Flexible Box 模型, 通常被称为 flexbox, 是一种一维的布局模型。它为flexbox中的子元素的对齐和分布提供了强大的能力。本文给出了flexbox的主要特性, 更多的细节将在别的文档中探索.
我们说flexbox是一种一维的布局，是因为一个flexbox一次只能处理一个维度上的元素布局，一行或者一列。作为对比的是另外一个二维布局 CSS Grid Layout, 可以同时处理行和列上的布局。
当使用flex布局时，首先想到的是两根轴线 — 主轴和垂直轴. 主轴由
flex-direction 定义, 另一根轴垂直于它. 我们使用flexbox的所有属性都跟这两根轴线有关, 所以有必要在一开始首先理解它.
column-reverse 时，你的主轴会沿着上下方向延伸 — 也就是 block 排列的方向.
The cross axis runs perpendicular to the main axis, therefore if your
flex-direction (main axis) is set to
row-reverse the cross axis runs down the columns.
If your main axis is
column-reverse then the cross axis runs along the rows.
Understanding which axis is which is important when we start to look at aligning and justifying flex items; flexbox features properties that align and justify content along one axis or the other.
Start and end lines
Another vital area of understanding is how flexbox makes no assumption about the writing mode of the document. In the past, CSS was heavily weighted towards horizontal and left-to-right writing modes. Modern layout methods encompass the range of writing modes and so we no longer assume that a line of text will start at the top left of a document and run towards the right hand side, with new lines appearing one under the other.
You can read more about the relationship between flexbox and the Writing Modes specification in a later article, however the following description should help explain why we do not talk about left and right and top and bottom when we describe the direction that our flex items flow in.
row and I am working in English, then the start edge of the main axis will be on the left, the end edge on the right.
If I were to work in Arabic, then the start edge of my main axis would be on the right and the end edge on the left.
In both cases the start edge of the cross axis is at the top of the flex container and the end edge at the bottom, as both languages have a horizontal writing mode.
After a while, thinking about start and end rather than left and right becomes natural, and will be useful to you when dealing with other layout methods such as CSS Grid Layout which follow the same patterns.
The flex container
An area of a document laid out using flexbox is called a flex container. To create a flex container, we set the value of the area's container's
display property to
inline-flex. As soon as we do this the direct children of that container become flex items. As with all properties in CSS, some initial values are defined, so when creating a flex container all of the contained flex items will behave in the following way.
- Items display in a row (the
flex-directionproperty's default is
- The items start from the start edge of the main axis.
- The items do not stretch on the main dimension, but can shrink.
- The items will stretch to fill the size of the cross axis.
flex-basisproperty is set to
flex-wrapproperty is set to
The result of this is that your items will all line up in a row, using the size of the content as their size in the main axis. If there are more items than can fit in the container, they will not wrap but will instead overflow. If some items are taller than others, all items will stretch along the cross axis to fill its full size.
You can see in the live example below how this looks. Try editing the items or adding additional items in order to test the initial behavior of flexbox.
flex-direction property to the flex container allows us to change the direction in which our flex items display. Setting
flex-direction: row-reverse will keep the items displaying along the row, however the start and end lines are switched.
If we change
column the main axis switches and our items now display in a column. Set
column-reverse and the start and end lines are again switched.
The live example below has
flex-direction set to
row-reverse. Try the other values —
column-reverse — to see what happens to the content.
Multi-line flex containers with flex-wrap
While flexbox is a one dimensional model, it is possible to cause our flex items to wrap onto multiple lines. In doing so, you should consider each line as a new flex container. Any space distribution will happen across that line, without reference to the lines either side.
To cause wrapping behaviour add the property
flex-wrap with a value of
wrap. Now, should your items be too large to all display in one line, they will wrap onto another line. The live sample below contains items that have been given a width, the total width of the items being too wide for the flex container. As
flex-wrap is set to
wrap, the items wrap. Set it to
nowrap, which is also the initial value, and they will instead shrink to fit the container because they are using initial flexbox values that allows items to shrink. Using
nowrap would cause an overflow if the items were not able to shrink, or could not shrink small enough to fit.
The flex-flow shorthand
You can combine the two properties
flex-wrap into the
flex-flow shorthand. The first value specified is
flex-direction and the second value is
In the live example below try changing the first value to one of the allowable values for
column-reverse, and also change the second to
Properties applied to flex items
To have more control over flex items we can target them directly. We do this by way of three properties:
We will take a brief look at these properties in this overview, and a future article will go into their behaviour in depth.
Before we can make sense of these properties we need to consider the concept of available space. What we are doing when we change the value of these flex properties is to change the way that available space is distributed amongst our items. This concept of available space is also important when we come to look at aligning items.
If we have three 100 pixel-wide items in a container which is 500 pixels wide, then the space we need to lay out our items is 300 pixels. This leaves 200 pixels of available space. If we don’t change the initial values then flexbox will put that space after the last item.
If we instead would like the items to grow and fill the space, then we need to have a method of distributing the leftover space between the items. This is what the
flex properties that we apply to the items themselves, will do.
flex-basis is what defines the size of that item in terms of the space it leaves as available space. The initial value of this property is
auto — in this case the browser looks to see if the items have a size. In the example above, all of the items have a width of 100 pixels and so this is used as the
If the items don’t have a size then the content's size is used as the flex-basis. This is why when we just declare
display: flex on the parent to create flex items, the items all move into a row and take only as much space as they need to display their contents.
flex-grow property set to a positive integer, flex items can grow along the main axis from their
flex-basis. This will cause the item to stretch and take up any available space on that axis, or a proportion of the available space if other items are allowed to grow too.
If we gave all of our items in the example above a
flex-grow value of 1 then the available space in the flex container would be equally shared between our items and they would stretch to fill the container on the main axis.
The flex-grow property can be used to distribute space in proportion. If we give our first item a
flex-grow value of 2 and the other items a value of 1, 2 parts will be given to the first item (100px out of 200px in the case of the example above), 1 part each the other two (50px each out of the 200px total).
flex-grow property deals with adding space in the main axis, the
flex-shrink property controls how it is taken away. If we do not have enough space in the container to lay out our items and
flex-shrink is set to a positive integer the item can become smaller than the
flex-basis. As with
flex-grow different values can be assigned in order to cause one item to shrink faster than others — an item with a higher value set for
flex-shrink will shrink faster that its siblings that have lower values.
The minimum size of the item will be taken into account while working out the actual amount of shrinkage that will happen, which means that flex-shrink has the potential to appear less consistent than flex-grow in behavior. We’ll therefore take a more detailed look at how this algorithm works in the article Controlling Ratios of items along the main axis.
Note that these values for
flex-shrink are proportions. Typically if we had all of our items set to flex:
1 1 200px and then wanted one item to grow at twice the rate, we would set that item to flex:
2 1 200px. However you could use flex:
10 1 200px and flex:
20 1 200px if you wanted.
Shorthand values for the flex properties
You will very rarely see the
flex-basis properties used individually; instead they are combined into the
flex shorthand. The
flex shorthand allows you to set the three values in this order —
The live example below allows you to test out the different values of the flex shorthand; remember that the first value is
flex-grow. Giving this a positive value means the item can grow. The second is
flex-shrink — with a positive value the items can shrink, but only if their total values overflow the main axis. The final value is
flex-basis; this is the value the items are using as their base value to grow and shrink from.
There are also some predefined shorthand values which cover most of the use cases. You will often see these used in tutorials, and in many cases these are all you will need to use. The predefined values are as follows:
flex: initial resets the item to the initial values of Flexbox. This is the same as
flex: 0 1 auto. In this case the value of
flex-grow is 0, so items will not grow larger than their
flex-basis size. The value of
flex-shrink is 1, so items can shrink if they need to rather than overflowing. The value of
auto. Items will either use any size set on the item in the main dimension, or they will get their size from the content size.
flex: auto is the same as using
flex: 1 1 auto; everything is as with
flex:initial but in this case the items can grow and fill the container as well as shrink if required.
flex: none will create fully inflexible flex items. It is as if you wrote
flex: 0 0 auto. The items cannot grow or shrink but will be laid out using flexbox with a
The shorthand you often see in tutorials is
flex: 1 or
flex: 2 and so on. This is as if you used
flex: 1 1 0. The items can grow and shrink from a
flex-basis of 0.
Try these shorthand values in the live example below.
Alignment, justification and distribution of free space between items
A key feature of flexbox is the ability to align and justify items on the main- and cross-axes, and to distribute space between flex items.
align-items property will align the items on the cross axis.
The initial value for this property is
stretch and this is why flex items stretch to the height of the tallest one by default. They are in fact stretching to fill the flex container — the tallest item is defining the height of that.
You could instead set
flex-start in order to make the items line up at the start of the flex container,
flex-end to align them to the end, or
center to align them in the centre. Try this in the live example — I have given the flex container a height in order that you can see how the items can be moved around inside the container. See what happens if you set the value of align-items to:
justify-content property is used to align the items on the main axis, the direction in which
flex-direction has set the flow. The initial value is
flex-start which will line the items up at the start edge of the container, but you could also set the value to
flex-end to line them up at the end, or
center to line them up in the centre.
You can also use the value
space-between to take all the spare space after the items have been laid out, and share it out evenly between the items so there will be an equal amount of space between each item. Or, to cause an equal amount of space on the right and left of each item use the value
Try the following values of
justify-content in the live example:
In a later article we will explore these properties in more depth, in order to have a better understanding of how they work. These simple examples however will be useful in the majority of use cases.
After reading this article you should have an understanding of the basic features of Flexbox. In the next article we will look at how this specification relates to other parts of CSS.