En komplet guide til Flexbox CSS-tricks

Baggrund

Den Flexbox Layout(Flexible Box) modul (en W3C Candidate Anbefaling fra oktober 2017) sigter mod at skabe en mere effektiv måde at lægge ud, align og distribuere plads blandt elementer i en beholder, selv når deres størrelse er ukendt og / eller dynamisk (dermed ord "flex").

Hovedideen bag flexlayoutet er at give containeren muligheden for at ændre dens genstanders bredde / højde (og rækkefølge) for bedst at udfylde den tilgængelige plads (for det meste til at rumme alle slags displayenheder og skærmstørrelser). En flexcontainer udvider emner for at udfylde ledig plads eller krymper dem for at forhindre overløb.

Vigtigst er det, at flexbox-layoutet er retningsagnostisk i modsætning til de almindelige layouts (blok, der er lodret og inline, som er vandret). Mens de fungerer godt for sider, mangler de fleksibilitet (ingen ordspil beregnet) til at understøtte store eller komplekse applikationer (især når det kommer til at ændre retning, ændre størrelse, strække, krympe osv.).

Bemærk: Flexbox-layout er mest hensigtsmæssigt for komponenterne i en applikation og småskala-layouts, mens Grid-layoutet er beregnet til layouter i større skala.

Grundlæggende og terminologi

Da flexbox er et helt modul og ikke en enkelt egenskab, involverer det mange ting inklusive hele dets sæt egenskaber. Nogle af dem er beregnet til at blive indstillet på beholderen (overordnet element, kendt som "flexcontainer"), mens de andre er beregnet til at blive indstillet på børnene (sagde "flex-emner").

Hvis "almindeligt" layout er baseret på både blok- og inline-strømningsretninger, er flex-layoutet baseret på "flex-flow-retninger". Se venligst denne figur fra specifikationen og forklar hovedideen bag flexlayoutet.

Elementer vil blive lagt efter enten main axis(fra main-starttil main-end) eller tværaksen (fra cross-starttil cross-end).

hovedakse - Hovedaksen for en flexcontainer er den primære akse, langs hvilken flexgenstande er lagt. Pas på, det er ikke nødvendigvis vandret; det afhænger af flex-directionejendommen (se nedenfor).
hovedstart | main-end - Flex-emnerne placeres i containeren startende fra main-start og går til main-end.
hovedstørrelse - En flex-vares bredde eller højde, alt efter hvilken der er i hoveddimensionen, er varens hovedstørrelse. Flex-elementets egenskab for hovedstørrelse er enten egenskaben 'bredde' eller 'højde', alt efter hvad der er i hoveddimensionen.
tværakse - Aksen vinkelret på hovedaksen kaldes tværaksen. Dens retning afhænger af hovedaksen retning.
krydsstart | tværende - Flex-linjer fyldes med genstande og placeres i beholderen, der starter på tværsiden af flexcontaineren og går mod tværsiden.
krydsstørrelse - Bredden eller højden på et fleksibelt emne, alt efter hvad der er i tværdimensionen, er varens tværstørrelse. Tværstørrelsesegenskaben er den, der er af 'bredde' eller 'højde', der er i tværdimensionen.

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Egenskaber til forældren
(flexcontainer)

Skærm

Dette definerer en flexcontainer; indbygget eller blokeret afhængigt af den givne værdi. Det muliggør en fleksibel kontekst for alle sine direkte børn.

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

Bemærk, at CSS-kolonner ikke har nogen effekt på en flexcontainer.

flex-retning

Dette etablerer hovedaksen og definerer således retningen flex-emner placeres i flexcontaineren. Flexbox er (bortset fra valgfri indpakning) et layout i en retning. Tænk på flex-emner som primært at lægge enten i vandrette rækker eller lodrette kolonner.

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

row(standard): venstre mod højre i ltr; højre mod venstre indrtl
row-reverse: højre mod venstre ind ltr; venstre mod højre indrtl
column: samme som rowmen top til bund
column-reverse: det samme som fra row-reversebund til top

flex-wrap

Som standard forsøger flex-emner alle at passe ind på en linje. Du kan ændre det og tillade, at varerne pakkes ind efter behov med denne egenskab.

.container ( flex-wrap: nowrap | wrap | wrap-reverse; )

nowrap (standard): alle flex-emner vil være på en linje
wrap: flex-emner vikles ind på flere linjer, fra top til bund.
wrap-reverse: flex-emner vikles ind på flere linjer fra bund til top.

Der er nogle visuelle demoer flex-wrapherfra.

flex-flow

Dette er en forkortelse for flex-directionog flex-wrapegenskaber, som tilsammen definerer det fleksible beholderens primære og cross akser. Standardværdien er row nowrap.

.container ( flex-flow: column wrap; )

retfærdiggør-indhold

Dette definerer justeringen langs hovedaksen. Det hjælper med at distribuere ekstra ledig plads tilovers, når enten alle flex-genstandene på en linje er ufleksible eller er fleksible, men har nået deres maksimale størrelse. Det udøver også en vis kontrol over justeringen af emner, når de overløber linjen.

.container ( justify-content: flex-start | flex-end | center | space-between | space-around | space-evenly | start | end | left | right… + safe | unsafe; )

flex-start (standard): emner pakkes mod starten af flexretningen.
flex-end: emner pakkes mod slutningen af flexretningen.
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.

Browsersupport

Brudt op af “version” af flexbox:

(ny) betyder den nylige syntaks fra specifikationen (f.eks. display: flex;)
(tweener) betyder en ulige uofficiel syntaks fra 2011 (f.eks. display: flexbox;)
(gammel) betyder den gamle syntaks fra 2009 (f.eks. display: box;)

Chrome	Safari	Firefox	Opera	IE	Edge	Android	iOS
20- (gammel) 21+ (ny)	3.1+ (gammel) 6.1+ (ny)	2-21 (gammel) 22+ (ny)	12.1+ (nyt)	10 (tweener) 11+ (ny)	17+ (nyt)	2.1+ (gammel) 4.4+ (ny)	3.2+ (gammel) 7.1+ (ny)

Blackberry browser 10+ understøtter den nye syntaks.