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Design Patterns using barcode encoder for none control to generate, create none image in none applications. Jasper Reports The second none none part of this chapter presents a JavaScript approach to a subset of the design patterns introduced by the book called Design Patterns: Elements of Reusable Object-Oriented Software, an influential book most commonly referred to as the Book of Four or the Gang of Four, or GoF (after its four authors). The patterns discussed in the GoF book are divided into three groups: Creational patterns that deal with how objects are created (instantiated) Structural patterns that describe how different objects can be composed in order to provide new functionality Behavioral patterns that describe ways for objects to communicate with each other. [ 280 ]. 8 . There are 23 patterns in the Book of Four, and more patterns have been identified since the book"s publication. It is way beyond the scope of this book to discuss all of them, so the remainder of the chapter will demonstrate only four of them, along with examples of the implementation of these four in JavaScript. Remember that the patterns are more about interfaces and relationships rather than implementation.

Once you have an understanding of a design pattern, it"s often not difficult to implement it, especially in a dynamic language such as JavaScript. The patterns discussed through the rest of the chapter are: Singleton Factory Decorator Observer. Singleton Singleton is a creational design pattern meaning that its focus is on creating objects. It is useful when you want to make sure there is only one object of a given kind or class. In a classical language, this would mean that an instance of a class is only created once and any subsequent attempts to create new objects of the same class would return the original instance.

In JavaScript, because there are no classes, a singleton is the default and most natural pattern. Every object is a single object. If you don"t copy it and don"t use it as a prototype of another object, it will remain the only object of its kind.

The most basic implementation of the singleton in JavaScript is the object literal:. var single = {};. Singleton 2 If you wan none for none t to use class-like syntax and still implement the singleton, things can become a bit more interesting. Let"s say you have a constructor called Logger() and you want to be able to do something like:. var my_log = new Logger(); my_log.log("some event"); // ..

. 1000 lines of code later ..

. var other_log = new Logger(); other_log.log("some new event"); alert(other_log === my_log); // true [ 281 ].

Coding and Design Patterns The idea i s that although you use new, only one instance needs to be created, and this instance is then returned in consecutive calls.. Global Variable One approa ch would be to use a global variable to store the single instance. Your constructor could look like this:. function L none for none ogger() { if (typeof global_log === "undefined") { global_log = this; } return global_log; }. Using this constructor gives the expected result:. var a = new Logger(); var b = new Logger(); alert(a === b); // true The drawba ck is, of course, the use of a global variable. It can be overwritten at any time, even accidentally, and you lose the instance. The opposite your global variable overwriting someone else"s is also possible.

. Property of the Constructor As you kno w, functions are objects and they have properties. You can assign the single instance to a property of the constructor function..

function L none for none ogger() { if (typeof Logger.single_instance === "undefined") { Logger.single_instance = this; } return Logger.

single_instance; }. If you wri te var a = new Logger(), a will point to the newly created Logger.single_instance property. A subsequent call var b = new Logger() will result in b pointing to the same Logger.

single_instance property, which is exactly what you wanted. This approach certainly solves the global namespace issue, because no global variables are created. The only drawback is that the property of the Logger constructor is publicly visible, so it can be overwritten at any time.

In such cases, the single instance can be lost or modified.. [ 282 ].
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