1.2.1. Flash Programs

To most computer users, a program is synonymous with an application, such as Adobe Photoshop or Macromedia Dreamweaver. Obviously, that's not what we're building when we program in Flash. Programmers, on the other hand, define a program as a collection of code (a "series of statements"), but that's only part of what we're building.

Our scripts include most of what you'd see in traditional programs without the operating-system-level stuff you would write in languages like C++ or Java to place graphics on the screen or cue sounds. We're spared the need to manage the nuts 'n' bolts of graphics and sound programming, which allows us to focus most of our effort on designing the behavior of our movies.

1.2.2. Expressions

Here we see the variable message:

var message = "Hi there, Flash!";

If we like, we can combine the variable expression message with the literal expression " How are you?" as follows:

message + " How are you?"

which becomes "Hi there, Flash! How are you?" when the program runs. You'll frequently see long expressions include shorter expressions when working with arithmetic, such as:

(2 + 3) * (4 / 2.5) - 1

It's important to be exposed to expressions early in your programming career because the term "expression" is often used in descriptions of programming concepts. For example, I might write, "To assign a value to a variable, type the name of the variable, then an equal sign followed by any expression."

1.2.3. Two Vital Statement Types: Conditionals and Loops

In nearly all programs, we'll use conditionals to add logic to our programs and loops to perform repetitive tasks.

1.2.4. Modular Code (Functions)

Suppose you want to write a quick script that calculates the area of a 4-sided figure. Without functions, your script might look like this:

var height = 10;
var width = 15;
var area = height * width;

Now suppose you want to calculate the area of five 4-sided figures. Your code quintuples in size:

var height1 = 10;
var width1 = 15;
var area1 = height1 * width1;
var height2 = 11;
var width2 = 16;
var area2 = height2 * width2;
var height3 = 12;
var width3 = 17;
var area3 = height3 * width3;
var height4 = 13;
var width4 = 18;
var area4 = height4 * width4;
var height5 = 20;
var width5 = 5;
var area5 = height5 * width5;

Because we're repeating the area calculation over and over, we are better off putting it in a function once and executing that function multiple times:

function area(height, width){
  return height * width;
}
area1 = area(10, 15);
area2 = area(11, 16);
area3 = area(12, 17);
area4 = area(13, 18);
area5 = area(20, 5);

return height * width;

After we create a function, we may run the code it contains from anywhere in our movie by using its name. In our example we called the area( ) function five times, passing it the height and width values it expects each time: area(10, 15), area(11, 16), and so on. The result of each calculation is returned to us and we store those results in the variables area1 through area5. Nice and neat, and much less work than the non-function version of our code.

1.2.5. Movie Clip Instances

Every movie clip has a symbol definition that resides in the Library of a Flash movie. We can add many copies, or instances, of a single movie clip symbol to a Flash movie by dragging the clip from the Library onto the Stage. A great deal of advanced Flash programming is simply a matter of movie clip instance control. A bouncing ball, for example, is nothing more than a movie clip instance being repositioned on the Stage repetitively. We can alter an instance's location, size, current frame, rotation, and so forth, through ActionScript during the playback of our movie.

If you're unfamiliar with movie clips and instances, consult Flash's documentation or Help files before continuing with the rest of this book.

1.2.6. The Event-Based Execution Model

In the early days of computing, a program's instructions were executed sequentially in the order that they appeared, starting with the first line and ending with the last line. The program was meant to perform some action and then stop. That kind of program, called a batch program, doesn't handle the interactivity required of an event-based programming environment like Flash.

Event-based programs don't run in a linear fashion as batch programs do. They run continuously (in an event loop), waiting for things (events) to happen and executing code segments in response to those events. In a language designed for use with a visual interactive environment (such as ActionScript or JavaScript), the events are typically user actions such as mouseclicks or keystrokes.

When an event such as a mouseclick occurs, the interpreter sounds an alarm. A program can then react to that alarm by asking the interpreter to execute an appropriate segment of code. For example, if a user clicks a button in a movie, we could execute some code that displays a different section of the movie (classic navigation) or submits variables to a database (classic form submission).

when (this event happens) { 
  execute these lines of code
}

This is typically written in the general form:

on (event) {
  statements
}

In practice, an event handler for a button that moves the playhead to frame 200 would read:

on (press) {
  gotoAndStop(200);
}

Events literally make things happen, which is why they come at the end of your first day of ActionScript language school. You've learned what's involved in writing scripts and what governs when those scripts will actually be executed (i.e., events). I'd say you're ready to try your first real conversation.