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Introducing ActionScript

Chapter Description

Get ready to be introduced to your new best friend: ActionScript! This sample chapter will show you some compelling reasons for learning ActionScripting, as well as what makes it tick.

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) to your Flash project. Like all languages, ActionScript contains many different elements, such as words, punctuation, and structure—all 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 more logical elements such as logical statements and expressions, will be dealt with in much more detail throughout the book.

To begin to understand how ActionScript works, take a look at the following script, which contains many of the essential elements that make up a typical script. Following 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
  mugCost = 5.00;
  //set the local sales tax percentage
  taxPercent = .06;
  //determine the dollar amount of tax
  totalTax = mugCost * taxPercent;
  //determine the total amount of the transaction
  totalCost = mugCost + totalTax;
  //display a custom message
  myTextBox.text = "The total cost of your transaction is " + totalTax;
  //send the cashRegister movie clip instance to frame 50
  cashRegister.gotoAndPlay (50);
}

Although at first glance this may look like Latin, once you become acquainted with some of its elements, you shouldn't find it too difficult to understand.

NOTE

There are additional script elements (for example, objects, functions, loops, properties, and methods), which we will discuss in greater detail throughout the book.

Events

These are things that occur, during the playback of a movie, which trigger the execution of a particular script. In our sample script above, the event that triggers the script is on (release). This 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 contain numerous events—everything from a button being pressed to text changing in a text field to a sound completing its playback, and more.

Actions

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

The following are a couple of examples of actions from the sample script above:

mugCost = 5.00;
cashRegister.gotoAndPlay (50); 

Generally speaking, actions comprise most of the lines in a script that are within curly braces ({}) and usually separated by semicolons (see below).

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 the following examples:

taxPercent = .06; assigns a 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, continue, delete, do, else, for, function, if, in, instanceOf, new, return, switch, this, typeOf, 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, its value can be accessed anywhere in the script simply by inserting its name.

In our sample script above, we gave a variable name of mugCost to the 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 speaking, anything that falls between opening and closing curly braces signifies an action or actions the script needs to perform when triggered. Think of curly braces as being used to say, "As a result of this – {do this}." For example:

on (release) {
  //set the cost of the mug
  mugCost = 5.00;
  //set the local sales tax percentage
  taxPercent = .06;
}

Semicolons

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

mugCost = 5.00;
taxPercent = .06;
totalTax = mugCost * taxPercent;
totalCost = mugCost + totalTax;
myTextBox = "The total cost of your transaction is " + totalTax;
cashRegister.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.indana.bloomington points to a movie on the main (_root) timeline named "usa," which contains a movie named "indiana," which contains a movie 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; thus to set the rotation property of a movie clip instance named wheel, you would use the following syntax:

wheel._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 following syntax:

wheel.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 them to set a specific amount that an action will use during its execution. Look at the last line of our sample script that tells the cashRegister movie clip instance to go to and play Frame 50:

cashRegister.gotoAndPlay (50); 

If the value within parentheses is changed from 50 to 20, the action still performs the same basic task (moving the timeline 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 in between them.

Quotation marks

These are used to denote textual data in the script. Because text is used to create 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. "Derek," on the other hand, signifies the actual word Derek.

Comments

These are lines in the script preceded by two forward-slash marks (//). When executing a script, Flash ignores lines containing comments. Instead, these are provided as a means to insert descriptive notes about what the script is doing at this point in its execution. By using comments, you can review a script months after it was written and still get a clear idea of its underlying logic.

Indentation/Spacing

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

on(release) {
mugCost = 5.00;
} 

Will execute the same as:

on(release) {
 mugCost = 5.00; 
} 

However, by indenting code, you make it easier to read. A good rule of thumb is to indent anything within curly braces to indicate that the code within those braces represent a code block, or chunk of code, that is to be executed at the same time. You can nest code blocks within other code blocks—a concept that will become clearer as you work through the exercises.

For the most part, white space is ignored within a script. Take a look at the following line of script:

totalCost = mugCost + totalTax ; 

This will execute in the same way as:

totalCost=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, however, a couple of exceptions to be aware of: Variable names cannot contain spaces, nor can you put a space between an object name and an associated property/method. Thus, although the following is acceptable:

myObject.propertyName 

This is not:

myObject. propertyName 
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