Tài liệu Understanding Inheritance pptx

So it makes sense to create a general class that programs these common capabilities—let's call it the Character class—and then create additional classes Hero and Enemy that inherit all t

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Understanding Inheritance

A class can gain (inherit) all members from another class This is called inheritance The class that's gaining the members is called a subclass and the class from which it inherits is called the superclass If B is a subclass of A, B is said to extend A

Inheritance promotes code reusability It allows you to give functionality to other classes without having to rewrite code that would be redundant This concept will \be much clearer by the end of this lesson

Imagine for a moment that you're writing the ActionScript for a game This game has a hero, controlled by the user, and lots of simple enemy characters The enemies are not the same as the hero, but they share a number of similarities, such as the ability to walk, get hurt, heal, and die If you program separate Hero and Enemy classes, you end up

rewriting much of the same code to handle these common capabilities So it makes sense

to create a general class that programs these common capabilities—let's call it the

Character class—and then create additional classes (Hero and Enemy) that inherit all the functionality of the Character class as well as extend that functionality, each in unique ways The Hero class might extend the Character class with the capability to be

controlled via a user interface, and the capability to wield many types of weapons The Enemy class might extend the Character class with the capability to use an artificial intelligence algorithm to govern its movement

In this example, the base superclass (Character) is written once, and two subclasses (Hero and Enemy) extend the functionality of the base class to make new and unique classes Let's look at how this is done

Let's assume that a Character class has already been created, given properties and

methods, and saved as Character.as A subclass of the Character class (we'll use the Hero class) is created by using the keyword extends in the class definition For example:

class Hero extends Character {

function Hero() {

//Constructor

}

}

The first word, class, says that what follows is a class definition The next word, Hero, gives a name to the class Next you see extends Character This statement instructs Flash

at compile time to give the Hero class all the methods and properties found in the

Character class (the Hero class inherits from the Character class) In addition to inheriting from the Character class, the Hero class can also be given functionality (properties and methods) unique to itself

A class can only extend (inherit from) one other class For example, if Hero extends Character, it cannot extend any other class; however, Hero can extend Character and then MyHero can extend Hero, and so on

When a class extends another class, the resulting subclass gains all the properties and

methods of the superclass; however, those properties and methods can still be changed or enhanced Replacing a property value or method of the superclass with a new one in the subclass is called overriding Let's look again at the Character class example to help you get a better understanding of this concept

Let's say that both the Hero and Enemy classes extend the Character class, and the

Character class has a die() method As a result of inheritance, both the Hero and Enemy classes inherit the functionality of the die() method, as well as all the other members of the Character class But let's say you want heroes and enemies to die in a different

manner than the inherited die() method allows In this case, you would simply define a new die() method in the Hero and Enemy classes Creating a method in a subclass with the same name as an inherited method from its superclass causes the functionality of the subclasses' method to take precedence over the inherited method

Being able to override properties and methods in this way is useful in creating oriented code

object-In the following exercise, you'll put into practice most of the concepts you've learned in this lesson so far You'll create a class called Animal and program the Animal class with several capabilities, including running and stopping, as most animals can do You'll also create a Cat class that extends Animal by giving Cat animals the capability to meow (Because the Cat class extends the Animal class, the Cat class inherits the functionality of the Animal class; Cat animals automatically can run and stop.) In addition, you'll create a Dog class that extends the Animal class in a manner similar to that of the Cat class,

except that Dog animals will be able to bark When you've finished scripting these

classes, you'll associate the Cat and Dog classes with different movie clips in the library

As a result of this association, each instance of one of those clips that you drag into your project will take on the characteristics of the class with which it's associated

In this exercise, you'll gain experience creating classes, extending a class, overriding methods and properties, and working with instances of custom-made classes Let's get

For this exercise, it's also important to understand the assets in the library because

they will play an integral role in how the final application works

2 Choose Window > Library to open the Library panel

The library contains a folder, two sound clips, and three movie clips The folder contains miscellaneous elements that you're free to examine, but you won't work with them directly in this exercise

The three movie clips are named Dog Clip, Cat Clip, and Balloon Dog Clip and

Cat Clip are the graphical and interactive representations of the Dog class and Cat class This is important to understand; things that we program the Dog and Cat classes to do will be graphically and interactively carried out by instances of the Dog Clip and Cat Clip movie clips The Balloon movie clip will be used and explained later in the exercise

The two sound clips named Meow.mp3 and Bark.mp3 represent the sounds that our Cat and Dog animals will make If you right-click (Control-click on a

Macintosh) either of these sounds and then choose Linkage from the menu that appears, you see that the Meow.mp3 sound has been given an identifier of Meow, and the Bark.mp3 sound has been given an identifier of Bark Steps 10 and 12 in this exercise will explain how these sounds are used

Now that you're familiar with the project's assets, it's time to begin creating the class files that it will use

NOTE

You will be creating and working with several files in this exercise As we

progress through the steps, keep all the files open You will be instructed when to tab between the files

3 Create a new ActionScript file called Animal.as in the Lesson07/Assets folder Add the following script to the Animal.as file:

extending it in a way that's appropriate for programming how Animals work This will become clearer as we progress through the steps

TIP

Any of Flash's built-in classes can be extended in this way With this capability, for example, you can create an enhanced Sound, TextField, or Array class to fit your needs exactly

The line following the class declaration defines a private variable named speed that will used by the class Remember that private variables can only be accessed and used by scripts within the class definition—not directly by instances of the class The speed variable will be used by a method named run() that we'll create in

a moment This method will be used to move a movie clip based on the value of speed

The final three lines of the ActionScript in this step define the constructor method for the Animal class Remember that scripts within the constructor are executed at the moment that an instance of the class is created Because the Dog and Cat classes inherit from the Animal class, any scripts placed here will be executed when an instance of the Dog class or Cat class is created The only action here is

to set the value of speed in relation to this this is a reference to an instance created from the class In other words, whenever an instance of the Animal class is

created, that instance is given an initial speed value of 5 We'll add more scripts to

the Animal class constructor later in the exercise

4 After the class constructor, add the following script to handle making an animal run:

The run() method is used to put instances of the Animal class into motion When

an instance of the Animal class invokes this method, an onEnterFrame event is attached to that instance (this) The action within the event moves the instance along its x axis by the value of the speed variable Remember that in Step 3 we set the initial value of speed in the constructor method to 5 When an instance of the Animal class invokes this method, it will begin moving five pixels at a time, 24 times per second (the frame rate of our movie) The instance will continue to move until stopped with the stop() method

5 Next, add the following stop() method:

This method stops the instance from moving, simply by deleting the onEnterFrame

event from the instance

6 End the definition of the Animal class with a closing curly brace (}); then choose File > Save to save the class file

You've created the Animal class Next you'll create the two subclasses (Dog and

Cat) that extend Animal

7 In the same directory as Animal.as, create a new ActionScript file named Cat.as Start the class definition with the following script:

continue extending classes

In the following steps, we'll program the Cat class to deal with functionalities unique to cats Running and stopping is something that most animals can do; that's why those functionalities were defined in the more general Animal class, from which the Cat class inherits The great thing about object-oriented inheritance is that we could create 50 more classes (based on different animals) that all extend the Animal class, but to change the way in which each class handles running we would simply edit the run() method in the Animal class file We'll demonstrate this principle later in the exercise

Let's set up the unique features of the Cat class

8 Below the line of script that defines the Cat class, create a property to store the sound object for a Cat instance:

the catSound property

9 Create the following constructor method for the Cat class:

When a new instance of the Cat class is created, its speed property is given a value

of 1 In Step 3, we created a variable named speed in the Animal class constructor method and gave it an initial value of 5 Because the Cat class inherits from the Animal class, it automatically inherited that property and its value You may be wondering why we're setting it again here Instances of the Animal class will still have a speed value of 5, but instances of the Cat class will have a speed value of 1 We're overriding the inherited value with a value specific to cats

Overriding an inherited property value involves nothing more than using the name

of the property you want to override—the speed property in this case—and

assigning it a new value As a result, when an instance of the Cat class is created, its speed property is set to 1

Properties and methods defined in a class always have precedence over properties and methods inherited from another class with the same name; therefore, instances

of the Cat class will see and use the speed value of 1 as opposed to a value of 5

The run() and stop() methods of the Animal class are still inherited, and still work with instances of the Cat class as they were defined in the Animal class file,

because we haven't overridden them with methods of the same name in the Cat

class (we won't override those methods in this exercise)

10 Create a method called meow() that plays a meowing sound when called:

identifier of Meow attached to it The sound is played by calling the start() method

of the sound object Because this method is defined in the Cat class, only instances

of the Cat class can call it

11 End the definition of the Cat class by adding the closing curly brace (}); then choose File > Save to save the class file

You have completed the first subclass of Animal Next you'll create another

subclass of Animal, the Dog class

12 In the same directory as Animal.as, create a new ActionScript file called Dog.as Define this class as follows:

13

14 class Dog extends Animal {

15

16 private var dogSound:Sound;

Similar to the meow() method discussed in Step 10, the bark() method creates a new sound object and stores it as dogSound The method attaches the sound with the linkage identifier of Bark and then plays the sound

In the end, Dogs and Cats both run() and stop(), as most Animals do, but Dogs run

at a speed of 2 and Bark, while Cats run at a speed of 1 and Meow

13 Choose File > Save to save the Dog class file

For the moment, our class files are complete We'll return to the Animal class file

shortly, but it's time to open the actual project file to plug in these class files and

their functionality

14 PetParade1.fla should already be open in the authoring environment Click its tab

to make PetParade1.fla the active window

The first order of business is to associate our Dog and Cat classes with the Dog

Clip and Cat Clip movie clips in the Library

15 With the Library panel open, right-click (Control-click on a Macintosh) the Cat Clip movie clip and choose Linkage from the menu that appears In the Linkage Properties dialog box, select the Export for ActionScript option; then enter cat in the Identifier field and Cat (the first letter must be uppercase) in the AS 2.0 Class field Click OK

Although you configured several settings in this step, the one you need to focus on

is the AS 2.0 Class field By entering a value of Cat in this field, you're associating the Cat Clip movie clip in the library to the Cat class you created This means that all instances of the Cat Clip movie clip you place in your project will take on the functionality defined in the Cat class file As a result, these instances can run(), stop(), and meow(), as you will soon see

16 Right-click (Control-click on a Macintosh) the Dog Clip movie clip and choose Linkage from the menu that appears In the Linkage Properties dialog box, select the Export for ActionScript option; then enter dog in the Identifier field and Dog (the first letter must be uppercase) in the AS 2.0 Class field Click OK

This step associates the Dog Clip movie clip with the Dog class As a result, all instances of the Dog Clip movie clip you place in your project can run(), stop(), and bark() This is a great feature of Flash that allows you to create highly

interactive class-based movie clips You simply create a custom class and then associate that class with a movie clip; you instantly have a custom movie clip NOTE

To use this feature, the associating class (which in this case is Dog) must inherit from the MovieClip class somewhere up the line

17 Click and drag instances of the Dog Clip and Cat Clip movie clips onto the stage Give the Dog Clip instance the name dog_mc and give the Cat Clip instance the name cat_mc

Next, we'll add scripts to cause these instances to perform the actions that cats and

dogs do, based on what we programmed into our custom class files

18 With the Actions panel open, select Frame 1 of the Actions layer and add the following button events to control the dog_mc movie clip instance: