Tài liệu Built-In Object Classes ppt

A constructor looks like this: nameOfInstance:nameOfClass = new nameOfClass; If you wanted to create an instance of the Sound class, the constructor would look like this: mySound:Sound =

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Built-In Object Classes

In the next section, we'll briefly review many of the built-in object classes that

ActionScript provides, as well as some ways they are used Before we do, however, it's important to discuss how to actually get instances of classes into your project in the first place, so you can begin to utilize their power and functionality

You can create object instances in one of two ways To create a new instance of the MovieClip, Button, or TextField class, you create it on the stage or drag it onto the stage from the library However, only MovieClip, Button, and TextField instances can be created in this way To create an instance of any other class, you must use a constructor—

a simple line of code that tells Flash to create an object instance of a particular class A constructor looks like this:

nameOfInstance:nameOfClass = new nameOfClass();

If you wanted to create an instance of the Sound class, the constructor would look like this:

mySound:Sound = new Sound();

Whenever you create an object instance, you must give it a unique name This allows you

to change a property or invoke a method as it relates to that particular instance

(remember, an instance inherits all the properties and methods of the class to which it belongs) We'll demonstrate the concept in examples As you gain programming

experience, you'll begin to understand when you should use constructors

While instances of the MovieClip, TextField, and Button classes can be created on the stage manually by dragging them from the library, they can also be created dynamically using ActionScript

NOTE

Object names follow the same naming conventions as variables, which means they can't

contain spaces, symbols, or a number as the first character

Some object classes are known as top-level classes, which means you can't create

instances of them using the methods we've shown What differentiates these classes from those of which you create instances? Top-level classes represent and control global

functionalities within your project Take, for example, the Mouse class, which controls cursor visibility (among other things) You have just one cursor, so it wouldn't make sense to be able to create instances of this class; instead, you use the methods available to

it, to do various things with the mouse Look at the example:

Mouse.hide()

This line of script hides the cursor Notice that the name of an instance is not referenced

in relation to the hide() method Instead, the name of the top-level class is referenced (in this case, Mouse), followed by the name of the method you wish to use Similar syntax is used with any top-level class As we go through the rest of this lesson, we'll introduce you to other top-level classes and the syntax required to use them

In the Actions panel under the Built-in Classes book, you can access all of Flash's built-in classes, each of which is contained in one of these five subbooks:

• Core These classes deal with information storage and manipulation, not including

information that's being moved into or out of Flash itself

• Media These classes assist with manipulating sound and video in your Flash

movie, such as playing sounds, gaining access to the system camera, and

streaming video

• Movie These classes deal with visual content and system-related information such

as movie clips, text fields, the stage, and accessibility

• Client/Server These classes control the movement of information in and out of

Flash

• Authoring These classes assist you in creating custom actions and custom

components

The following describes many of the built-in classes available in ActionScript as well as where and how you might use them We'll indicate whether a class is a top-level class (creating instances is not required), or not, in which case you must create individual instances

Accessibility Class (Top-Level)

This class contains read-only information about the computer's ability to use a screen reader:

Accessibility.isActive();

This script returns a result of either true or false If the result is true, the user's computer can employ a screen reader

Array Class (Instances)

An array is a storage device for multiple pieces of information Arrays store information that can be set and referenced using a numbering system For example:

var cakeType:Array = new Array();

cakeType[0] = "Chocolate";

cakeType[1] = "Angel Food";

cakeType[2] = "Baked Alaska";

The first line creates a new instance of the Array class called cakeType using the Array constructor The next lines place data inside that array

The Array class contains many useful methods that will help you add, remove, and sort array items from instances of that class

NOTE

For more on arrays, see Lesson 6, "Creating and Manipulating Data."

Boolean Class (Instances)

Instances of the Boolean class store one of two values, true or false You can create a Boolean object by using the Boolean constructor or by using the = assign operator For example:

var toggle:Boolean = new Boolean(false);

and

var toggle:Boolean = false;

create identical objects

Button Class (Instances)

When you place a button on the stage, you create an instance of the Button class Only MovieClip and TextField objects are created in a similar fashion—that is, by placing actual instances on the stage The Button class contains properties and methods that allow you to control the appearance, tab order, functionality, and more of button instances

Capabilities Class (Top-Level)

This class contains information about the user's computer, such as screen resolution and whether it can play sounds The following script places the horizontal resolution of the

user's computer into myVariable:

var myVariable:Number = System.capabilities.screenResolutionX;

TIP

Being able to access computer information allows you to create movies that tailor

themselves to the capabilities of your user's computer For example, you can determine whether a handheld computer is accessing the movie and, if so, redirect the user to a page designed expressly for handheld devices

Color Class (Instances)

You use an instance of this class to change a movie clip's color dynamically When you create a Color object, you point it at a particular movie clip Using the Color class's methods, you can alter your movie clip's color You create a Color object using the Color class constructor method:

var myColor:Color = new Color(pathToTimeline);

Later in this lesson you'll complete an exercise using an instance of the Color class

ContextMenu Class (Instances)

The context menu is the menu seen in the Flash player when you right-click (Control-click on the Macintosh) This class is used in conjunction with instances of the

ContextMenuItems class (described shortly) to create customized context menus (with custom commands), which appear when the user right-clicks (or Control-clicks) a visual element in the Flash player window This class also allows you to enable or disable any and all of the built-in context menu commands (such as Play, Stop, and Print) even as your movie plays

This script creates a new ContextMenu object named myCustomMenu:

var myCustomMenu:ContextMenu = new ContextMenu();

myCustomMenu.hideBuiltInItems();

myCustomMenu.builtInItems.print = true;

myMovieClip_mc.menu = myCustomMenu;

myButton_btn.menu = myCustomMenu;

myTextField_txt.menu = myCustomMenu;

The second line of the script uses the hideBuiltInItems() method to hide all the built-in menu commands, and the third line enables the Print command so that it will appear when the menu is opened The last three lines assign the custom menu to a movie clip, button, and text field instance Right-clicking (or Control-clicking) any of these instances will cause the custom menu to appear

ContextMenuItems Class (Instances)

This class is used in conjunction with the ContextMenu class (described just above) to create items that appear in a custom context menu Your Flash project can be scripted to capture when a user clicks a custom menu item so that you can have a specific action or actions occur For example, you can create a custom context menu that will allow a user

to right-click in your project and choose to mute the volume

NOTE

You will see more on the ContextMenu and ContextMenuItems classes in Lesson 20,

"Maximum-Strength SWFs"

Date Class (Instances)

With this class you can access the current time as local time or Greenwich Mean Time, as well as easily determine the current day, week, month, or year To create a new instance

of the Date class, you use the Date class constructor method This example demonstrates one use of the Date class:

var now:Date = new Date();

var largeNumber:Number = now.getTime();

The example creates a variable called largeNumber, whose value is the number of

milliseconds since midnight January 1, 1970

NOTE

We will use the Date object in Lesson 16, "Time- and Frame-Based Dynamism."

Error Class (Instances)

The Error class was introduced in Flash MX 2004 to help with managing errors in your projects An error is whatever you define an error to be (a number was too large or too small, for example) When an error occurs, a new instance of the Error class is

instantiated—this is known as "throwing" an error With the Error class you can capture errors and write code to handle them so that your application behaves well, rather than acting in an unpredictable way

NOTE

You will see more on the Error class in Lesson 19, "Testing and Debugging."

Key Class (Top-Level)

You use the Key class to determine the state of the keys on the keyboard—for example, whether the Caps Lock key is toggled on, which key was pressed last, and which key or keys are currently pressed

You will complete an exercise using this object later in this lesson

LoadVars Class (Instances)

Flash allows you to load data into a movie from an external source Using the LoadVars class, Flash can load in variables from a specified URL (which can be a standard text file) For example:

var myObj:LoadVars = new LoadVars();

myObj.load("http://www.mysite.com/myFiles/file.txt");

In the example, all of the loaded variables become properties of the myObj LoadVars instance

Math Class (Top-Level)

With the Math class, you can perform many useful calculations and have the result

returned Here's one:

var positiveNumber:Number = Math.abs(-6);

The script uses the absolute value method of the Math object to convert the–6 to a

positive number

Mouse Class (Top-Level)

The Mouse class controls cursor visibility and allows you to set up Listeners to track mouse activity Here is an example:

Mouse.hide();

The script hides the mouse from view The mouse is still active, but it is not visible

MovieClip Class (Instances)

You create instances of this most familiar class either in the authoring environment (by placing them on the stage), or with ActionScript actions such as createEmptyMovieClip() and duplicateMovieClip()—not by using the constructor function Movie clip instances

have many properties and methods that are used frequently in an interactive project Here's an example:

myClip_mc.gotoAndStop("Menu");

With this script, a movie clip with an instance name of myClip_mc will be sent to the frame labeled Menu

MovieClipLoader Class (Instances)

This class provides a way for you to easily load and gain access to information during the load of an SWF or JPG into a target movie clip or level With an instance of the

MovieClipLoader class, you know the file size of the external asset you are loading as well as how much of it has been loaded By continually checking to see how much of the asset has been loaded, you can build a progress bar that indicates how far along an asset

is in the loading process

The MovieClipLoader class also provides a way for you to be informed of when the asset has finished loading

NOTE

This class will be used in Lesson 18, "Loading External Assets."

NetConnection Class (Instances)

The NetConnection class is used together with the NetStream class to play external Flash Video (FLV) files from an HTTP address or a hard drive

NetStream Class (Instances)

The NetStream class provides methods and properties for controlling the playback of external Flash Video (FLV) files

NOTE

You will see more on the NetConnection and NetStream classes in Lesson 18, "Loading External Assets."

Number Class (Top-Level)

You can create a Number class instance by using its constructor method or by assigning a number as the value of a variable For instance:

var age:Number = new Number(26);

is equivalent to:

var age:Number = 26;

The new Number() constructor method is rarely used, however, because creating a new number without the constructor takes less effort and achieves the same result

Object Class (Instances)

No, it's not a typo: there is an Object class! You can use this generic class—which is also known as ActionScript's root class (meaning it's the highest in the class hierarchy) in various ways By employing the properties and methods available to it, you can affect and modify other object classes (such as those listed in this section) It also comes in handy for creating object instances that hold information about the current user, or

instances that track chunks of related data (to name just a couple of uses)

The following is the syntax for creating a generic object:

var names:Object = new Object();

names.cat = "Hayes";

The first line of script creates a new object called names The second line adds a variable (property) to the object called cat The variable is considered a property of this object

classes of objects (better than generic objects!) as well as how to create properties and methods for your custom class Once you know how to do this, you can create objects and classes that do precisely what you want

PrintJob Class (Instances)

Previous versions of Flash left much to be desired in the area of printing For example, various frames on the timeline had to be specified as printable before the movie was exported to an SWF file In addition, printing content from multiple timelines opened multiple Print dialog boxes Flash MX 2004 provides a much-improved way to handle printing With the PrintJob class you are able to dynamically specify frames from various timelines to print from a single Print dialog box

NOTE

This class is used in Lesson 21, "Printing and Context Menus."

Selection Class (Top-Level)

You use the Selection class to retrieve information or set characteristics relating to

selected items in your movies, especially text in text fields When the cursor is in an area

of a text field, that field is said to be "in focus." You can employ the Selection class to set the focus to a specific text field, to find out which text field is currently in focus, or even

to programmatically select specific chunks of text in a field so that it can be manipulated

in some way Here's an example of one use of the Selection class:

Selection.setFocus("firstName");

The script sets into focus the input text field with the instance name of firstName

You'll complete an exercise using this class later in this lesson

Sound Class (Instances)

You use instances of the Sound class to control sounds—for example, setting volume and adjusting left and right speaker pan settings To learn more about this class, see Lesson

17, "Scripting for Sound."