Smalltalk and Object Orientation: an Introduction - Free

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made”. (However implementation still tends to possess unexpected desi gn problems which must be solved. These decisions should be subject to, and determined by, the processes described above). Because of this, OMT places a limited amount of emphasis on the implementation phase, concentrating instead on stylistic points. You should treat the implementation of an object oriented system in just the same way as you would treat the implementation of any software system. This means that it should be subject to, and controlled by, the same processes as any other implementation. In a ddition it should be subjected to similar testing (as discussed earlier in this book). This is where the use cases may come back into use in helping identifying suitable test scenarios. 20.5 Summary As can be seen from this chapter, OMT concentrates the majorit y of its guidance on the analysis phase, the design and implementation phases are far less well supported. It is however one of the most widely used object oriented design methods and is likely to have a very large influence on any method developed by Booch, Rumbaugh and Jacobson to support the UML. 170
21. Frameworks and Patterns for Object Oriented Design 21.1 Introduction Designing complex software systems is hard. It is a great deal easier to reuse an existing software system, merely modifying it where necessary, than to build it from scratch. These two facts have led to a great deal of interest in what has become termed software frameworks in the object oriented community. A software framework is “the reusable design of a system or a part of a system expressed as a set of abstract classes [and concrete classes] and the way instances of (subclasses of) those classes collaborate” [Beck and Johnson 1994]. However such frameworks are notoriously difficult to document. For example the Model View Controller (MVC) framewo rk in Smalltalk is very powerful, but it has proved difficult to explain in a clear and simple manner how the MVC should be used [Krasner and Pope 1988]. In the object oriented community a number of researchers have explored the work of an architect who d esigned a language for encoding knowledge of the design and construction of buildings [Alexander et al 1977, Alexander 1979]. The knowledge is described in terms of patterns which capture both a recurring architectural arrangement and a rule for how and wh en to apply this knowledge. That is, they incorporate knowledge about the design as well as the basic design relations. The result is that there is now a growing community exploring how software frameworks can be documented using (software) design patterns (for example, [Johnson 1992] and [Birrer and Eggenschmiler 1993]). Johnson’s paper describes the form that these design patterns take and the problems encountered in applying them. The remainder of the chapter is structured in the following manner: Sectio n two considers what a framework is and the role of patterns in documenting frameworks. Section three introduces the HotDraw framework and Section four presents a very simple pattern documenting how you can use HotDraw to construct a simple editor. 21.2 Patterns and frameworks 21.2.1 What is a framework? A framework is a reusable design of a program or a part of a program expressed as a set of classes. That is, a framework is a set of prefabricated software building blocks that programmers can use, extend, or customize for specific computing solutions. With frameworks, software developers don’t have to start from scratch each time they write an application. Frameworks are built from a collection of objects, so both the design and code of a framework m ay be reused. However frameworks are not necessarily easy to design or implement. Questions such as “how much will it cost” to produce a framework and “how much will we benefit” from the network are difficult to answer [Moser and Nierstrasz 1996]. In addition reusing frameworks instead of libraries can cause subtle architectural changes in an application calling for innovative management [Sparks, Benner and Faris, 1996]. Frameworks can provide solutions to different types of problem domain. These include a pplication frameworks, domain frameworks and support frameworks: • Application frameworks encapsulate expertise applicable to a wide variety of programs. These frameworks encompass a horizontal slice of functionality that can be applied across client domains. An application framework might provide the basic facilities of a payroll system, or a 171
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Smalltalk and Object Orientation: A
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Chapter 7: Smalltalk Constructs Thi
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The user interface construction fac
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Preface 1. INTRODUCTION TO OBJECT O
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12.5 A CLASS SIDE METHOD...........
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27.3 VISUALWORKS WINDOW PAINTING TO
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Part One Introduction to Object Ori
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interfaces between different parts
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een to ease the transition from one
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encapsulation. Attempting to introd
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is a controversial subject which is
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will become second nature to many o
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or return something. It has since b
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defining the behavior of different
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In time most developers start to ac
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2.4.3.4 Smalltalk environments are
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3. Constructing an Object Oriented
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Wash wipe switch Wiper motor Relay
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Point 2 above is more complicated.
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services working? working? working?
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Part Two The Smalltalk Language 42
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other systems. As the browsers, ins
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VisualAge (a version of Smalltalk w
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4.5.2 The VisualWorks Launcher and
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Class categories. These are group s
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5. A Little Smalltalk 5.1 Introduct
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Notifiers can be displayed under a
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These concepts of messages, receive
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Select the “Debug” option. This
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6. Smalltalk Building Blocks 6.1 In
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6.3.1 Class definitions In Smalltal
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Object Class1 Class 2 method search
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“Now calculate y “ y := x * 23.
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7. Smalltalk Constructs 7.1 Introdu
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They are more efficient for storage
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7.5.3 true, false and nil variables
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anArray at: 1 max: 10. This will tr
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Figure 8.1: Defining a new class 8.
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this convention at the moment, howe
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9. Control and Iteration 9.1 Introd
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9.3.3 Block temporary variables Blo
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The brackets round the 1 to: 10 are
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OrderedCollection class (respective
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holding other objects. Typically, c
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| temp | temp := Set new. temp add:
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Next we will define a message proto
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11. Further Collection Classes 11.1
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for (i = 0; i
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x inspect. This has some similariti
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11.9.3 The reject: message The reje
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• asSortedCollection. This create
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Figure 12.2: The Organizer class de
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12.6 The private-updating protocol
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do: [:item | Transcript show: item
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13. Streams and Files 13.1 Introduc
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Notice that in this example we used
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to reconstruct the data into object
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1. The ability to save the current
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displayed. The second component add
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27. Graphical User Interface Constr
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27.3.1 Palette The Palette tool pro
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uild a running user interface from
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names) and the actions for buttons
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Edit button, then the user interfac
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27.11 Visual reuse The aim of visua
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Figure 27.17: Defining the parent c
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important, as these facilities hand
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28.3 The AddressBook class 28.3.1 T
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The aspect methods can then be simp
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the name is in the address book, it
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29.2 The custom view widget 29.2.1
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DrawingView model SmallDraw drawing
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The action of adding a new graphic
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The next method we shall consider i
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In the updating protocol we need to
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The displaying protocol contains th
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30. Memory Management and Garbage C
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30.3.3 PermSpace This is used to ho
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31. Concurrency in Smalltalk 31.1 I
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The Semaphore class provides facili
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Once the process wakes up it can de
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31.4.3 Acknowledgments Acknowledgme
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“There are two kinds of objects i
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Object Object class Collection Coll
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ehavior, but from the point of view
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32.5.5 The whole enchilada Behaviou
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33. The Future for Object Technolog
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solutions are needed which suit the
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scripting and query language to use
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34. Appendix: The Smalltalk Languag
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A capitalization convention is used
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• marries: is the message selecto
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Array OrderedCollection SortedColle
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[Coleman et al 1994] D. Coleman, P.
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OOPSLA/ECOOP’90, Joint Conference
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detect.............................
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True...............................
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