Recipe 2.7.
Creating Constants
Problem
You want
to declare a constant.
Solution
Declare it just like you would declare a
property, except use the const keyword in place of
var.
Discussion
As the name constant implies, constant values do not
change. Constants are useful when you have complex values that you
want to be able to reference by a simple identifier or when you
want to be able to use compile-time error checking for values.
Math.PI is an
example of a constant that contains a complex value (which is the
value of pi, or 3.14159). MouseEvent.MOUSE_UP, which
contains the value mouseUp, is an example of a constant
that allows you to use error-checking. When you add an event
listener for the mouse up event, you can use the string value
mouseUp. However, if you accidentally have a typo, you
won't be notified of an error, and your code won't work as
expected:
// This is valid code, but because of the typo (mousUp instead of mouseUp) the
// code won't work as expected.
addEventListener("mousUp", onMouseUp);
Using a constant helps. If you accidentally
misspell the constant, you will receive a compile error that helps
you track down the error:
// This causes a compile error.
addEventListener(MouseEvent.MOUS_UP, onMouseUp);
The syntax for declaring a constant is very
similar to that for declaring a standard property. However, rather
than using the var keyword you use the const
keyword. Although not required, the majority of constants also
happen to be public and static. If you want a constant to be public
and static, you must use the correct attributes. Additionally, you
must assign a value for a constant when declaring it:
static public const EXAMPLE:String = "example";
By convention, constant names are all in
uppercase. This convention makes it easy to identify and
differentiate constants from properties.
See Also
Recipe
2.4
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