Inside Flash MX 2004March 2004

Gary Grossman, Director of Engineering, Flash

The Essence of Flash

What is the essence of Flash?That is, what are the key values that led to the

product’s extraordinary success?

Flash is

freedom of designrich interactivity

compact, ubiquitous player

Flash is the only product thatbrings

design and developmenttogether

What’s in a SWF file?

SWF is tag-based file format What is a tag?

Uniform data block for easy parsing Perform basic operations in Flash Player “Machine code” of Flash Player

What are typical tags? stagDefineShape stagPlaceObject stagShowFrame

SWF file format is documented athttp://www.macromedia.com/software/flash/open/licensing/fileformat/

Best illustrated by a simple example...

Motion Tween SWF

simple shape

tweened tomovefrom left, toright, to leftagain in loop

Deconstructing Motion Tween SWF

shapes and sprites are characters each character has a unique ID (#1, #2) shape #1 is defined sprite #2 (movie clip) is defined using the shape initial place tag creates instance of sprite each timeline frame is delimited by show frame subsequent place tags move the existing instance main timeline automatically loops when end of SWF tag

stream is reached

Flash execution model

Flash Player starts at beginning of SWF and executes tags one after the other, like reading a tape

Entire SWF resides in memory, but playback can start when only one frame is loaded, so SWF’s can stream

Player tries to display graphics on Frame 1 even when not completely loaded; results in some quirks

When a showFrame tag is hit, the frame is displayed to the user

Rewinding on a long Timeline can be slow, because entire SWF must be re-parsed from beginning to the desired point

Motion Tween: A touch of ActionScript

Addedstop()action:animationwill stopafter oneiteration

How doesit look inthe SWF?

ActionScript in the SWF file

some tags contain action records action records contain stream of

action bytecodes Flash compiles ActionScript source

code into stream of action bytecodes action bytecodes can represent

simple actions (stop), up to sophisticated scripts

FLASM is a tool that can disassemble the bytecodes in a SWFhttp://flasm.sourceforge.net

ActionScript execution model

Frame actions, on/onClipEvent, callbacks and listeners are all queued up and executed before frame is displayed

Flash Player is not multi-threaded Typically runs in same thread as browser, via

system timer callback Scripts must exit nicely, thus 15 seconds

warning if they don’t One reason we don’t have “updateStage” like

Director

Do frames ever get skipped?

Frame rate is not guaranteed Frames are dropped only when

using streaming sound, to maintain sync All ActionScript is executed, even on

dropped frames

Interval timers

Does setInterval guarantee millisecond accuracy? No. There is a system timer used for all

timeline execution. The same timer is used for all setIntervals.

Accuracy is typically to 10X frame rate and no more

In future, setInterval may be more accurate

onEnterFrame may give smoother results

Unicode

Software and Web are becoming global commodities Growing number of companies deliver products in

many languages Example: Flash MX 2004 available in 11 languages

SWF file format has been Unicode (UTF-8) since Flash Player 6

Flash MX 2004 authoring tool is now fully Unicode Create multilingual content without switching system

languages

Font Chaining

Over 1.1 million Unicode characters Font chaining is means of avoiding

gibberish characters If a needed glyph is not in the selected

font, another font is found which has it Operating system determines which font

is used for chained glyphs Eliminates need for language-specific

device fonts Can use _sans, _serif, _typewriter instead

System.useCodePage

Means of working with data in non-Unicode encodings (EUC-KR)

Must be set on Frame 1 and not changed later.Behavior is undefined if you alter it later!

All client/server communication will be conducted in system code page: XML XMLSocket LoadVars loadVariables

Internally, everything is still converted to Unicode.

Alias Text

A key advantage of Flash over HTML has always been ability to use any fonts you desire, and embed them in the SWF

Flash text looks blurry at small point sizes due to lack of hint information

Solution has been to use device fonts for small text Alias Text feature makes small text crisp & readable Backwards compatible: Flash Player 7 not required!

Example: Lucida Sans Unicode, 10 point text, zoomed 400%

ActionScript optimizations: Register allocation

ActionScript interpreter is called “AVM”, ActionScript VM

AVM is stack machine and also has registers 4 initial registers were added in Flash 5 but not

used heavily by compiler until Flash MX 2004 Flash MX 2004 uses registers to get massive

performance boost for ActionScript code Flash Player 6 r65 and Flash Player 7 support up

to 256 registers to further boost performance

Flash Player 6 register allocation

Only 4 registers in AVM for Flash Player 6 r:0, r:1, r:2, r:3 r:0 reserved for compiler use So, 3 registers available for local variables

Compiler determines 3 most popular variables and allocates to registers

If you open a Flash MX FLA file in FMX 2004 and export... It will run faster! Even if “Optimize for r65” is not checked

Optimizing for Flash Player 6 r65

New checkbox in Publish Settings...What’s with it?

Flash Player r65 Player release code named “Firefox” Added “defineFunction2” opcode, making large

performance boost possible w/ 256 registers Need Flash MX 2004 to export SWF’s that

leverage Flash Player 6 r65 Higher penetration than Flash Player 7 (for now) Use new Flash Player Detection feature to

check for minor version r65

Performance gains in Flash Player 7

New performance features like register allocation

Flash Player 7 has been fine-tuned for performance throughout Performance work done by Flash Player team

“raises all boats” ... even existing content that hasn’t been re-exported now runs faster!

For supercharged performance, target your Flash movie to Flash Player 7!

Local variables

function factorial(x:Number):Number {

var sum:Number = 1; for (var i:Number=2; i<=x; i++) { sum *= i;

} return sum;

}

Use local variables (var) Good programming practice Compiler can optimize local variables to

registers Significant performance benefits In following example, every variable and

parameter can be optimized into a register

Variable names

It used to improve performance to use shorter variable names

Still true for timeline variables and object members

Not true for local variables allocated to registers Name of variable doesn’t even go in SWF Also, harder to decompile your SWF

Best practice:Use variable names that are reasonable but not overly verbose

The Dot or Slash Notation Debate

Dot notation (Flash 5 and later):results.obj.value

Slash notation (Flash 4):results/obj:value

Some Flashcoders discovered that slash notation is sometimes faster than dot notation

Not necessarily true now, because slash notation cannot take advantage of register allocation

Slash notation had to be taken out of AS2 to support strong typing

Common sub-expression elimination

The following code contains common sub-expressions:

for (var i=0; i<srcItems.length; i++) { destItems[i].name = srcItems[i].name; destItems[i].value = srcItems[i].value; destItems[i].id = srcItems[i].id; destItems[i].data = srcItems[i].data; }

The code runs faster if we factor out the CSE’s:

for (var i=0; i<srcItems.length; i++) { var destItem = destItems[i]; var srcItem = srcItems[i]; destItem.name = srcItem.name; destItem.value = srcItem.value; destItem.id = srcItem.id; destItem.data = srcItem.data; }

Many compilers can factor out CSE’s automatically.ActionScript can’t do this yet, so factoring CSE’s yourself will

provide performance benefits.Expect to see many compiler optimizations in the future.

ActionScript 2.0

Real OOP much better for managing complexity Cleaner, easier to read code Greater maintainability

Demo: Puzzle, TipCalculator examples

AS2 compiles down to AS1, so it is backwards compatible to Flash Player 6

In future, compiler will optimize for language model

One can go crazy and “overengineer” with too many classes. One must strike the right balance.

Benefits of Typed Variables

Consider the following AS1 code:

var myXML = new XML();myXML.loadd("menu.xml");

The compiler will not detect any error.

In AS2, you can specify variable type:

var myXML:XML = new XML();myXML.loadd("menu.xml");

The compiler will detect that "loadd" is not a method of XML, saving you debugging time.

Pitfalls of Typed Variables

This code uses a common AS1 pattern:

class Menu{ public function initXML() { var myXML:XML = new XML(); myXML.owner = this; myXML.onLoad = function () { this.owner.callback(); } myXML.load("menu.xml"); }public function callback() { ... }

}

The compiler will give error on "owner“because owner is not a member of XML.

How to solve?

Pitfalls of Typed Variables: Solution

One solution is to leverage variable scoping rules

class Menu{ public function initXML() { var myXML:XML = new XML(); var owner = this; myXML.onLoad = function () { owner.callback(); } myXML.load("menu.xml"); }public function callback() { ... }

}

Local variable "owner" is visible to function defined within a function

V2 components

Built in AS2 for use in AS2 Macromedia HALO look and feel Advanced features for Rich Internet Application

development Focus Manager Depth Manager Popup Manager CSS styles Accessibility Integrated with Data Binding

For small use cases, updated V1 components available as extension on Flash support center:http://www.macromedia.com/cfusion/exchange/index.cfm?view=sn111&extID=1009423

V2 Components: Event Model

Listener/broadcaster pattern More flexible, scalable, and object-

oriented than V1 callback function approach

Centralize event handling code in AS2 classes

Pitfalls of V2 Event Model

This code has two common errors

class Menu extends mx.screens.Form{var myButton:mx.controls.Button;

function Menu() { myButton.addEventListener(“click”, onButtonClicked);

}public function onButtonClicked() { ... }

}

Constructor is too early to add event listeners... wait until onLoad

Due to behavior of ActionScript function references, onButtonClicked will be executed in myButton’s context, which is not desired behavior.

Pitfalls of V2 Event Model: Solution

Many different strategies exist for event handling

One approach is EventProxy,posted on Mike Chambers’s bloghttp://www.markme.com/mesh/archives/004286.cfm

class Menu extends mx.screens.Form{var myButton:mx.controls.Button;

public function onLoad() { myButton.addEventListener(“click”, new EventProxy(this, onButtonClicked));

}public function onButtonClicked() { ... }

}

Integrating Flash into production environment

The new JSAPI in Flash MX 2004 offers lots of possibilities for automating Flash

Example:Command-line compilation with FlashCommand by Mike Chambershttp://www.markme.com/mesh/archives/003656.cfm

FlashCommand can be used to create an automated build system for a team developing with Flash

Code Security

Many people want to know:How can I prevent people from decompiling my SWF?

As the SWF specification is open, it is not possible to prevent decompilers like ASV.

Suggestions: Obfuscation by register allocation will help a little. All client/server communication should require proper

authentication on the server side Push sensitive logic that could be harmfully spoofed up

to the server side; Flash should be “presentation tier” only.

Macromedia is aware of this issue and is looking for solutions.

Dynamic Generation of SWF

Demo

Questions?