ActionScript Elements

ActionScript is a language that bridges the gap between what you understand and what Flash understands. As such, it allows you to provide both action-oriented instructions (do this) and logic-oriented instructions (analyze this before doing that) in your Flash project. Like all languages, ActionScript contains many different elements, such as words, punctuation, and structureall of which you must employ properly to get your Flash project to behave the way you want it to. If you don't employ ActionScript correctly, you'll find that interactivity either won't occur or won't work the way you intended. Many of these elements, as well as several other elements such as logical statements and expressions, will be covered in more detail throughout the book.

To begin to understand how ActionScript works, look at this sample script, which contains many of the essential elements that make up a typical script. After the script is a discussion of these elements and their role in the script's execution.

We can assume that this script is attached to a button:


on (release) {

  //set the cost of the mug

  var mugCost:Number = 5.00;

  //set the local sales tax percentage

  var taxPercent:Number = .06;

  //determine the dollar amount of tax

  var totalTax:Number = mugCost * taxPercent;

  //determine the total amount of the transaction

  var totalCost:Number = mugCost + totalTax;

  //display a custom message

  myTextBox_txt.text = "The total cost of your transaction is " + totalCost;

  //send the cashRegister_mc movie clip instance to frame 50

  cashRegister_mc.gotoAndPlay (50);

}

Although at first glance this may look like Latin, once you become acquainted with some of its elements, you'll understand.

NOTE

Other script elements (for example, objects, functions, loops, properties, and methods) are discussed in detail throughout the book.

Events

Events occur during the playback of a movie and trigger the execution of a particular script. In our sample script, the event that triggers the script is on (release). This event signifies that when the button to which this script is attached is released, the script will execute. Every script is triggered by an event, and your movie can react to numerous eventseverything from a button being pressed to text changing in a text field to a sound completing its playback, and more. We will discuss events in depth in Lesson 2, "Using Event Handlers."

Actions

These form the heart of your script. An action is usually considered to be any line that instructs Flash to do, set, create, change, load, or delete something.

Here are some examples of actions from the sample script:


var mugCost:Number = 5.00;

cashRegister_mc.gotoAndPlay (50);

The first line creates a variable named mugCost, sets its data type as Number (indicating the variable will hold a numeric value), and sets the value of the variable to 5.00. The second line tells the cashRegister_mc movie clip instance to begin playing at Frame 50 of its timeline.

Generally speaking, most of the lines in a script that are within curly braces ({ } ) are actions. These lines are usually separated by semicolons (we'll discuss punctuation shortly).

Operators

These include a number of symbols (=, <, >, +, , *, &&, etc.) and are used to connect two elements in a script in various ways. Take a look at these examples:

var taxPercent:Number = .06; assigns a numeric value of .06 to the variable named taxPercent
amountA < amountB asks if amountA is less than amountB
value1 * 500 multiplies value1 times 500

Keywords

These are words reserved for specific purposes within ActionScript syntax. As such, they cannot be used as variable, function, or label names. For example, the word on is a keyword and can only be used in a script to denote an event that triggers a script, such as on (press), on (rollOver), on (rollOut), and so on. Attempting to use keywords in your scripts for anything other than their intended purpose will result in errors. Other keywords include break, case, class, continue, default, delete, do, dynamic, else, extends, finally, for, function, get, if, implements, import, interface, in, instanceof, new, null, private, public, return, set, static, switch, this, throw, try, typeof, undefined, var, void, while, and with.

Data

A dynamic script almost always creates, uses, or updates various pieces of data during its execution. Variables are the most common pieces of dynamic data found in scripts and represent pieces of data that have been given unique names. Once a variable has been created and assigned a value, that value can be accessed anywhere in the script simply by inserting the variable's name.

NOTE

Variable names are case sensitive: myVariable and MyVariable are not the same.

In our sample script, we created a variable named mugCost and assigned it a value of 5.00. Later in the script, the name of that variable is used to refer to the value it contains.

Curly Braces

Generally, anything between opening and closing curly braces signifies an action or set of actions the script needs to perform when triggered. Think of curly braces as saying, "As a result of this{ do this} ." For example:


on (release) {

  //set the cost of the mug

  var mugCost:Number = 5.00;

  //set the local sales tax percentage

  var taxPercent:Number = .06;

}

Semicolons

Appearing at the end of most lines of scripts, semicolons are used to separate multiple actions that may need to be executed as the result of a single event (similar to the way semicolons are used to separate thoughts in a single sentence). This example denotes six actions, separated by semicolons:


var mugCost:Number = 5.00;

var taxPercent:Number = .06;

var totalTax:Number = mugCost * taxPercent;

var totalCost:Number = mugCost + totalTax;

myTextBox_txt.text = "The total cost of your transaction is " + totalCost;

cashRegister_mc.gotoAndPlay (50);

Dot Syntax

Dots (.) are used within scripts in a couple of ways: One is to denote the target path to a specific timeline. For example, _root.usa.indiana.bloomington points to a movie clip on the main (_root) timeline named usa, which contains a movie clip named indiana, which contains a movie clip named bloomington.

Because ActionScript is an object-oriented language, most interactive tasks are accomplished by changing a characteristic (property) of an object or by telling an object to do something (invoking a method). When changing a property or when invoking a method, dots are used to separate the object's name from the property or method being worked with. For example, movie clips are objects; to set the rotation property of a movie clip instance named wheel_mc, you would use the syntax:


wheel_mc._rotation = 90;

Notice how a dot separates the name of the object from the property being set.

To tell the same movie clip instance to play, invoking the play() method, you would use the syntax:


wheel_mc.play()

Once again, a dot separates the name of the object from the method invoked.

Parentheses

These are used in various ways in ActionScript. For the most part, scripts employ parentheses to set a specific value that an action will use during its execution. Look at the last line of our sample script that tells the cashRegister_mc movie clip instance to go to and play Frame 50:


cashRegister_mc.gotoAndPlay (50);

If the value within parentheses is changed from 50 to 20, the action still performs the same basic task (moving the cashRegister_mc movie clip instance to a specified frame number); it just does so according to the new value. Parentheses are a way of telling an action to work based on what's specified between the parentheses.

Quotation Marks

These are used to denote textual data in the script. Because text is used in the actual creation of the script, quotation marks provide the only means for a script to distinguish between instructions (pieces of data) and actual words. For example, Derek (without quotes) signifies the name of a piece of data. On the other hand, "Derek" signifies the actual word "Derek."

Comments

These are lines in the script preceded by two forward slashes (//). When executing a script, Flash ignores lines containing comments. They indicate descriptive notes about what the script is doing at this point in its execution. Comments enable you to review a script months after it was written and still get a clear idea of its underlying logic.

You can also create multi-line comments using the syntax:


/* everything between

here is considered

a comment */

Indenting/Spacing

Although not absolutely necessary, it's a good idea to indent and space the syntax in your code. For example:


on (release) {

var mugCost:Number = 5.00;

}

will execute the same as:


on (release) {

  var mugCost:Number = 5.00;

}

However, by indenting code, you make it easier to read. A good rule is to indent anything within curly braces to indicate that the code within those braces represents a code block, or chunk of code, that is to be executed at the same time. (The AutoFormat feature of the Actions panel takes care of most of this for you.) You can nest code blocks within other code blocksa concept that will become clearer as you work through the exercises.

For the most part, white space is ignored within a script. For example:


var totalCost:Number = mugCost + totalTax ;

will execute in the same way as:


var totalCost:Number =mugCost+totalTax;

While some programmers feel that extra white space makes their code easier to read, others believe it slows them down to insert spaces. For the most part, the choice is yours. There are a couple of exceptions: variable names cannot contain spaces; nor can you put a space between an object name and an associated property or method. While this syntax is acceptable: