Chapter 9. Functions
I'm almost
giddy to tell you about functions because they're such a
powerful part of ActionScript. A function is simply a chunk of code
that can be reused throughout a program. Not only do functions lend
enormous flexibility and convenience to our scripts, they also give
us control over Flash movie elements. I can hardly imagine
programming without functions -- they ease everything from sorting
words to calculating the distance between two movie clips.
We'll introduce functions in this chapter before learning how
to create complex, powerful programs using functions with objects in
Chapter 12, "Objects and Classes".
We'll focus first on program
functions -- the functions we create ourselves in
our scripts. By learning to create our own functions, we'll
become familiar with these fundamentals:
- Function declaration
Creating functions to use in our scripts.
- Function invocation
Causing functions to execute. In other words, running the code in a
function. Also known as calling a function.
- Function arguments and parameters
Providing functions with data to manipulate upon invocation.
- Function termination
Ending the execution of a function and optionally returning a result.
- Function scope
Determining the availability and life span of a function, and the
accessibility of variables referenced in a function body.
Once we understand those aspects of functions, we'll consider
how they apply to internal
functions, functions that come built into
ActionScript. Let's get to it!
9.1. Creating Functions
To
make
a basic function we simply need a function name
and a block of statements to perform, like this:
function funcName ( ) {
statements
}
The function keyword starts the
declaration
of
our new function. Next comes our function name,
funcName, which we'll use later to
invoke our
function. funcName must be a legal
identifier.[1] Next, we
supply a pair of parentheses, ( ), that enclose
any optional parameters, which we'll discuss later. If our
function does not have any parameters, we leave the parentheses
empty. Finally, we provide the function
body
(i.e., statement block), which contains the code that's
executed when our function is
called.
Let's make a (very) simple function:
Start a new Flash
movie. On frame 1 of the main movie timeline, attach the following code:
function sayHi ( ) {
trace("Hi there!");
}
That was easy -- we've just created a function named
sayHi( ). When we run it, the trace(
) statement in its body will be executed. Don't
close the movie you've created, we'll learn to run our
function next.
 |  |  | | 8.8. Onward! |  | 9.2. Running Functions |
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