MACROMEDIA FLASH MX 2004 - ACTIONSCRIPT - Page 576

576 Chapter 12: ActionScript Dictionary
You can add getter/setter properties to prototype objects. If you add a getter/setter property to a
prototype object, all object instances that inherit the prototype object inherit the getter/setter
property. This makes it possible to add a getter/setter property in one location, the prototype
object, and have it propagate to all instances of a class (much like adding methods to prototype
objects). If a get/set function is invoked for a getter/setter property in an inherited prototype
object, the reference passed to the get/set function will be the originally referenced object, not the
prototype object.
If invoked incorrectly,
Object.addProperty() may fail with an error. The following table
describes errors that may occur:
Example
Usage 1: An object has two internal methods, setQuantity() and getQuantity(). A property,
bookcount, can be used to invoke these methods when it is either set or retrieved. A third internal
method,
getTitle(), returns a read-only value that is associated with the property bookname:
function Book() {
this.setQuantity = function(numBooks) {
this.books = numBooks;
}
this.getQuantity = function() {
return this.books;
}
this.getTitle = function() {
return "Catcher in the Rye";
}
this.addProperty("bookcount", this.getQuantity, this.setQuantity);
this.addProperty("bookname", this.getTitle, null);
}
myBook = new Book();
myBook.bookcount = 5;
order = "You ordered " + myBook.bookcount + " copies of " + myBook.bookname;
When a script retrieves the value of myBook.bookcount, the ActionScript interpreter
automatically invokes
myBook.getQuantity(). When a script modifies the value of
myBook.bookcount, the interpreter invokes myObject.setQuantity(). The bookname property
does not specify a set function, so attempts to modify bookname are ignored.
Error condition What happens
prop is not a valid property name; for instance, an
empty string.
Returns false and the property is not added.
getFunc is not a valid function object. Returns false and the property is not added.
setFunc is not a valid function object. Returns false and the property is not added.