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Original author: 
David Storey

  

Flexible box layout (or flexbox) is a new box model optimized for UI layout. As one of the first CSS modules designed for actual layout (floats were really meant mostly for things such as wrapping text around images), it makes a lot of tasks much easier, or even possible at all. Flexbox’s repertoire includes the simple centering of elements (both horizontally and vertically), the expansion and contraction of elements to fill available space, and source-code independent layout, among others abilities.

Flexbox has lived a storied existence. It started as a feature of Mozilla’s XUL, where it was used to lay out application UI, such as the toolbars in Firefox, and it has since been rewritten multiple times. The specification has only recently reached stability, and we have fairly complete support across the latest versions of the leading browsers.

There are, however, some caveats. The specification changed between the implementation in Internet Explorer (IE) and the release of IE 10, so you will need to use a slightly different syntax. Chrome currently still requires the -webkit- prefix, and Firefox and Safari are still on the much older syntax. Firefox has updated to the latest specification, but that implementation is currently behind a runtime flag until it is considered stable and bug-free enough to be turned on by default. Until then, Firefox still requires the old syntax.

When you specify that an element will use the flexbox model, its children are laid out along either the horizontal or vertical axis, depending on the direction specified. The widths of these children expand or contract to fill the available space, based on the flexible length they are assigned.

Example: Horizontal And Vertical Centering (Or The Holy Grail Of Web Design)

Being able to center an element on the page is perhaps the number one wish among Web designers — yes, probably even higher than gaining the highly prized parent selector or putting IE 6 out of its misery (OK, maybe a close second then). With flexbox, this is trivially easy. Let’s start with a basic HTML template, with a heading that we want to center. Eventually, once we’ve added all the styling, it will end up looking like this vertically and horizontally centered demo.


<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="utf-8"/>
   <title>Centering an Element on the Page</title>
</head>
<body>
   <h1>OMG, I’m centered</h1>
</body>
</html>

Nothing special here, not even a wrapper div. The magic all happens in the CSS:


html {
   height: 100%;
} 

body {
   display: -webkit-box;   /* OLD: Safari,  iOS, Android browser, older WebKit browsers.  */
   display: -moz-box;   /* OLD: Firefox (buggy) */ 
   display: -ms-flexbox;   /* MID: IE 10 */
   display: -webkit-flex;    /* NEW, Chrome 21+ */
   display: flex;       /* NEW: Opera 12.1, Firefox 22+ */

   -webkit-box-align: center; -moz-box-align: center; /* OLD… */
   -ms-flex-align: center; /* You know the drill now… */
   -webkit-align-items: center;
   align-items: center;

    -webkit-box-pack: center; -moz-box-pack: center; 
   -ms-flex-pack: center; 
   -webkit-justify-content: center;
   justify-content: center;

   margin: 0;
   height: 100%;
   width: 100% /* needed for Firefox */
} 

h1 {
   display: -webkit-box; display: -moz-box;
   display: -ms-flexbox;
   display: -webkit-flex;
   display: flex;
 
   -webkit-box-align: center; -moz-box-align: center;
   -ms-flex-align: center;
   -webkit-align-items: center;
   align-items: center;

   height: 10rem;
}

I’ve included all of the different prefixed versions in the CSS above, from the very oldest, which is still needed, to the modern and hopefully final syntax. This might look confusing, but the different syntaxes map fairly well to each other, and I’ve included tables at the end of this article to show the exact mappings.

This is not exactly all of the CSS needed for our example, because I’ve stripped out the extra styling that you probably already know how to use in order to save space.

Let’s look at the CSS that is needed to center the heading on the page. First, we set the html and body elements to have 100% height and remove any margins. This will make the container of our h1 take up the full height of the browser’s window. Firefox also needs a width specified on the body to force it to behave. Now, we just need to center everything.

Enabling Flexbox

Because the body element contains the heading that we want to center, we will set its display value to flex:


body {
   display: flex;
}

This switches the body element to use the flexbox layout, rather than the regular block layout. All of its children in the flow of the document (i.e. not absolutely positioned elements) will now become flex items.

The syntax used by IE 10 is display: -ms-flexbox, while older Firefox and WebKit browsers use display: -prefix-box (where prefix is either moz or webkit). You can see the tables at the end of this article to see the mappings of the various versions.

What do we gain now that our elements have been to yoga class and become all flexible? They gain untold powers: they can flex their size and position relative to the available space; they can be laid out either horizontally or vertically; and they can even achieve source-order independence. (Two holy grails in one specification? We’re doing well.)

Centering Horizontally

Next, we want to horizontally center our h1 element. No big deal, you might say; but it is somewhat easier than playing around with auto margins. We just need to tell the flexbox to center its flex items. By default, flex items are laid out horizontally, so setting the justify-content property will align the items along the main axis:


body {
   display: flex;
   justify-content: center;
}

For IE 10, the property is called flex-pack, while for older browsers it is box-pack (again, with the appropriate prefixes). The other possible values are flex-start, flex-end, space-between and space-around. These are start, end, justify and distribute, respectively, in IE 10 and the old specification (distribute is, however, not supported in the old specification). The flex-start value aligns to the left (or to the right with right-to-left text), flex-end aligns to the right, space-between evenly distributes the elements along the axis, and space-around evenly distributes along the axis, with half-sized spaces at the start and end of the line.

To explicitly set the axis that the element is aligned along, you can do this with the flex-flow property. The default is row, which will give us the same result that we’ve just achieved. To align along the vertical axis, we can use flex-flow: column. If we add this to our example, you will notice that the element is vertically centered but loses the horizontal centering. Reversing the order by appending -reverse to the row or column values is also possible (flex-flow: row-reverse or flex-flow: column-reverse), but that won’t do much in our example because we have only one item.

There are some differences here in the various versions of the specification, which are highlighted at the end of this article. Another caveat to bear in mind is that flex-flow directions are writing-mode sensitive. That is, when using writing-mode: vertical-rl to switch to vertical text layout (as used traditionally in China, Japan and Korea), flex-flow: row will align the items vertically, and column will align them horizontally.

Centering Vertically

Centering vertically is as easy as centering horizontally. We just need to use the appropriate property to align along the “cross-axis.” The what? The cross-axis is basically the axis perpendicular to the main one. So, if flex items are aligned horizontally, then the cross-axis would be vertical, and vice versa. We set this with the align-items property (flex-align in IE 10, and box-align for older browsers):


body {
   /* Remember to use the other versions for IE 10 and older browsers! */
   display: flex;
   justify-content: center;
   align-items: center;
}

This is all there is to centering elements with flexbox! We can also use the flex-start (start) and flex-end (end) values, as well as baseline and stretch. Let’s have another look at the finished example:

figure1.1_mini
Simple horizontal and vertical centering using flexbox. Larger view.

You might notice that the text is also center-aligned vertically inside the h1 element. This could have been done with margins or a line height, but we used flexbox again to show that it works with anonymous boxes (in this case, the line of text inside the h1 element). No matter how high the h1 element gets, the text will always be in the center:


h1 {
   /* Remember to use the other versions for IE 10 and older browsers! */
   display: flex;
   align-items: center;
   height: 10rem;
}

Flexible Sizes

If centering elements was all flexbox could do, it’d be pretty darn cool. But there is more. Let’s see how flex items can expand and contract to fit the available space within a flexbox element. Point your browser to this next example.

figure1.2_mini
An interactive slideshow built using flexbox. Larger view.

The HTML and CSS for this example are similar to the previous one’s. We’re enabling flexbox and centering the elements on the page in the same way. In addition, we want to make the title (inside the header element) remain consistent in size, while the five boxes (the section elements) adjust in size to fill the width of the window. To do this, we use the new flex property:


section {
   /* removed other styles to save space */
   -prefix-box-flex: 1; /* old spec webkit, moz */
   flex: 1;
   height: 250px;
}

What we’ve just done here is to make each section element take up 1 flex unit. Because we haven’t set any explicit width, each of the five boxes will be the same width. The header element will take up a set width (277 pixels) because it is not flexible. We divide the remaining width inside the body element by 5 to calculate the width of each of the section elements. Now, if we resize the browser window, the section elements will grow or shrink.

In this example, we’ve set a consistent height, but this could be set to be flexible, too, in exactly the same way. We probably wouldn’t always want all elements to be the same size, so let’s make one bigger. On hover, we’ve set the element to take up 2 flex units:


section:hover {
   -prefix-box-flex: 2;
   flex: 2;
   cursor: pointer;
}

Now the available space is divided by 6 rather than 5, and the hovered element gets twice the base amount. Note that an element with 2 flex units does not necessarily become twice as wide as one with 1 unit. It just gets twice the share of the available space added to its “preferred width.” In our examples, the “preferred width” is 0 (the default).

Source-Order Independence

For our last party trick, we’ll study how to achieve source-order independence in our layouts. When clicking on a box, we will tell that element to move to the left of all the other boxes, directly after the title. All we have to do is set the order with the order property. By default, all flex items are in the 0 position. Because they’re in the same position, they follow the source order. Click on your favorite person in the updated example to see their order change.

figure1.3_mini
An interactive slideshow with flex-order. Larger view.

To make our chosen element move to the first position, we just have to set a lower number. I chose -1. We also need to set the header to -1 so that the selected section element doesn’t get moved before it:


header {
   -prefix-box-ordinal-group: 1; /* old spec; must be positive */
   -ms-flex-order: -1; /* IE 10 syntax */
   order: -1; /* new syntax */
} 

section[aria-pressed="true"] {
   /* Set order lower than 0 so it moves before other section elements,
      except old spec, where it must be positive.
 */
   -prefix-box-ordinal-group: 1;
   -ms-flex-order: -1;
   order: -1;

   -prefix-box-flex: 3;
   flex: 3;
   max-width: 370px; /* Stops it from getting too wide. */
}

In the old specification, the property for setting the order (box-ordinal-group) accepts only a positive integer. Therefore, I’ve set the order to 2 for each section element (code not shown) and updated it to 1 for the active element. If you are wondering what aria-pressed="true" means in the example above, it is a WAI-ARIA attribute/value that I add via JavaScript when the user clicks on one of the sections.

This relays accessibility hints to the underlying system and to assistive technology to tell the user that that element is pressed and, thus, active. If you’d like more information on WAI-ARIA, check out “Introduction to WAI-ARIA” by Gez Lemon. Because I’m adding the attribute after the user clicks, this example requires a simple JavaScript file in order to work, but flexbox itself doesn’t require it; it’s just there to handle the user interaction.

Hopefully, this has given you some inspiration and enough introductory knowledge of flexbox to enable you to experiment with your own designs.

Syntax Changes

As you will have noticed throughout this article, the syntax has changed a number of times since it was first implemented. To aid backward- and forward-porting between the different versions, we’ve included tables below, which map the changes between the specifications.

Specification versions

Specification
IE
Opera
Firefox
Chrome
Safari

Standard
11?
12.10+ *
Behind flag
21+ (-webkit-)

Mid
10 (-ms-)

Old

3+ (-moz-)
<21 (-webkit-)
3+ (-webkit-)

* Opera will soon switch to WebKit. It will then require the -webkit- prefix if it has not been dropped by that time.

Enabling flexbox: setting an element to be a flex container

Specification
Property name
Block-level flex
Inline-level flex

Standard
display
flex
inline-flex

Mid
display
flexbox
inline-flexbox

Old
display
box
inline-box

Axis alignment: specifying alignment of items along the main flexbox axis

Specification
Property name
start
center
end
justify
distribute

Standard
justify-content
flex-start
center
flex-end
space-between
space-around

Mid
flex-pack
start
center
end
justify
distribute

Old
box-pack
start
center
end
justify
N/A

Cross-axis alignment: specifying alignment of items along the cross-axis

Specification
Property name
start
center
end
baseline
stretch

Standard
align-items
flex-start
center
flex-end
baseline
stretch

Mid
flex-align
start
center
end
baseline
stretch

Old
box-align
start
center
end
baseline
stretch

Individual cross-axis alignment: override to align individual items along the cross-axis

Specification
Property name
auto
start
center
end
baseline
stretch

Standard
align-self
auto
flex-start
center
flex-end
baseline
stretch

Mid
flex-item-align
auto
start
center
end
baseline
stretch

Old
N/A

Flex line alignment: specifying alignment of flex lines along the cross-axis

Specification
Property name
start
center
end
justify
distribute
stretch

Standard
align-content
flex-start
center
flex-end
space-between
space-around
stretch

Mid
flex-line-pack
start
center
end
justify
distribute
stretch

Old
N/A

This takes effect only when there are multiple flex lines, which is the case when flex items are allowed to wrap using the flex-wrap property and there isn’t enough space for all flex items to display on one line. This will align each line, rather than each item.

Display order: specifying the order of flex items

Specification
Property name
Value

Standard
order

Mid
flex-order
<number>

Old
box-ordinal-group
<integer>

Flexibility: specifying how the size of items flex

Specification
Property name
Value

Standard
flex
none | [ <flex-grow> <flex-shrink>? || <flex-basis>]

Mid
flex
none | [ [ <pos-flex> <neg-flex>? ] || <preferred-size> ]

Old
box-flex
<number>

The flex property is more or less unchanged between the new standard and the draft supported by Microsoft. The main difference is that it has been converted to a shorthand in the new version, with separate properties: flex-grow, flex-shrink and flex-basis. The values may be used in the same way in the shorthand. However, the default value for flex-shrink (previously called negative flex) is now 1. This means that items do not shrink by default. Previously, negative free space would be distributed using the flex-shrink ratio, but now it is distributed in proportion to flex-basis multiplied by the flex-shrink ratio.

Direction: specifying the direction of the main flexbox axis

Specification
Property name
Horizontal
Reversed horizontal
Vertical
Reversed vertical

Standard
flex-direction
row
row-reverse
column
column-reverse

Mid
flex-direction
row
row-reverse
column
column-reverse

Old
box-orient

box-direction
horizontal

normal
horizontal

reverse
vertical

normal
vertical

reverse

In the old version of the specification, the box-direction property needs to be set to reverse to get the same behavior as row-reverse or column-reverse in the later version of the specification. This can be omitted if you want the same behavior as row or column because normal is the initial value.

When setting the direction to reverse, the main flexbox axis is flipped. This means that when using a left-to-right writing system, the items will display from right to left when row-reverse is specified. Similarly, column-reverse will lay out flex items from bottom to top, instead of top to bottom.

The old version of the specification also has writing mode-independent values for box-orient. When using a left-to-write writing system, horizontal may be substituted for inline-axis, and vertical may be substituted for block-axis. If you are using a top-to-bottom writing system, such as those traditional in East Asia, then these values would be flipped.

Wrapping: specifying whether and how flex items wrap along the cross-axis

Specification
Property name
No wrapping
Wrapping
Reversed wrap

Standard
flex-wrap
nowrap
wrap
wrap-reverse

Mid
flex-wrap
nowrap
wrap
wrap-reverse

Old
box-lines
single
multiple
N/A

The wrap-reverse value flips the start and end of the cross-axis, so that if flex items are laid out horizontally, instead of items wrapping onto a new line below, they will wrap onto a new line above.

At the time of writing, Firefox does not support the flex-wrap or older box-lines property. It also doesn’t support the shorthand.

The current specification has a flex-flow shorthand, which controls both wrapping and direction. The behavior is the same as the one in the version of the specification implemented by IE 10. It is also currently not supported by Firefox, so I would recommend to avoid using it when specifying only the flex-direction value.

Conclusion

Well, that’s a (flex-)wrap. In this article, I’ve introduced some of the myriad of possibilities afforded by flexbox. Be it source-order independence, flexible sizing or just the humble centering of elements, I’m sure you can find ways to employ flexbox in your websites and applications. The syntax has settled down (finally!), and implementations are here. All major browsers now support flexbox in at least their latest versions.

While some browsers use an older syntax, Firefox looks like it is close to updating, and IE 11 uses the latest version in leaked Windows Blue builds. There is currently no word on Safari, but it is a no-brainer considering that Chrome had the latest syntax before the Blink-WebKit split. For the time being, use the tables above to map the various syntaxes, and get your flex on.

Layout in CSS is only getting more powerful, and flexbox is one of the first steps out of the quagmire we’ve found ourselves in over the years, first with table-based layouts, then float-based layouts. IE 10 already supports an early draft of the Grid layout specification, which is great for page layout, and Regions and Exclusions will revolutionize how we handle content flow and layout.

Flexbox can be used today if you only need to support relatively modern browsers or can provide a fallback, and in the not too distant future, all sorts of options will be available, so that we can use the best tool for the job. Flexbox is shaping up to be a mighty fine tool.

Further Reading

(al)

© David Storey for Smashing Magazine, 2013.

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I am no longer committed to supporting any Flash related open-source projects.

Here is why. When I started using the Flash Player it was quite easy to reach its limits. However you were able to get around those limitations with clever hacks and debatable optimization techniques. I was always keen to share my knowledge with the community and to explore all possible options to achieve best performance.

The Flash Player has been hibernating for half a decade now. The only glimpse of performance was finally a set of specialized op-codes which allow you to modify an array of bytes. In layman’s terms this means it was finally possible to do a[b] = c with an acceptable performance. So I wrote a tool which allows you to do just that and many other things. I have spent a good time of my free time trying to improve the performance of the Flash Player and contributing all my code to the community.

As a reminder: I showed some drastic performance improvements at Flash on the Beach in 2009. That was three years ago. It was not necessary to modify the Flash Player and it was not necessary to modify the ActionScript language.

The Adobe roadmap for the Flash runtimes states that Flash Player “Dolores”

  • will support ActionScript Workers
  • comes with improved performance for Apple iOS
  • and ActionScript 3 APIs to access the fast-memory op-codes

This player should be released in the second half of 2012. The “Next” Flash Player will finally include

  • modernizing the core of the Flash runtime
  • work on the VM
  • updates to the ActionScript language

This is planned for 2013 apparently. And what can we expect? Type inference, static typing as a default, and hardware-oriented numeric types. Hooray, so it will be finally possible in 2013 to write a[b] = c without having to use some weird fast-memory op-codes. If we look back to the year 2009 this makes me really sad.

With the introduction of the speed tax you will now have to license your application. No matter if you make money out of it or not. Now I think that 9% is a decent number and I can understand Adobe’s position on this. In fact it is much more friendly than the 30% Google or Apple take. However the AppStore was an invention. What is the invention here? Squeezing money out of an already existing feature, and suddenly making it unavailable after people have been relying on it for years to push the boundaries of the platform and actually innovate?

But for the hell of it, a[b] = c is not a premium feature. Nor are hardware accelerated graphics. That is what I would expect from any decent runtime.

Limiting the capabilities of a runtime — by defaulting back to software rendering for instance — will make it less attractive to use it in the first place. You are probably not interested to go through a signing progress for a small demo. So your performance might be crap, people will complain about the Flash Player taking 100% CPU because its using software rendering (YEY! 2013!), laptop fans will start to dance and you will look like a bad developer because that other guy got the same thing running with hardware acceleration. Or you could use a different technology.

Why is this bad? Because apparently this signing with a $50k threshold targets the enterprise and small developers seem to be acceptable collateral damage. However thinking about the next five to ten years: who is going to write ActionScript code if it is no longer attractive to play around with it in the first place?

We still rely on the Flash Player at audiotool.com. I am still developing for it and we will probably have to use it as long as there is no alternative. Me no longer supporting open-source tools is just me no longer spending my personal time for a platform that I would not use for private stuff. Work is of course not always about fun. But fortunately I am able to spend 90% of my time writing Scala code.

I will finish this blog post with some bad karma:

It’s also worth noting that the new Adobe license will prohibit scenarios where you’d have the first level of a game in the Flash Player, and the full experience inside the Unity Web Player. Alas, this is something you’ll need to be aware of if you were considering such a route.

You will not only pay for the features. You are also welcome to cede some of your rights.

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Welcome to the preview release of codename "Alchemy." Alchemy is a research project that allows users to compile C and C++ code that is targeted to run on the open source ActionScript Virtual Machine (AVM2). The purpose of this preview is to assess the level of community interest in reusing existing C and C++ libraries in Web applications that run on Adobe® Flash® Player and Adobe AIR®.

With Alchemy, Web application developers can now reuse hundreds of millions of lines of existing open source C and C++ client or server-side code on the Flash Platform.  Alchemy brings the power of high performance C and C++ libraries to Web applications with minimal degradation on AVM2.  The C/C++ code is compiled to ActionScript 3.0 as a SWF or SWC that runs on Adobe Flash Player 10 or Adobe AIR 1.5.

Alchemy is primarily intended to be used with C/C++ libraries that have few operating system dependencies. Ideally suited for computation-intensive use cases, such as audio/video transcoding, data manipulation, XML parsing, cryptographic functions or physics simulation, performance can be considerably faster than ActionScript 3.0 and anywhere from 2-10x slower than native C/C++ code. Alchemy is not intended for general development of SWF applications using C/C++.

Download and Discuss


With Alchemy, it is easy bridge between C/C++ and ActionScript 3.0 to expand the capabilities of applications on the Flash Platform, while ensuring that the generated SWCs and SWFs cannot bypass existing Flash Player security protections.

Adobe is providing some example libraries, and developers are encouraged to share their ported libraries.

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