Design Patterns

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198 CHAPTER 14 ■ THE PROXY PATTERN /* PublicLibrary class. */ var PublicLibrary = function(books) { // implements Library this.catalog = {}; for(var i = 0, len = books.length; i < len; i++) { this.catalog[books[i].getIsbn()] = { book: books[i], available: true }; } }; PublicLibrary.prototype = { findBooks: function(searchString) { var results = []; for(var isbn in this.catalog) { if(!this.catalog.hasOwnProperty(isbn)) continue; if(searchString.match(this.catalog[isbn].getTitle()) || searchString.match(this.catalog[isbn].getAuthor())) { results.push(this.catalog[isbn]); } } return results; }, checkoutBook: function(book) { var isbn = book.getIsbn(); if(this.catalog[isbn]) { if(this.catalog[isbn].available) { this.catalog[isbn].available = false; return this.catalog[isbn]; } else { throw new Error('PublicLibrary: book ' + book.getTitle() + ' is not currently available.'); } } else { throw new Error('PublicLibrary: book ' + book.getTitle() + ' not found.'); } }, returnBook: function(book) { var isbn = book.getIsbn(); if(this.catalog[isbn]) { this.catalog[isbn].available = true; } else { throw new Error('PublicLibrary: book ' + book.getTitle() + ' not found.'); } } };
CHAPTER 14 ■ THE PROXY PATTERN 199 This is a fairly simple class. It allows you to search the catalog, check out books, and then return them later. A proxy for the PublicLibrary class that implements no access control would look like this: /* PublicLibraryProxy class, a useless proxy. */ var PublicLibraryProxy = function(catalog) { // implements Library this.library = new PublicLibrary(catalog); }; PublicLibraryProxy.prototype = { findBooks: function(searchString) { return this.library.findBooks(searchString); }, checkoutBook: function(book) { return this.library.checkoutBook(book); }, returnBook: function(book) { return this.library.returnBook(book); } }; It implements the same interface and all of the same methods as PublicLibrary. When it is instantiated, it instantiates a copy of PublicLibrary and stores it as an attribute. Anytime a method of PublicLibraryProxy is invoked, it will use this attribute to invoke the same method on its PublicLibrary instance. This type of proxy could also be created dynamically by examining the interface of the real subject and creating corresponding methods for each one. This is similar to the way we created a decorator with a dynamic interface in Chapter 12. As we said before, this type of proxy isn’t very useful. One of the most useful types of proxies is the virtual proxy. A virtual proxy controls access to a real subject that is expensive to create. It will defer instantiation of its real subject until a method is called, and sometimes provide feedback on the status of the instantiation. It can also function as a stand-in until the real subject is loaded. As an example, let’s say the PublicLibrary class is too slow to instantiate immediately at page load. You could create a virtual proxy that would defer instantiation until needed: /* PublicLibraryVirtualProxy class. */ var PublicLibraryVirtualProxy = function(catalog) { // implements Library this.library = null; this.catalog = catalog; // Store the argument to the constructor. }; PublicLibraryVirtualProxy.prototype = { _initializeLibrary: function() { if(this.library === null) { this.library = new PublicLibrary(this.catalog); } }, findBooks: function(searchString) { this._initializeLibrary();
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The EXPERT’s VOIce ® in Web Deve
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Pro JavaScript Design Patterns Cop
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Contents at a Glance About the Auth
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viii ■CONTENTS ■CHAPTER 3 Encap
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x ■CONTENTS ■CHAPTER 8 The Brid
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xii ■CONTENTS Example: Tooltip Ob
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Acknowledgments Thanks to our intre
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xxii ■INTRODUCTION them. This typ
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CHAPTER 1 ■ ■ ■ Expressive Ja
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CHAPTER 1 ■ EXPRESSIVE JAVASCRIPT
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CHAPTER 2 ■ ■ ■ Interfaces Th
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CHAPTER 2 ■ INTERFACES 19 The Int
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CHAPTER 2 ■ INTERFACES 21 1. Incl
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CHAPTER 2 ■ INTERFACES 23 Pattern
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26 CHAPTER 3 ■ ENCAPSULATION AND
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42 CHAPTER 4 ■ INHERITANCE Classi
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CHAPTER 5 ■ ■ ■ The Singleton
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CHAPTER 5 ■ THE SINGLETON PATTERN
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CHAPTER 6 ■ ■ ■ Chaining In t
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CHAPTER 6 ■ CHAINING 85 Since all
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CHAPTER 6 ■ CHAINING 87 (function
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CHAPTER 6 ■ CHAINING 89 Using Cal
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PART 2 ■ ■ ■ Design Patterns
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94 CHAPTER 7 ■ THE FACTORY PATTER
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100 CHAPTER 7 ■ THE FACTORY PATTE
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110 CHAPTER 8 ■ THE BRIDGE PATTER
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CHAPTER 9 ■ ■ ■ The Composite
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CHAPTER 9 ■ THE COMPOSITE PATTERN
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264 ■INDEX classical inheritance,
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266 ■INDEX as objects, 6-8, 37 pr
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268 ■INDEX performance issues, 12
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