Web layout is a key application of CSS.

The traditional solution to layout, based on the box model, relies on the display property + position property + float property. It is very inconvenient for special layouts, such as vertical center, which is not easy to implement.

In 2009, the W3C introduced a new solution, —-Flex layout, that enables easy, complete, and responsive implementation of a variety of page layouts. It is currently supported by all browsers, which means it is safe to use it now.

Flex layouts will be the preferred solution for future layouts. This article covers its syntax, and the next article gives you Flex writing for common layouts. JailBreak has created a Demo for all the examples in this article.

A Complete Guide to Flexbox and A Visual Guide to CSS3 Flexbox Properties.

What is the Flex layout?

Flex, short for Flexible Box, is designed to provide maximum flexibility to Box models.

Any container can be specified as a Flex layout.

.box{
  display: flex;
}
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Inline elements can also be laid out using Flex.

.box{
  display: inline-flex;
}
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Webkit-kernel browsers must be prefixed with -webkit.

.box{
  display: -webkit-flex; /* Safari */
  display: flex;
}
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Note that with Flex layout, the float, clear, and vertical-align attributes of the child elements are invalidated.

2. Basic concepts

Elements with a Flex layout are called Flex containers, or “containers” for short. All of its child elements automatically become container members and are called Flex items, or “items.”

The container has two axes by default: a horizontal main axis and a vertical cross axis. The starting position of the spindle (where it intersects with the border) is called main start and the ending position is called main end; The starting position of the intersecting axis is called cross start and the ending position is called cross end.

By default, items are arranged along the main axis. The main axis space occupied by a single project is called main size, and the cross axis space occupied is called cross size.

Properties of the container

The following six properties are set on the container.

flex-direction
flex-wrap
flex-flow
justify-content
align-items
align-content
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3.1 the flex – direction attribute

The flex-direction attribute determines the direction of the main axis (that is, the alignment of items).

.box {
  flex-direction: row | row-reverse | column | column-reverse;
}
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It could have four values.

Row (default) : The main axis is horizontal and the starting point is on the left. Row-reverse: The main axis is horizontal and the starting point is at the right end. Column: The main axis is in the vertical direction, and the starting point is in the upper edge. Column-reverse: the main axis is vertical and the starting point is at the bottom.Copy the code

3.2 the flex – wrap attributes

By default, projects are arranged on a line (also known as an “axis”). The flex-wrap property defines how to wrap a line if an axis does not fit.

.box{
  flex-wrap: nowrap | wrap | wrap-reverse;
}
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It could take three values.

(1) NowRap (default) : no line breaks.

(2) Wrap: the first line is at the top.

(3) wrap-reverse: the first line is at the bottom.

3.3 the flex – flow

The flex-flow property is a short form of the flex-direction and flex-wrap properties. The default value is Row Nowrap.

.box {
  flex-flow: <flex-direction> || <flex-wrap>;
}
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3.4 the justify – content attribute

The context-content attribute defines the alignment of items on the main axis.

.box {
  justify-content: flex-start | flex-end | center | space-between | space-around;
}
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It might take five values, aligned in a way that depends on the direction of the axis. So let’s say that the principal axis is going from left to right.

Flex-start (default) : left-align Flex-end: right-align center: center space-between: align both ends with equal intervals between items. Space-around: Equal spacing on both sides of each item. As a result, the spacing between items is twice as large as the spacing between items and the border.Copy the code

3.5 the align – the items property

The align-items property defines how items are aligned on the cross axis.

.box {
  align-items: flex-start | flex-end | center | baseline | stretch;
}
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It could take five values. The exact alignment depends on the direction of the intersecting axis, which is assumed to run from top to bottom.

Flex-start: Alignment of the starting point of the cross axes. Flex-end: alignment of ends of crossed axes. Center: Alignment of the midpoint of the cross axis. Baseline: The baseline alignment of the first line of text of the project. Stretch (default) : If the height is not set or auto is set, the project will occupy the entire height of the container.Copy the code

3.6 the align – content attribute

The align-content property defines the alignment of multiple axes. This property has no effect if the project has only one axis.

.box {
  align-content: flex-start | flex-end | center | space-between | space-around | stretch;
}
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The property may take six values.

Flex-start: align with the starting point of the cross axis. Flex-end: aligns with the end of the cross axis. Center: aligns with the midpoint of the intersecting axis. Space-between: aligned with both ends of the intersecting axes, with evenly distributed spacing between axes. Space-around: The spacing on both sides of each axis is equal. Therefore, the spacing between axes is twice as large as the spacing between axes and borders. Stretch (default) : Axis takes up the entire cross axis.Copy the code

4. Project attributes

The following six properties are set on the project.

order
flex-grow
flex-shrink
flex-basis
flex
align-self
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4.1 the order attribute

The order attribute defines the order of items. The smaller the value is, the more advanced it is. The default value is 0.

.item {
  order: <integer>;
}
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4.2 the flex – turns attributes

The Flex-Grow property defines the project’s zoom scale, which defaults to 0, or no zoom if there is free space.

.item {
  flex-grow: <number>; /* default 0 */
}
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If all projects have a flex-Grow attribute of 1, they divide the remaining space equally, if any. If one project has a flex-grow attribute of 2 and all the other projects are 1, the former takes up twice as much free space as the other items.

4.3 the flex – the shrink properties

The Flex-shrink attribute defines the scale by which a project shrinks. The default is 1, that is, if there is insufficient space, the project shrinks.

.item {
  flex-shrink: <number>; /* default 1 */
}
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If all projects have a Flex-shrink attribute of 1, they are scaled equally when space is insufficient. If the flex-shrink attribute is 0 for one project and 1 for all other projects, the former does not shrink when space is insufficient.

Negative values have no effect on this property.

4.4 the flex – the basis of attributes

The Flex-basis property defines the main size of the project before allocating extra space. Based on this property, the browser calculates whether the main axis has extra space. Its default value is Auto, the original size of the project.

.item {
  flex-basis: <length> | auto; /* default auto */
}
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It can be set to the same value as the width or height attribute (such as 350px), and the project will take up a fixed space.

4.5 the flex property

The flex attribute is short for flex-grow, flex-shrink, and flex-basis. The default value is 0 1 Auto. The last two attributes are optional.

.item {
  flex: none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'>]}Copy the code

This property has two shortcut values: auto (1 1 auto) and None (0 0 auto).

It is recommended to use this attribute in preference to writing three separate attributes, as the browser will infer related values.

4.6 the align – self attribute

The align-self property allows a single item to have a different alignment than other items, overriding the align-items property. The default value is auto, which inherits the align-items property of the parent element. If there is no parent element, it equals stretch.

.item {
  align-self: auto | flex-start | flex-end | center | baseline | stretch;
}
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This property may take six values, all identical to the align-items property except auto.

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Next up:Flex Web Layout ii – Writing tutorial

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