Kompletní průvodce Flexboxem Triky CSS

Pozadí

Modul Flexbox Layout(Flexible Box) (doporučení kandidáta W3C z října 2017) má za cíl poskytnout efektivnější způsob rozvržení, zarovnání a distribuce prostoru mezi položkami v kontejneru, i když jejich velikost není známa a / nebo dynamická (tedy slovo „flex“).

Hlavní myšlenkou flex rozložení je dát kontejneru možnost měnit šířku / výšku (a pořadí) svých položek tak, aby co nejlépe vyplnily dostupné místo (většinou tak, aby vyhovovaly všem druhům zobrazovacích zařízení a velikostí obrazovek). Flex kontejner rozšiřuje položky tak, aby vyplnily dostupné volné místo, nebo je zmenší, aby se zabránilo přetečení.

Nejdůležitější je, že rozložení flexboxu je směrově agnostické na rozdíl od běžných rozvržení (blok, který je vertikálně založen a inline, který je horizontálně založen). I když fungují dobře pro stránky, chybí jim flexibilita (není určena slovní hříčka) pro podporu velkých nebo složitých aplikací (zejména pokud jde o změnu orientace, změnu velikosti, roztažení, zmenšení atd.).

Poznámka: Rozvržení Flexboxu je nejvhodnější pro komponenty aplikace a rozvržení v malém měřítku, zatímco rozvržení mřížky je určeno pro rozvržení ve větším měřítku.

Základy a terminologie

Vzhledem k tomu, že flexbox je celý modul a není jedinou vlastností, zahrnuje spoustu věcí, včetně celé jeho sady vlastností. Některé z nich jsou určeny k tomu, aby byly umístěny na kontejneru (nadřazený prvek, známý jako „flex kontejner“), zatímco ostatní jsou určeny k tomu, aby byly umístěny na podřízených (řečeno „flex položky“).

Pokud je „normální“ rozložení založeno na blokových i inline směrech toku, pak je flex rozložení založeno na „směrech flex-flow“. Podívejte se prosím na tento obrázek ze specifikace a vysvětlete hlavní myšlenku za flex rozložením.

Položky budou rozloženy po buď main axis(od main-startdo main-end) nebo příčnou osou (z cross-startk cross-end).

hlavní osa - Hlavní osa flex kontejneru je primární osa, podél které jsou rozloženy flex položky. Pozor, nemusí to být nutně horizontální; záleží na flex-directionvlastnosti (viz níže).
hlavní start | main-end - Položky flexu se umisťují do kontejneru počínaje od main-startu až po main-end.
main size - Šířka nebo výška položky flexu, podle toho, která je v hlavní dimenzi, je hlavní velikostí položky. Vlastností hlavní velikosti položky flex je vlastnost 'width' nebo 'height', podle toho, co je v hlavní dimenzi.
příčná osa - Osa kolmá k hlavní ose se nazývá příčná osa. Jeho směr závisí na směru hlavní osy.
cross-start | cross-end - čáry Flex jsou vyplněny položkami a umístěny do kontejneru počínaje na straně cross-start flex kontejneru a směřující ke straně cross-end.
velikost kříže - Šířka nebo výška položky flex, podle toho, která je v rozměru kříže, je velikost kříže položky. Vlastnost velikosti kříže je libovolná z „šířky“ nebo „výšky“, která je v rozměru kříže.

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Vlastnosti pro rodiče
(flex kontejner)

Zobrazit

To definuje flex kontejner; inline nebo block v závislosti na dané hodnotě. Umožňuje kontext flexe pro všechny jeho přímé potomky.

.container ( display: flex; /* or inline-flex */ )

Všimněte si, že sloupce CSS nemají žádný vliv na flex kontejner.

flex-direction

Tím se vytvoří hlavní osa, čímž se definuje směr, ve kterém jsou flex položky umístěny do flex kontejneru. Flexbox je (kromě volitelného balení) koncept jednosměrného rozvržení. Přemýšlejte o flexibilních položkách jako o primárním rozložení buď ve vodorovných řadách, nebo ve svislých sloupcích.

.container ( flex-direction: row | row-reverse | column | column-reverse; )

row(výchozí): zleva doprava dovnitř ltr; zprava doleva dovnitřrtl
row-reverse: zprava doleva dovnitř ltr; zleva doprava dovnitřrtl
column: stejné jako, rowale shora dolů
column-reverse: stejné jako, row-reverseale zdola nahoru

flex-wrap

Ve výchozím nastavení se všechny položky flexu budou snažit vejít na jeden řádek. Můžete to změnit a povolit, aby se položky podle potřeby zabalily s touto vlastností.

.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.

flex-flow

This is a shorthand for the flex-direction and flex-wrap properties, which together define the flex container’s main and cross axes. The default value is row nowrap.

.container ( flex-flow: column wrap; )

justify-content

This defines the alignment along the main axis. It helps distribute extra free space leftover 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 | start | end | left | right… + safe | unsafe; )

flex-start (default): items are packed toward the start of the flex-direction.
flex-end: items are packed toward the end of the flex-direction.
start: items are packed toward the start of the writing-mode direction.
end: items are packed toward the end of the writing-mode direction.
left: items are packed toward left edge of the container, unless that doesn’t make sense with the flex-direction, then it behaves like start.
right: items are packed toward right edge of the container, unless that doesn’t make sense with the flex-direction, then it behaves like end.
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.

Note that that browser support for these values is nuanced. For example, space-between never got support from some versions of Edge, and start/end/left/right aren’t in Chrome yet. MDN has detailed charts. The safest values are flex-start, flex-end, and center.

There are also two additional keywords you can pair with these values: safe and unsafe. Using safe ensures that however you do this type of positioning, you can’t push an element such that it renders off-screen (e.g. off the top) in such a way the content can’t be scrolled too (called “data loss”).

align-items

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 | first baseline | last baseline | start | end | self-start | self-end +… safe | unsafe; )

stretch (default): stretch to fill the container (still respect min-width/max-width)
flex-start / start / self-start: items are placed at the start of the cross axis. The difference between these is subtle, and is about respecting the flex-direction rules or the writing-mode rules.
flex-end / end / self-end: items are placed at the end of the cross axis. The difference again is subtle and is about respecting flex-direction rules vs. writing-mode rules.
center: items are centered in the cross-axis
baseline: items are aligned such as their baselines align

The safe and unsafe modifier keywords can be used in conjunction with all the rest of these keywords (although note browser support), and deal with helping you prevent aligning elements such that the content becomes inaccessible.

align-content

This aligns a flex container’s lines within when there is extra space in the cross-axis, similar to how justify-content aligns individual items within the main-axis.

Note: This property only takes effect on multi-line flexible containers, where flex-flow is set to either wrap or wrap-reverse). A single-line flexible container (i.e. where flex-flow is set to its default value, no-wrap) will not reflect align-content.

.container ( align-content: flex-start | flex-end | center | space-between | space-around | space-evenly | stretch | start | end | baseline | first baseline | last baseline +… safe | unsafe; )

normal (default): items are packed in their default position as if no value was set.
flex-start / start: items packed to the start of the container. The (more supported) flex-start honors the flex-direction while start honors the writing-mode direction.
flex-end / end: items packed to the end of the container. The (more support) flex-end honors the flex-direction while end honors the writing-mode direction.
center: items centered in the container
space-between: items evenly distributed; the first line is at the start of the container while the last one is at the end
space-around: items evenly distributed with equal space around each line
space-evenly: items are evenly distributed with equal space around them
stretch: lines stretch to take up the remaining space

Properties for the Children
(flex items)

order

By default, flex items are laid out in the source order. However, the order property controls the order in which they appear in the flex container.

.item ( order: 5; /* default is 0 */ )

flex-grow

This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.

If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least).

.item ( flex-grow: 4; /* default 0 */ )

Negative numbers are invalid.

flex-shrink

This defines the ability for a flex item to shrink if necessary.

.item ( flex-shrink: 3; /* default 1 */ )

Negative numbers are invalid.

flex-basis

This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The auto keyword means “look at my width or height property” (which was temporarily done by the main-size keyword until deprecated). The content keyword means “size it based on the item’s content” - this keyword isn’t well supported yet, so it’s hard to test and harder to know what its brethren max-content, min-content, and fit-content do.

.item ( flex-basis: | auto; /* default auto */ )

If set to 0, the extra space around content isn’t factored in. If set to auto, the extra space is distributed based on its flex-grow value. See this graphic.

flex

This is the shorthand for flex-grow, flex-shrink and flex-basis combined. The second and third parameters (flex-shrink and flex-basis) are optional. The default is 0 1 auto, but if you set it with a single number value, it’s like 1 0.

.item ( flex: none | ( ? || ) )

It is recommended that you use this shorthand property rather than set the individual properties. The shorthand sets the other values intelligently.

align-self

This allows the default alignment (or the one specified by align-items) to be overridden for individual flex items.

Please see the align-items explanation to understand the available values.

.item ( align-self: auto | flex-start | flex-end | center | baseline | stretch; )

Note that float, clear and vertical-align have no effect on a flex item.

Examples

Let’s start with a very very simple example, solving an almost daily problem: perfect centering. It couldn’t be any simpler if you use flexbox.

.parent ( display: flex; height: 300px; /* Or whatever */ ) .child ( width: 100px; /* Or whatever */ height: 100px; /* Or whatever */ margin: auto; /* Magic! */ )

This relies on the fact a margin set to auto in a flex container absorb extra space. So setting a vertical margin of auto will make the item perfectly centered in both axes.

Now let’s use some more properties. Consider a list of 6 items, all with fixed dimensions, but can be auto-sized. We want them to be evenly distributed on the horizontal axis so that when we resize the browser, everything scales nicely, and without media queries.

.flex-container ( /* We first create a flex layout context */ display: flex; /* Then we define the flow direction and if we allow the items to wrap * Remember this is the same as: * flex-direction: row; * flex-wrap: wrap; */ flex-flow: row wrap; /* Then we define how is distributed the remaining space */ justify-content: space-around; )

Done. Everything else is just some styling concern. Below is a pen featuring this example. Be sure to go to CodePen and try resizing your windows to see what happens.

Let’s try something else. Imagine we have a right-aligned navigation element on the very top of our website, but we want it to be centered on medium-sized screens and single-columned on small devices. Easy enough.

/* Large */ .navigation ( display: flex; flex-flow: row wrap; /* This aligns items to the end line on main-axis */ justify-content: flex-end; ) /* Medium screens */ @media all and (max-width: 800px) ( .navigation ( /* When on medium sized screens, we center it by evenly distributing empty space around items */ justify-content: space-around; ) ) /* Small screens */ @media all and (max-width: 500px) ( .navigation ( /* On small screens, we are no longer using row direction but column */ flex-direction: column; ) )

Let’s try something even better by playing with flex items flexibility! What about a mobile-first 3-columns layout with full-width header and footer. And independent from source order.

.wrapper ( display: flex; flex-flow: row wrap; ) /* We tell all items to be 100% width, via flex-basis */ .wrapper> * ( flex: 1 100%; ) /* We rely on source order for mobile-first approach * in this case: * 1. header * 2. article * 3. aside 1 * 4. aside 2 * 5. footer */ /* Medium screens */ @media all and (min-width: 600px) ( /* We tell both sidebars to share a row */ .aside ( flex: 1 auto; ) ) /* Large screens */ @media all and (min-width: 800px) ( /* We invert order of first sidebar and main * And tell the main element to take twice as much width as the other two sidebars */ .main ( flex: 2 0px; ) .aside-1 ( order: 1; ) .main ( order: 2; ) .aside-2 ( order: 3; ) .footer ( order: 4; ) )

Prefixing Flexbox

Flexbox requires some vendor prefixing to support the most browsers possible. It doesn’t just include prepending properties with the vendor prefix, but there are actually entirely different property and value names. This is because the Flexbox spec has changed over time, creating an “old”, “tweener”, and “new” versions.

Perhaps the best way to handle this is to write in the new (and final) syntax and run your CSS through Autoprefixer, which handles the fallbacks very well.

Alternatively, here’s a Sass @mixin to help with some of the prefixing, which also gives you an idea of what kind of things need to be done:

@mixin flexbox() ( display: -webkit-box; display: -moz-box; display: -ms-flexbox; display: -webkit-flex; display: flex; ) @mixin flex($values) ( -webkit-box-flex: $values; -moz-box-flex: $values; -webkit-flex: $values; -ms-flex: $values; flex: $values; ) @mixin order($val) ( -webkit-box-ordinal-group: $val; -moz-box-ordinal-group: $val; -ms-flex-order: $val; -webkit-order: $val; order: $val; ) .wrapper ( @include flexbox(); ) .item ( @include flex(1 200px); @include order(2); )

Related Properties

A Complete Guide to Grid
Almanac entries on Grid properties, like grid-row / grid-column

Other Resources

Flexbox in the CSS specifications
Flexbox at MDN
Flexbox at Opera
Diving into Flexbox by Bocoup
Mixing syntaxes for best browser support on CSS-Tricks
Flexbox by Raphael Goetter (FR)
Flexplorer by Bennett Feely

Bugs

Flexbox is certainly not without its bugs. The best collection of them I’ve seen is Philip Walton and Greg Whitworth’s Flexbugs. It’s an open-source place to track all of them, so I think it’s best to just link to that.

Podpora prohlížeče

Rozděleno podle „verze“ flexboxu:

(new) means the recent syntax from the specification (eg display: flex;)
(tweener) znamená zvláštní neoficiální syntax z roku 2011 (např. display: flexbox;)
(old) znamená starou syntaxi z roku 2009 (např. display: box;)

Chrome	Safari	Firefox	Opera	TJ	Okraj	Android	iOS
20- (starý) 21+ (nový)	3.1+ (starý) 6.1+ (nový)	2-21 (starý) 22+ (nový)	12.1+ (nový)	10 (tweener) 11+ (nový)	17+ (nové)	2.1+ (starý) 4.4+ (nový)	3,2+ (starý) 7,1+ (nový)

Prohlížeč Blackberry 10+ podporuje novou syntaxi.