C# 3.0 Design Patterns PHẦN 3 pps

42 | Chapter 2: Structural Patterns: Decorator, Proxy, and Bridge The second implementation of the Bridge interface in this example also happens to be a Proxy to SpaceBook. Most of the Proxy mechanisms have been stripped out, and all that remains is the smart proxy operation of keeping a serial count of users. Here is OpenBook in its most rudimentary form: public class MyOpenBook : Bridge { // Combination of a virtual and authentication proxy SpaceBook mySpaceBook; string name; static int users; public MyOpenBook (string n) { name = n; users++; mySpaceBook = new SpaceBook(name+"-"+users); } public void Add(string message) { mySpaceBook.Add(message); } public void Add(string friend, string message) { mySpaceBook.Add(friend, name + " said: "+message); } public void Poke(string who) { mySpaceBook.Poke(who,name); } } Note that the Bridge defines the operations that will be supported by all Portal mem- bers. Now, suppose OpenBook wanted to add some cool operations, like SuperPoke: public void SuperPoke (string who, string what) { myOpenBook.Add(who, what+" you"); } SuperPoke is implemented rather crudely on top of Add, as this is a SpaceBook- supported feature. If we put SuperPoke in OpenBook, the compiler will accept it, but we won’t be able to call it because in the main program tom is a reference to a Portal object and SuperPoke is not in the Portal. We can solve this problem in two ways: • Add the new operation to the Portal (Abstraction), but not to the Bridge,soit does not affect other implementations. • If we cannot alter the Portal, we can create an extension method to extend it as follows: static class OpenBookExtensions { public static void SuperPoke (this Portal me, string who, string what) { me.Add(who, what+" you"); } } Example: OpenBook | 43 and call it the same way we call the other methods: tom.SuperPoke("Judith-1","hugged"); Extension methods are one of the new features in C# 3.0. The full program for the OpenBook extension is given in Example 2-6. The SpaceBook and MySpaceBook classes from Example 2-4 were not altered in any way and are given as headers only to save space. In this output, Judith has switched to Open- Book and therefore has the name Judith-1. Tom, as the second OpenBook user, is given the name Tom-2. C# 3.0 Feature—Extension Methods Extension methods allow developers to add new methods to an existing type without having to create an inherited class or to recompile the original. Thus, you can add methods to classes for which you might not even have the sources (e.g., System. String ). An extension method is defined the same way as any other, with two stipulations: • It is declared as static in an outer-level static, nongeneric class. • The type it is extending is declared as the first parameter, preceded by this. The method can then be called as an instance method on an object of the type that has been extended. cf. C# Language Specification Version 3.0, September 2007, Section 10.6.9 Example 2-6. Bridge pattern example code—OpenBook using System; using System.Collections.Generic; // Bridge Pattern Example Judith Bishop Dec 2006, Aug 2007 // Extending SpaceBook with a second implementation via a Portal // Abstraction class Portal { Bridge bridge; public Portal (Bridge aSpaceBook) { bridge = aSpaceBook; } public void Add(string message) {bridge.Add(message);} public void Add(string friend, string message) {bridge.Add(friend,message);} public void Poke(string who) {bridge.Poke(who);} } 44 | Chapter 2: Structural Patterns: Decorator, Proxy, and Bridge // Bridge interface Bridge { void Add(string message); void Add(string friend, string message); void Poke(string who); } class SpaceBookSystem { // The Subject private class SpaceBook { } // The Proxy public class MySpaceBook { } // A Proxy with little to do // Illustrates an alternative implementation of the Bridge pattern public class MyOpenBook : Bridge { // Combination of a virtual and authentication proxy SpaceBook myOpenBook; string name; static int users; public MyOpenBook (string n) { name = n; users++; myOpenBook = new SpaceBook(name+"-"+users); } public void Add(string message) { myOpenBook.Add(message); } public void Add(string friend, string message) { myOpenBook.Add(friend, name + " : "+message); } public void Poke(string who) { myOpenBook.Poke(who,name); } } } static class OpenBookExtensions { public static void SuperPoke (this Portal me, string who, string what) { me.Add(who, what+" you"); } } // The Client class BridgePattern : SpaceBookSystem { static void Main ( ) { Example 2-6. Bridge pattern example code—OpenBook (continued) Example: OpenBook | 45 Portal me = new Portal(new MyOpenBook("Judith")); me.Add("Hello world"); me.Add("Today I worked 18 hours"); Portal tom = new Portal(new MyOpenBook("Tom")); tom.Poke("Judith-1"); tom.SuperPoke("Judith-1","hugged"); tom.Add("Judith-1","Poor you"); tom.Add("Hey, I'm on OpenBook - it's cool!"); } } /* Output ======== Judith-1's SpaceBook ========= Hello world =================================== ======== Judith-1's SpaceBook ========= Hello world Today I worked 18 hours =================================== ======== Judith-1's SpaceBook ========= Hello world Today I worked 18 hours Tom poked you Tom : hugged you =================================== ======== Judith-1's SpaceBook ========= Hello world Today I worked 18 hours Tom poked you Tom : hugged you Tom : Poor you =================================== ======== Tom-2's SpaceBook ========= Hey, I'm on OpenBook - it's cool! =================================== */ Example 2-6. Bridge pattern example code—OpenBook (continued) 46 | Chapter 2: Structural Patterns: Decorator, Proxy, and Bridge Use Bridge is a very simple, but very powerful pattern. Given a single implementation, we can add a second one together with a Bridge and an Abstraction and achieve considerable generality over the original design. A well-quoted use of the Bridge pattern is in graphics, where different displays have different capabilities and drivers. These would be the implementations of the Bridge pattern, and the Bridge would be an interface of their essential capabilities. The Client calls the Abstraction to display something, and the Abstraction can examine the properties of the one or more Bridge instances (drivers) it is holding and select the most appropriate one for the task. Exercise 1. Although some limited operations can be added to OpenBook, the fact that some fundamental ones, such as the page header, are embedded in the private SpaceBook class makes real change difficult. Assuming that you can negotiate an upgrade with the developers of SpaceBook, make a proposal, including a UML diagram, for the long-term design of an extensible system. Implement it. Pattern Comparison The careful reader might have noticed that the three patterns described in this chap- ter seem to offer much the same service. At a high level, they are all helping to extend classes in novel ways, and they all provide alternatives to inheritance. A summary of when each pattern might be used was provided at the end of each section. Because this is also a programming book, we’ll now summarize the object mechanisms the patterns use (see Table 2-2) and draw conclusions about their comparative opera- tion. This summary is based on the UML diagrams and theory code for each of the patterns. In each pattern, we can identify four roles, which can fall under the headings origi- nal, new, interface, and client. The actual names given to these meta-roles are shown in the first three rows of the table. Note that the Bridge pattern can work in two ways. In our examples, we used the “Bridge-up” option. We assumed that we had Use the Bridge pattern when… You can: • Identify that there are operations that do not always need to be implemented in the same way. You want to: • Completely hide implementations from clients. • Avoid binding an implementation to an abstraction directly. • Change an implementation without even recompiling an abstraction. • Combine different parts of a system at runtime. Pattern Comparison | 47 one implementation already and wanted to share its interface with other implemen- tations yet to be built. To do so, we needed to create an Abstraction that was closely connected to the Bridge interface. An equally valid application for the Bridge pattern would be to have an original abstraction in mind and to build it hand-in-hand with the implementations (the “Bridge-down” approach). As you can see in the fourth row of the table, variations are found in the clients. For example, the Decorator pattern aggregates the interface so that it can share decora- tions; it provides the original as a construction parameter. The Bridge-up pattern does the same. To make this more concrete, here are the two statements from the clients: // Decorator Display(new DecoratorA(new IComponent( )); // Bridge-up Console.WriteLine(new Abstraction(new ImplementationA( )).Operation( )); The fifth row shows what objects are invoked in the pattern. The Decorator pattern can call the original components or the decorators, but the Bridge pattern variations only call one or the other. The Proxy pattern both aggregates and invokes only the new classes. Table 2-2. Comparison of Decorator, Proxy, and Bridge patterns Decorator Proxy Bridge-down Bridge-up Original Component Subject Abstraction Implementation Interface IComponent ISubject Bridge Bridge New Decorator Proxy Implementation Abstraction Client aggregates New with interface New Original with new New with original Client activates Original and new New Original New Original changed by Implementing the interface No change Aggregating the interface Implementing the interface New classes Aggregate the interface Implement the interface Aggregate the original Implement the interface Implement the interface Aggregate the interface Operation routed From new to original From new to original From original to new From new to original 48 | Chapter 2: Structural Patterns: Decorator, Proxy, and Bridge The next row itemizes how the original changes as a result of the pattern. Only the Proxy pattern enables the original to remain completely unchanged. The Decorator pattern relies on there being an interface that everyone implements, which might have to be added after the original classes are developed. The Bridge pattern is more closely coupled, and there is an understanding that the original must incorporate considerable references to the rest of the system. All the patterns rely on rerouting operations. The last row in the table shows that the rerouting is always done from the new code back to the original; in the case of the Bridge pattern, it just depends on which class is called new and which class is called the original. It is worth noting that in real-time applications, where reaction time is important, the overhead of the time for rerouting the operations might not be acceptable. 49 Chapter 3 CHAPTER 3 Structural Patterns: Composite and Flyweight 3 The Composite and Flyweight structural patterns apply to systems that have many data objects. The Composite pattern has wide applicability, and its composite lists can also make use of the Flyweight pattern. The Flyweight pattern shares identical objects behind the scenes to save space. In their implementations, these patterns make use of the following novel C# features: • Generics • Properties • Structs • Indexers • Implicit typing • Initializers • Anonymous types Composite Pattern Role The Composite pattern arranges structured hierarchies so that single components and groups of components can be treated in the same way. Typical operations on the components include add, remove, display, find, and group. Illustration Computer applications that specialize in grouping data abound these days. Consider a music playlist in iTunes or a digital photo album in Flickr or iPhoto (Figure 3-1). Items are put in a large list, which is then given structure separately. 50 | Chapter 3: Structural Patterns: Composite and Flyweight Looking at the screenshot from iPhoto, we can see that there are different ways of viewing the photos that have been imported: chronologically or by named event. A sin- gle photo can appear in many albums (“Last Roll,” “2007,” and “Switzerland,” for example). Creating an album forms a composite object but does not entail actually copying the photos. In this context, the important point about the Composite pattern is that the operations carried out on photos and albums of photos should have the same names and effects, even though the implementations are different. For example, the user should be able to display a photo or an album (that contains photos). Simi- larly, deletions of a single photo or an album should behave in the same way. Design The Composite pattern is one of the simplest there is, on the face of it. It has to deal with two types: Components and Composites of those components. Both types agree to conform to an interface of common operations. Composite objects consist of Components, and in most cases, operations on a Composite are implemented by calling the equivalent operations for its Component objects. See Figure 3-2 for the UML dia- gram for this pattern. The essential players in the Composite pattern UML diagram are: IComponent Defines the operations for objects in the composition, and any default behavior that will be applicable across objects of both types Figure 3-1. Composite pattern illustration—iPhoto Composite Pattern | 51 Operation Performs an operation on IComponent-conforming objects Component Implements the operations as applicable to single objects that cannot be further decomposed Composite Implements the operations as applicable to composite objects, using (or accessing) a list of components kept locally and probably using the equivalent Component operations The client deals only with the IComponent interface, which simplifies its task. Figure 3-2. Composite pattern UML diagram QUIZ Match the Composite Pattern Players with the iPhoto Illustration To test whether you understand the Composite pattern, cover the lefthand column of the table below and see if you can identify the players among the items from the illus- trative example (Figure 3-1), as shown in the righthand column. Then check your answers against the lefthand column. IComponent Component Composite Operation A visible entity in iPhoto A single photo An album of photos Open and view Client <<interface>> IComponent +Operation() Component +Operation() Composite –list : IComponent +Operation() For each component call its operation [...]... property cf C# Language Specification Version 3. 0, September 2007, Section 10.7 Now, let’s consider how the Component class would be implemented in accordance with the IComponent interface, as in the namespace shown in Example 3- 1 Composite Pattern | 53 Example 3- 1 Composite pattern—namespace code 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40... C# Language Specification Version 3. 0, September 2007, Section 7.5.10.4 C# 3. 0 Feature—Object and Collection Initializers Initializers specify values for fields or properties of objects or collections Examples of initializers include: Point p = new Point {X = 0, Y = 1}; List digits = new List { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; In C# 2.0, initialization syntax was valid only for arrays cf C#. .. list; { public T Name {get; set;} public Composite (T name) Name = name; { Chapter 3: Structural Patterns: Composite and Flyweight Example 3- 1 Composite pattern—namespace code (continued) 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 } 100 } list = new List ( ); } public void Add(IComponent... digits = new List { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; In C# 2.0, initialization syntax was valid only for arrays cf C# Language Specification Version 3. 0, September 2007, Section 7.5.10.1 -3 70 | Chapter 3: Structural Patterns: Composite and Flyweight C# 3. 0 Feature—Anonymous Types Anonymous types are created from object initializers An anonymous type is a nameless class type that inherits directly from... ease of image handling, we’ll use the Application.Run model from C# The Windows object calls LoadGroups explicitly, and DisplayGroups is invoked implicitly through the PaintEventHandler once the Form is drawn LoadGroups illustrates the use of some of the implicit typing and initializing features of C# 3. 0 (See the following sidebars.) C# 3. 0 Feature—Implicit Typing Variables can be declared as fields... Our implementation of the Flyweight pattern makes use of two interesting features from C# 1.0 and three from C# 3. 0 They are: • Structs • Indexers • Implicit typing for local variables and arrays • Object and collection initializers • Anonymous types Flyweight Pattern | 63 In addition, it uses generic collections from C# 2.0, as discussed in the section on the Composite pattern I will introduce these... were successful The last line of the interface is interesting because it introduces a new syntax for properties in classes Before C# 3. 0, this new syntax was only present in interfaces It has now been extended to classes as well and makes programs shorter and easily readable C# 3. 0 Feature—Properties and Accessors A property gives controlled access to the private local state of an object, either directly... application that produced the output in Figure 3- 4 Its code, preceded by the FlyweightPattern namespace developed above, is shown in Example 3- 4 Instructions for compiling a namespace separately from the program that uses it are included in the discussion of the Façade pattern in Chapter 4 66 | Chapter 3: Structural Patterns: Composite and Flyweight Example 3- 4 Flyweight pattern example—Photo Group including... Flyweights can receive and act on intrinsic state Flyweight Pattern | 61 Figure 3- 3 Flyweight pattern illustration—Photo Group Client –unSharedState FlyweightFactory –flyweights : Dictionary IFlyweight +Operation( ) Flyweight –intrinsicState +Operation( ) Figure 3- 4 Flyweight pattern UML diagram 62 | Chapter 3: Structural Patterns: Composite and Flyweight exintrinsicState computed at runtime FlyweightFactory... Summer.jpg Trees.jpg BetterFlowers.jpg Quit Will remain at Garden To end of Garden The Client class that implements these commands is shown in Example 3- 3 It is a simple command interpreter that makes good use of C# s switch on string feature Example 3- 3 Composite pattern example code—Photo Library using using using using System; System.Collections.Generic; System.IO; CompositePattern; // The Client . directly"); 30 return this; 31 } 32 33 public string Display(int depth) { 34 return new String('-', depth) + Name+" "; 35 } 36 37 public IComponent <T> Find (T s) { 38 if. call to set a property. cf. C# Language Specification Version 3. 0, September 200 7, Section 10. 7 54 | Chapter 3: Structural Patterns: Composite and Flyweight Example 3- 1. Composite pattern—namespace. the generic parameters, as in <string>. cf. C# Language Specification Version 3. 0, September 200 7, Section 10. 1 .3 Composite Pattern | 53 Because the types will be in a separate namespace,