Macromedia flex 2-migrating applications to flex 2 Manual De Usuario

30

ActionScript 2.0 to 3.0

Default values

The following example shows the default values for variables of different types:

var dog:int;

// defaults to 0

var dog:uint;

// defaults to 0 

var dog:Boolean;// defaults to false
var dog:Number; // defaults to NaN
var dog:Object; // defaults to undefined
var cat;

// type defaults to Object, so default value is undefined

For all other classes (such as String, XML, MovieClip, Sprite, or any user-defined class), 

null

 

is the default value for uninitialized variables of that type.

var dog:String;

// defaults to null

var dog:UserDefinedClass;

// defaults to null

var dog:MovieClip; 

// defaults to null

With the exception of Object, the types with a non-null default value are the types which can 
never hold 

null

 as a value. They get what a typecast of 

null

 to that type would result in. 

To check for a possibly uninitialized variable, compare against the defaults described in 

. Alternatively, assign an initial, impossible value to your variable’s 

declaration which you can then test against; for example:

var length:int = -1;

if (someTest())
length = 2;

if (someOtherTest())

length = 3;

if (length == -1)

length = lastResort();

For instances of classes, use the following pattern:

if (o)

// ActionScript 3.0

if (o != null)

// ActionScript 2.0

and

if (!o)

// ActionScript 3.0

if (o == null)

// ActionScript 2.0

Because a Boolean can only be true or false, and never undefined, write the following when 
checking if a Boolean is true:

if (b)

// ActionScript 3.0

if (b == true)

// ActionScript 2.0

if (b != false) // ActionScript 2.0