Tài liệu Understanding Inheritance pptx

< Day Day Up > 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 object- oriented code. 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 o f 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 started! 1. Open PetParade1.fla. The first order of business is to become familiar with the contents of this FLA. There are three layers in this project file. The Background layer holds the background graphics; the Assets layer currently contains six buttons named dogRun_btn, dogStop_btn, dogSound_btn, catRun_btn, catStop_btn, and catSound_btn. The Actions layer is currently empty, but will eventually contain script. 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: 4. 5. class Animal extends MovieClip { 6. 7. private var speed:Number; 8. 9. function Animal() { 10. 11. this.speed = 5 12. 13. } 14. The first line gives the class a name, Animal, and then specifies that this class extends the MovieClip class. This means that instances of the Animal class inherit all the functionalities (properties and methods) of movie clips—properties such as _x and _name, as well as methods such as gotoAndPlay() and loadMovie(). This is your first experience creating a new class that inherits from another class. You can think of this technique as taking the basic functionality of movie clips and 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: 5. 6. function run() { 7. 8. this.onEnterFrame = function() { 9. 10. this._x += this.speed; 11. 12. }; 13. 14. } 15. 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: 6. 7. function stop() { 8. 9. delete this.onEnterFrame; 10. 11. } 12. 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: 8. 9. class Cat extends Animal { 10. The first two words tell Flash that you're creating a new class called Cat. The next two words, extends Animal, tell Flash that this class inherits every method and property of the Animal class, including run() and stop(). We used similar syntax when defining the Animal class; it extended the MovieClip class. As a result, not only does the Cat class get all the capabilities of the Animal class, but those of the MovieClip class as well. Inheritance trickles down like this as long as you 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: 9. 10. private var catSound:Sound; 11. We won't need to access the catSound property from outside the class, so it's declared as a private variable. The meow() method (created in Step 10) will use the catSound property. 9. Create the following constructor method for the Cat class: 10. 11. function Cat() { 12. 13. this.speed = 1; 14. 15. } 16. 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: 11. 12. function meow() { 13. 14. catSound = new Sound(this); 15. 16. catSound.attachSound("Meow"); 17. 18. catSound.start(); 19. 20. } 21. When called, this method creates a new sound object and stores it in the private variable catSound. This object has the sound in the Library with a linkage 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. [...]... can both run, stop, and make sounds, all based on the code in our class files We're not finished quite yet To help solidify your understanding of inheritance, we're going to make a few enhancements to the Animal class so you can see how easy it is to update an object-oriented /inheritance- based project 1 The Library panel should still be open from the preceding exercise Double-click the Balloon movie clip... single file—the Animal class file All instances that inherited from this class automatically inherited this new behavior when the class file was updated We hope that you can now appreciate the power of inheritance, and how it allows you to create more manageable projects 13 Close the test movie and save your work as PetParade2.fla Due to page constraints, most of the projects in the remainder of the . < Day Day Up > Understanding Inheritance A class can gain (inherit) all members from another class. This is called inheritance. The class that's. class files. We're not finished quite yet. To help solidify your understanding of inheritance, we're going to make a few enhancements to the Animal