Flex SDK coding conventions and best practices

Note: This page is not yet complete and has some sections marked TBD. But there's plenty here to get you started!

Introduction

This document lays out the coding standards for writing open-source Flex framework components in ActionScript 3. Adhering to these standards makes the source code look consistent, well-organized, and professional.

Some of these standards are completely arbitrary, since there is not always a “best way” to code. Nevertheless, in the interest of consistency, all commits to the Flex SDK project will be expected to follow these conventions.

Contents

Naming
Language Usage
File Organization
Formatting
ASDoc

Naming

Choosing good names is critical to creating code that is easy to use and easy to understand. You should always take the time to think about whether you have chosen the right name for something, especially if it is part of the public API.

Our naming standards are mostly consistent with those of ECMAScript and Flash Player 9.

Abbreviations

Avoid them as a general rule. For example, calculateOptimalValue() is a better method name than calcOptVal().

Being clear is more important than minimizing keystrokes. And if you don't abbreviate, developers won't have to remember whether you shortened a word like “qualified” to “qual” or “qlfd”.

However, we have standardized on a few abbreviations:

• acc for accessibility, as in ButtonAccImpl
• auto for automatic, as in autoLayout
• auto for automatic, as in autoLayout
• eval for evaluate, as in EvalBindingResponder
• impl for implementation, as in ButtonAccImpl
• info for information, as in GridRowInfo
• num for number of, as in numChildren
• min for minimum, as in minWidth
• max for maximum, as in maxHeight
• nav for navigation, as in NavBar
• regexp for regular expression, as in RegExpValidator
• util for utility, as in StringUtil

This list probably does not include all abbreviations that are currently in use. If you're considering using an abbreviation that isn't listed here, please search the source code to determine whether it is already in use. If you don't find it, think twice about whether abbreviating is really appropriate.

Occasionally we are (deliberately) inconsistent about abbreviations. For example, we spell out “horizontal” and “vertical” in most places, such as horizontalScrollPolicy and verticalScrollPolicy but we abbreviate them to H and V in the very-commonly-used container names HBox and VBox.

Acronyms

Various acronyms are common in Flex, such as AIR, CSS, HLOC, IME, MX, MXML, RPC, RSL, SWF, UI, UID, URL, WSDL, and XML.

An acronym is always all-uppercase or all-lowercase (e.g., SWF or swf, but never Swf). The only time that all-lowercase is used is when the acronym is used by itself as an identifier, or at the beginning of an identifier, and the identifier should start with a lowercase letter. See the rules below for which identifiers should start with which case.

Examples of identifiers with acronyms are CSSStyleDeclaration, IUID, uid, IIME, and imeMode.

Word boundaries

When an identifier contains multiple words, we use two ways of indicating word boundaries: intercaps (as in LayoutManager or measuredWidth) and underscores (as in object_proxy). See the rules below for which method to use.

Sometimes it isn't clear whether a word combination has become its own single word, and we are unforunately inconsistent about this in some places: dropdown, popUp, pulldown.

Follow the acronym-casing rules even in the rare case that two acronyms must be adjacent. An example (which isn't actually in use) would be something like loadCSSURL(). But try to avoid such names.

Type-specifying names

If you want to incorporate the type into the name, make it the last “word”. Don't use the old ActionScript 1 convention of concatenating abbreviated type suffixes such as _mc to indicate type. For example, name a border Shape border, borderSkin, or borderShape, but not border_mc.

Often, the best name for an object is simply the same as its type, with different casing:

var button:Button = new Button();

Package names

Start them with a lowercase letter and use intercaps for subsequent words: controls, listClasses.

Package names should always be nouns or gerunds (the -ing noun form of a verb), not verbs, adjectives, or adverbs.

A package implementing lots of similar things should have a name which is the plural form of the thing: charts, collections, containers, controls, effects, events, formatters, managers, preloaders, resources, skins, states, styles, utils, validators.

It is common to use a gerund for the name of a package which implements a concept: binding, logging, messaging, printing. Otherwise, they are generally "concept nouns": accessibility, core, graphics, rpc.

A package containing classes that support component FooBar should be called fooBarClasses.

File names

For importable APIs, the file name must be the same as the public API inside. But include files don't have to follow this rule.

Start the names of include files for [Style(...)] metadata with an uppercase letter, use intercaps for subsequent words, and make the last word “Styles”: BorderStyles.as, ModalTransparencyStyles.as.

Start the names of individual asset files with a lowercase letter and use underscores between words: icon_align_left.png.

Namespace names

Start them with a lowercase letter and use underscores between words: mx_internal, object_proxy.

Interface names

Start them with I and use intercaps for subsequent words: IList, IFocusManager, IUID.

Class names

Start them with an uppercase letter and use intercaps for subsequent words: Button, FocusManager, UIComponent.

Name Event subclasses FooBarEvent.

Name Error subclasses FooBarError.

Name the EffectInstance subclass associated with effect FooBar FooBarInstance.

Name Formatter subclasses FooBarFormatter.

Name Validator subclasses FooBarValidator.

Name skinning classes FooBarBackground, FooBarBorder, FooBarSkin, FooBarIcon, FooBarIndicator, FooBarSeparator, FooBarCursor, etc.

Name utility classes FooBarUtil (not FooBarUtils; the package is plural but the class is singular).

It is common to name a base class FooBarBase: ComboBase, DateBase, DataGridBase, ListBase.

Event names

Start them with a lowercase letter and use intercaps for subsequent words: "move", "creationComplete".

Style names

Start them with a lowercase letter and use intercaps for subsequent words: color, fontSize.

Enumerated values for String properties

Start them with a lowercase letter and use intercaps for subsequent words: "auto", "filesOnly",

Constant names

Use all uppercase letters with underscores between words: OFF, DEFAULT_WIDTH.

The words in the identifier must match the words in the constant value if it is a String:

public static const FOO_BAR:String = "fooBar";

Property (variable and getter/setter) names

Start them with a lowercase letter and use intercaps for subsequent words: i, width, numChildren.

Use i for a loop index and n for its upper limit. Use j for an inner loop index and m for its upper limit.

for (var i:int = 0; i < n; i++)
{
    for (var j:int = 0; j < m; j++)
    {
        ...
    }
}

Use p (for “property”) for a for-in loop variable:

for (var p:String in o)
{
    ...
}

If a class overrides a getter/setter and wants to continue to expose the base getter/setter, it should do so by implementing a property whose name is the base name with a $ prepended. This getter/setter should be marked final and should do nothing more than call the supergetter/setter.

mx_internal final function get $numChildren():int
{
    return super.numChildren;
}

Storage variable names

Give the storage variable for the getter/setter foo the name _foo.

Method names

Start them with a lowercase letter and use intercaps for subsequent words: measure(), updateDisplayList().

Method names should always be verbs.

Parameterless methods should generally not be named getFooBar() or setFooBar(); these should be implemented as getter/setters instead. However, if getFooBar() is a slow method requiring a large amount of computation, it should be named findFooBar(), calculateFooBar(), determineFooBar(), etc. to suggest this, rather than being a getter.

If a class overrides a method and wants to continue to expose the base method, it should do so by implementing a method whose name is the base name with a $ prepended. This method should be marked final and should do nothing more than call the supermethod.

mx_internal final function $addChild(child:DisplayObject):DisplayObject
{
    return super.addChild(child);
}

Event handler names

Event handlers should be named by concatenating “Handler” to the type of the event: mouseDownHandler().

If the handler is for events dispatched by a subcomponent (i.e., not this), prefix the handler name with the subcomponent name and an underscore: textInput_focusInHandler().

Argument names

Use value for the argument of every setter:

Do this:

public function set label(value:String):void

Not this:

public function set label(lab:String):void

Or this:

public function set label(labelValue:String):void

Or this:

public function set label(val:String):void

Use event (not e, evt, or eventObj) for the argument of every event handler:

protected function mouseDownHandler(event:Event):void

Resource bundle names

If a resource bundle contains resources for a particular package, name the bundle the same as the package: controls, {formatters}}, validators.

Resource key names

Start them with a lowercase letter and use intercaps for subsequent words: pm, dayNamesShort.

Miscellaneous nomenclature

Avoid “object” because it is vague.

An “item” is a data item, not a DisplayObject.

A “renderer” is a DisplayObject that displays a data item.

A “type” is an AS3 type; use "kind" otherwise.

Language Usage

This section discusses how we use the language constructs of ActionScript 3, especially when there are multiple ways to express the same thing.

Compilation options

Compile with the options -strict and -show-actionscript-warnings. (These are the defaults in the flex-config.xml file.)

Property-based APIs

Favor property-based APIs rather than method-based APIs, because these are more suitable for declarative-style MXML programming.

Type declarations

Write a type annotation for every constant, variable, function argument, and function return value, even if the annotation is simply :* to indicate “no type”.

Do this:

var value:*;

Not this:

var value;

Use the narrowest type that is appropriate. For example, a loop index should be a int, not a Number, and certainly not an Object or *. As another example, a mouseDownHandler should declare its argument as event:MouseEvent, not event:Event.

Use int for integers, even if they can't be negative. Use uint only for RGB colors, bit masks, and other non-numeric values.

Use * only if the value can be undefined. You should generally use Object rather than *, with null being the “object doesn't exist” value.

If you declare something to be of type Array, add a comment of the form /* of ElementType */ immediately after Array indicate the type of the array elements. A future version of the language is likely to have typed arrays.

Do this:

var a:Array /* of String */ = [];

Not this:

var a:Array = [];

And this:

function f(a:Array /* of Number */):Array /* of Object */
{
    ...
}

Not this:

function f(a:Array):Array

Literals

undefined

Avoid using this when possible. It is only necessary when dealing with values whose compile-time is type is *, and you should be using * sparingly as well.

int and uint literals

Do not use a decimal point in a integer.

Do this:

2

Not this:

2.

Use a lowercase x and uppercase A-Z in hexadecimal numbers.

Do this:

0xFEDCBA

Not this:

0Xfedcba  

Always write an RGB color as a six-digit hexadecimal number.

Do this:

private const BLACK:uint = 0x000000;

Not this:

private const BLACK:uint = 0;  

When dealing with indices, use the value -1 to mean “no index”.

Number literals

If a Number value typically can be fractional, indicate this by using a decimal point, and follow the decimal point by a single trailing zero.

Do this:

alphaFrom = 0.0;
alphaTo = 1.0;

Not this:

alphaFrom = 0;
alphaTo = 1;

However, don't do this for pixel coordinates, which are by convention integral even though they can in principle be fractional.

Do this:

var xOffset:Number = 3;

Not this:

var xOffset:Number = 3.0;

Use e, not E, when using exponential notation.

Do this:

1.0e12

Not this:

1.0E12

Use the default value NaN as the “not set” value for a Number.

String literals

Use quotation marks (double quotes), not apostrophes (single quotes), to delimit strings, even if that string contains a quotation mark as a character.

Do this:

"What's up, \"Big Boy\"?"

Not this:

'What\'s up, "Big Boy"?'

Use \u, not \U, for unicode escape sequences.

Array literals

Use Array literals rather than new Array().

Do this:

[]

Not this:

new Array()

And this:

[ 1, 2, 3 ]

Not this:

new Array(1, 2, 3)

Use the Array constructor only to allocate an array of a prespecified size, as in new Array(3), which means [ undefined, undefined, undefined ], not [ 3 ].

Object literals

Use Object literals rather than new Object().

Do this:

{}

Not this:

new Object()

And this:

o = { a: 1, b: 2, c: 3 };

Not this:

o = new Object();
o.a = 1;         
o.b = 2;         
o.c = 3;

Or this:

o = {};
o.a = 1;         
o.b = 2;         
o.c = 3;

Function literals

Avoid using function literals to define anonymous functions; use a class method or package function instead.

If you must use a function literal, declare a return type, and terminate the last statement inside the function block with a semicolon.

Do this:

function(i:int):void { doIt(i - 1); doIt(i + 1); }

Not this:

function(i:int) { doIt(i - 1); doIt(i + 1) }

RegExp literals

Use the literal notation rather than constructing a RegExp instance from a String.

Do this:

var pattern:RegExp = /\d+/g;

Not this:

var pattern:RegExp = new RegExp("\\d+", "g");

XML and XMLList literals

Use the literal notation rather than constructing an XML instance from a String.

Do this:

var node:XML = <name first="Jane" last="Doe"/>;

Not this:

var node:XML = new XML("<name first=\"Jane\" last=\"Doe\"/>");

Use double-quotes rather than single-quotes around XML attribute values:

Do this:

var node:XML = <name first="Jane" last="Doe"/>;

Not this:

var node:XML = <name first='Jane' last='Doe'/>;

Class literals

Use a fully-qualified class literal only if necessary to disambiguate between two imported classes with the same unqualified name.

Do this:

import mx.controls.Button;
...
var b:Button = new Button();

Not this:

import mx.controls.Button;
...
var b:Button = new mx.controls.Button();

But here a fully-qualified name is required and therefore qppropriate:

import mx.controls.Button;
import my.controls.Button;
...
var b:Button = new mx.controls.Button();

Expressions

Parentheses

Don't use unnecessary parentheses with common operators such as +, -, *, /, &&, ||, <, <=, >, >=, ==, and !=.

Do this:

var e:Number = a * b / (c + d);

Not this:

var e:Number = (a * b) / (c + d);

And this:

var e:Boolean = a && b || c == d;

Not this:

var e:Boolean = ((a && b) || (c == d));

The precedence rules for other operators are harder to remember, so parentheses can be helpful with them.

Coercion

Don't compare a Boolean value to true or false; it already is one or the other.

Do this:

if (flag)

Not this:

if (flag == true)

Do this:

var flag:Boolean = a && b;

Not this:

var flag:Boolean = (a && b) != false;

Explicitly coerce an int, uint, Number or String to a Boolean:

Do this:

if (n != 0)

Not this:

if (n)

And this:

if (s != null && s != "")

Not this:

if (s)

Let object references implicitly coerce to a Boolean:

Do this:

if (child)

Not this:

if (child != null)

And this:

if (!child)

Not this:

if (child == null)

Prefer the use of a cast to the use of the as operator. Use the as operator only if the coercion might fail and you want the expression to evaluate to null instead of throwing an exception.

Do this:

IUIComponent(child).document

Not this:

(child as IUIComponent).document

Comparison

Write comparisons in the order that they read most naturally:

Do this:

if (n == 3) // "if n is 3"

Not this:

if (3 == n) // "if 3 is n"

++ and -- operators

In cases where the postfix and prefix forms are equivalent, use the postfix form. Use the prefix form only when you need to use the value before it is incremented.

Do this:

for (var i:int = 0; i < n; i++)

Not this:

for (var i:int = 0; i < n; ++i)

Ternary operator

Use a ternary operator in place of a simple if/else statement, especially for null checks:

Do this:

return item ? item.label : null;

Not this:

if (!item)
    return null;
return item.label;

But don't use nested ternary operators in place of complex if/else logic.

Do this:

if (a < b)
    return -1;
else if (a > b)
    return 1;
return 0;

Not this:

return a < b ? -1 : (a > b ? 1 : 0);

new operator

Use parentheses after the class reference, even if the constructor takes no arguments.

Do this:

var b:Button = new Button();

Not this:

var b:Button = new Button;

Statements

Terminate each statement with a semicolon. Do not use the optional-semicolon feature of ActionScript 3.

Do this:

a = 1;
b = 2;
c = 3;

Not this:

a = 1
b = 2
c = 3

include statements

Use include, not the deprecated #include. Terminate the include statement with a semicolon, like any other statement.

Do this:

include "../core/ComponentVersion.as";

Not this:

#include "../core/ComponentVersion.as"

Use relative, not absolute, paths.

import statements

Import specific classes, interfaces, and package-level functions rather than using the * wildcard.

Do this:

import mx.controls.Button;
import flash.utils.getTimer;

Not this:

import mx.core.*;

use namespace statements

Avoid them; use :: syntax instead on each reference to something in a non-open namespace.

Do this:

import mx.core.mx_internal;

// Later, in some method...
mx_internal::doSomething();

Not this:

import mx.core.mx_internal;
use namespace mx_internal;

// Later, in some method...
doSomething();

if statements

If the various branches of an if/else statement involve single statements, don't make them into blocks.

Do this:

if (flag)
    doThing1();

Not this:

if (flag)
{
    doThing1();
}

And this:

if (flag)
    doThing1();
else
    doThing2():

Not this:

if (flag)
{
    doThing1();
}
else
{
    doThing2();
}

But if any branch has multiple statements, make all of them into blocks.

Do this:'

if (flag)
{
    doThing1();
}
else
{
    doThing2();
    doThing3();
}

Not this:

if (flag)
    doThing1();
else
{
    doThing2();
    doThing3();
}

When doing multiple error checks, use sequential if statements that test for failure and return early. The successful execution flow is then down the page, with the succesful return at the end of the method. Do not use nested tests for success, which make the execution flow drift across the page.

Do this:

if (!condition1)
    return false;
...
if (!condition2)
    return false;
...
if (!condition2)
    return false;
...
return true;

Not this:

if (condition1)
{
    ...
    if (condition2)
    {
        ...
        if (condition3)
        {
            ...
            return true;
        }
    }
}
return false;

for statements

Make the body of a for loop be a block, even if it consists of only one statement.

Do this:

for (var i:int = 0; i < 3; i++)
{
   doSomething(i);
}

Not this:

for (var i:int = 0; i < 3; i++)
    doSomething(i);

Store the upper limit for a for-loop variable in a local variable so that it isn't re-evaluated every time through the loop (unless, of course, it needs to be re-evaluated on each interation).

Do this:

var n:int = a.length;
for (var i:int = 0; i < n; i++)
{
    ...
}

Not this:

for (var i:int = 0; i < a.length; i++)
{
    ...
}

Declare the loop var inside the parentheses of the for statement, unless it is reused elsewhere.

Do this:

for (var i:int = 0; i < 3; i++)

Not this:

var i:int;
for (i = 0; i < 3; i++)
{
   ...
}

while statements

Make the body of a while loop be a block, even if it consists of only one statement.

Do this:

while (i < n)
{
   doSomething(i);
}

Not this:

while (i < n)
    doSomething(i);

do statements

Make the body of a do loop be a block, even if it consists of only one statement.

Do this:

do
{
   doSomething(i);
}
while (i < n);

Not this:

do
    doSomething(i);
while (i < n);

switch statements

Make the body of each case clause, and of the default clause, be a block. Put the break or return statement within the block, not after it. If you are returning, don't put a break after the return. Treat the default clause similarly to the case clauses; break or return from it rather than falling through the bottom of the switch.

Do this:

switch (n)
{
    case 0:
    {
        foo();
        break;
    }

    case 1:
    {
        bar();
        return;
    }

    case 2:
    {
        baz();
        return;
    }

    default:
    {
        blech();
        break;
    }
}

Not this:

switch (n)
{
    case 0:
        foo();
        break;

    case 1:
    {
        bar();
    }
    break;

    case 2:
        baz();
        return;
        break;

    default:
        blech();
}

return statements

Do not enclose a return value in unnecessary parentheses.

Do this:

return n + 1;

Not this:

return (n + 1);

Returning from the middle of a method is OK.

Declarations

Don't declare multiple constants or variables in a single declaration.

Do this:

var a:int = 1;
var b:int = 2;

Not this:

var a:int = 1, b:int = 2;

The override keyword

If present, put this first, before the access specifier.

Do this:

override protected method measure():void

Not this:

protected override method measure():void

Access specifiers

Put an explicit access specifier everywhere that one is allowed. Do not use the fact that internal is the implicit access specifier if none is written.

Before making an API public or protected, think hard about whether it is really needs to be. Public and protected APIs must be documented. They must also be supported for several releases before being formally deprecated.

The static keyword

If present, put this after the access specifier.

Do this:

public static const MOVE:String = "move"

Not this:

static public const MOVE:String = "move";

The final keyword

If present, put this after the access specifier.

Do this:

public final class BoxDirection

Not this:

final public class BoxDirection

Declare all “enum classes” to be final.

Also declare “base” properties and methods (those starting with $) to be final.

Constants

All constants should be static. There is no reason to use an instance constant, since all instances would store the same value.

Do this:

public static const ALL:String = "all";

Not this:

public const ALL:String = "all";

Variables

If a variable needs to be initialized to a non-default value, do this in the declaration, not in the constructor.

Do this:

private var counter:int = 1;

Not this:

private var counter:int;
...
public function MyClass()
{
    super();
    ...
    counter = 1;
}

Local variables

Declare local variables at or just before the point of first use. Don't declare them all at the top of the function.

Do this:

private function f(i:int, j:int):int
{
    var a:int = g(i - 1) + g(i + 1);
    var b:int = g(a - 1) + g(a + 1);
    var c:int  = g(b - 1) + g(b + 1);

    return (a * b * c) / (a + b + c);
}

Not this:

private function f(i:int, j:int):int
{
    var a:int;
    var b:int;
    var c:int;

    a = g(i - 1) + g(i + 1);
    b = g(a - 1) + g(a + 1);
    c = g(b - 1) + g(b + 1);

    return (a * b * c) / (a + b + c);
}

Declare local variables only one per function. ActionScript 3 doesn't have block-scoped locals.

Do this:

var a:int;
if (flag)
{
    a = 1;
    ...
}
else
{
    a = 2;
    ...
}

Not this:

if (flag)
{
    var a:int = 1;
    ...
}
else
{
    var a:int = 2;
    ...
}

And this:

var i:int;
for (i = 0; i < n; i++)
{
    ...
}

for (i = 0; i < n; i++)
{
    ...
}

Not this:

for (var i:int = 0; i < n; i++)
{
    ...
}

for (var i:int = 0; i < n; i++)
{
    ...
}

Classes

If a class simply extends Object, omit the extends Object clause.

The only “bare statements” in a class should be calls to static class initialization methods, such as loadResources().

Constructors

If a classes has instance members, write a constructor, and make it explicitly call super(), even if it does nothing else.

If the constructor takes arguments that set instance vars, give the the same names as the instance vars.

Do this:

public function MyClass(foo:int, bar:int)
{
    this.foo = foo;
    this.bar = bar;
}

Not this:

public function MyClass(fooVal:int, barVal:int)
{
    foo = fooVal;
    bar = barVal;
}

Don't set the classes' instance vars in the constructor; do this in the declarations of the instance vars. However, if you need to reset the values of inherited instance vars, do this in the consturctor.

Interfaces

TBD

Namespaces

TBD

Implementing properties

TBD

Metadata

TBD

Packages

One public API (usually a class, sometimes a namespace or function) inside the package statement.

Helper classes

bare statements

File Organization

This section presents the order in which a Flex framework file should be organized.

Copyright notice

Include a copyright notice at the top of every .as file in the framework. The format for the 2008 open-source copyright is given below.

////////////////////////////////////////////////////////////////////////////////
//
//  ADOBE SYSTEMS INCORPORATED
//  Copyright 2008 Adobe Systems Incorporated
//  All Rights Reserved.
//
//  NOTICE: Adobe permits you to use, modify, and distribute this file
//  in accordance with the terms of the license agreement accompanying it.
//
////////////////////////////////////////////////////////////////////////////////

Note that it is 80 characters wide.

package statement

TBD

import statements

TBD

use namespace statement

TBD

Class metadata

Organize the class metadata into sections, in the order Events, Styles, Effects, Excluded APIs, and Other Metadata.

Put a minor section header before each section. Note that the minor section headers are 40 characters wide and that there are two spaces between the // and the section name.

Alphabetize the metadata by name="..." within each section. In the Other Metadata section, alphabetize them by metadata tag name.

//--------------------------------------
//  Events
//--------------------------------------
/
**
 *  ASDoc comment.
 */
[Event

/**
 *  ASDoc comment.
 */
[Event

//--------------------------------------
//  Styles
//--------------------------------------

/**
 *  ASDoc comment.
 */
[Style

/**
 *  ASDoc comment.
 */
[Style]

//--------------------------------------
//  Effects
//--------------------------------------

/**
 *  ASDoc comment.
 */
[Effect

/**
 *  ASDoc comment.
 */
[Eff