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Basic concepts of flexbox


The flexible box layout module (usually referred to as flexbox) is a one-dimensional layout model for distributing space between items and includes numerous alignment capabilities. This article gives an outline of the main features of flexbox, which we will explore in more detail in the rest of these guides.

When we describe flexbox as being one-dimensional we are describing the fact that flexbox deals with layout in one dimension at a time — either as a row or as a column. This can be contrasted with the two-dimensional model of CSS Grid Layout, which controls columns and rows together.



In this article

The two axes of flexbox

Start and end lines

The flex container

Multi-line flex containers with flex-wrap

The flex-flow shorthand

Properties applied to flex items

Alignment, justification and distribution of free space between items

Next steps



The two axes of flexbox

When working with flexbox you need to think in terms of two axes — the main axis and the cross axis. The main axis is defined by the flex-direction property, and the cross axis runs perpendicular to it. Everything we do with flexbox refers back to these axes, so it is worth understanding how they work from the outset.



The main axis

The main axis is defined by flex-direction, which has four possible values:

row

row-reverse

column

column-reverse

Should you choose row or row-reverse, your main axis will run along the row in the inline direction.



Choose column or column-reverse and your main axis will run in the block direction, from the top of the page to the bottom.





The cross axis

The cross axis runs perpendicular to the main axis. Therefore, if your flex-direction (main axis) is set to row or row-reverse the cross axis runs down the columns.



If your main axis is column or column-reverse then the cross axis runs along the rows.





Start and end lines

Another vital area of understanding is how flexbox makes no assumption about the writing mode of the document. Flexbox doesn't just assume that all lines of text start at the top left of a document and run towards the right-hand side, with new lines appearing one under the other. Rather, it supports all writing modes, like other logical properties and values.

You can read more about the relationship between flexbox and writing modes 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.

If the flex-direction is 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 that is laid out using flexbox is called a flex container. To create a flex container, set the area's display property to flex. When we do this, the direct children of that container become flex items. You can explicitly control whether the container itself is displayed inline or in block formatting context using inline flex or inline-flex for inline flex containers or block flex or flex for block level flex containers.



Initial values

As with all properties in CSS, some initial values are defined, so the contents of a new flex container will behave in the following way:

Items display in a row (the flex-direction property's default value is row).

The items start from the start edge of the main axis.

The items do not stretch on the main dimension but can shrink (a flex-item's flex-grow property's default value is 0 and its flex-shrink property's default value is 1).

The items will stretch to fill the size of the cross-axis (the align-items property's default value is stretch).

The flex-item's flex-basis property's default value is auto. This means that, in each case, it will be equal to the flex item width in horizontal writing mode, and the flex item height in vertical writing mode. If the corresponding width/height is also set to auto, the flex-basiscontent value is used instead.

All the items will be in a single row (the flex-wrap property's default value is nowrap), overflowing their container if their combined width/height exceeds the containing element width/height.

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 full length of the cross-axis.

You can see in the live sample below how this looks. Click "Play" to open the example in the MDN Playground and edit the items or add new items to try out the initial behavior of flexbox:


html
<div class="box"> <div>One</div> <div>Two</div> <div>Three <br />has <br />extra <br />text</div> </div>


css
.box > * { border: 2px solid rgb(96 139 168); border-radius: 5px; background-color: rgb(96 139 168 / 0.2); } .box { border: 2px dotted rgb(96 139 168); display: flex; }







Changing flex-direction

Adding the 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 flex-direction to 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 sample below has flex-direction set to row-reverse. Try the other values — row, column and column-reverse — to see what happens to the content.


html
<div class="box"> <div>One</div> <div>Two</div> <div>Three</div> </div>


css
.box > * { border: 2px solid rgb(96 139 168); border-radius: 5px; background-color: rgb(96 139 168 / 0.2); } .box { border: 2px dotted rgb(96 139 168); display: flex; flex-direction: row-reverse; }







Multi-line flex containers with flex-wrap

While flexbox is a one dimensional model, it is possible to make flex items wrap across multiple lines. If you do this, you should consider each line as a new flex container. Any space distribution will happen across each line, without reference to the previous or subsequent lines.

To cause wrapping behavior add the property flex-wrap with a value of wrap. Now, if your items are 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 is too wide for the flex container. As flex-wrap is set to wrap, the items wrap across multiple lines. If you set it to nowrap, which is the initial value, they will shrink to fit the container. They shrink because they are using initial flexbox values, including flex-shrink: 1, 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.


html
<div class="box"> <div>One</div> <div>Two</div> <div>Three</div> </div>


css
.box > * { border: 2px solid rgb(96 139 168); border-radius: 5px; background-color: rgb(96 139 168 / 0.2); width: 200px; } .box { width: 500px; border: 2px dotted rgb(96 139 168); display: flex; flex-wrap: wrap; }





Find out more about wrapping flex items in the guide Mastering wrapping of flex items.



The flex-flow shorthand

You can combine the two properties flex-direction and flex-wrap into the flex-flow shorthand.

In the live sample below, try changing the first value to one of the allowable values for flex-direction - row, row-reverse, column or column-reverse, and also change the second to wrap and nowrap.


html
<div class="box"> <div>One</div> <div>Two</div> <div>Three</div> </div>


css
.box > * { border: 2px solid rgb(96 139 168); border-radius: 5px; background-color: rgb(96 139 168 / 0.2); width: 200px; } .box { width: 500px; border: 2px dotted rgb(96 139 168); display: flex; flex-flow: row wrap; }







Properties applied to flex items

To control the inline-size of each flex item, we target them directly via three properties:

flex-grow

flex-shrink

flex-basis

We will take a brief look at these properties below, but if you want more comprehensive information, take a look at the Controlling ratios of flex items on the main axis guide.

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. The flex properties that we apply to the items themselves, enable dictating how that available space should be distributed among the sibling flex items.



The flex-basis property

The 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 item has a size. In the example above, all of the items have a width of 100 pixels. This is used as the flex-basis.

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.



The flex-grow property

With the flex-grow property set to a positive integer, if there is available space, the flex item can grow along the main axis from its flex-basis. Whether the item stretches to take up all the available space on that axis, or just a portion of the available space depends on if the other items are allowed to grow too and the value of their flex-grow properties.

Each item with a positive value consumes a portion of any available space based on their flex-grow value. 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. If we give our first item a flex-grow value of 2, and the other items a value of 1 each, there are a total of 4 parts; 2 parts of the available space will be given to the first item (100px out of 200px in the case of the example above) and 1 part each the other two (50px each out of the 200px total).



The flex-shrink property

Where the 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, then 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 than its siblings that have lower values.

An item can shrink down to its min-content size. This minimum size is 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: These values for flex-grow and 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 also 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-grow, flex-shrink, and 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 — flex-grow, flex-shrink, flex-basis.

The live sample 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.


html
<div class="box"> <div class="one">One</div> <div class="two">Two</div> <div class="three">Three</div> </div>
…(内容过长已截断)

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