defines the default behavior for how flex items are laid out along the cross axis on the current line. Think of it as the justify-content version for the cross-axis (perpendicular to the main-axis).

.container {
  align-items: stretch | flex-start | flex-end | center | baseline;
}

stretch (default): stretch to fill the container (still respect min-width/max-width)flex-start: cross-start margin edge of the items is placed on the cross-start lineflex-end: cross-end margin edge of the items is placed on the cross-end linecenter: items are centered in the cross-axisbaseline: items are aligned such as their baselines align