Smalltalk and Object Orientation: an Introduction - Free

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CheckButtonView class and its subclasses. This class has five subclasses including MacCheckButtonView, Win3CheckButtonView and MotifCheckButtonView. In this case when a user requests that a CheckButtonView is created, one of its subclasses will be used depending on the user interface look requested by the user. For example, either an Apple Macintosh, Windows 3 or X/Motif style button will be displayed. Here different views can be used, without any modification of the model or the associated controller. Figure 26.4: Part of the View class hierarchy 26.2.2.3 Controllers Controllers handle the user interaction with the application. They work in tandem with the view so that when a user clicks on a button (displayed by a view) the controller can decide what to do. For example, the controller can ask the view to confirm that the cursor was within the active area of the button and then call the method associated with that button. That is, when the button is “pressed”, the controller will ensure that the appropriate method is evaluated. As was said e arlier, all views have an associated controller (even if it is an instance of the NoController class). Therefore, in many situations there are controller classes available which mirror the view classes. For example, there is a MenuButtonController (to go w ith the MenuButtonView), there is a ScrollBarController (to go with the ScrollBar view) and there are various ButtonControllers. The StandardSystemController and the ApplicationStandardSystemController are used with the top most application view while many of the other controllers are used to handle input and output for small subviews (such as buttons). 216
Figure 26.5: Part of the Controller class hierarchy It might seem understandable that the m odel should be separated out from the user interface, but why is the input separated from the output? This is for two reasons: • Firstly, it allows the controllers and the views to inherit from different class hierarchies. This allows the developer to overc ome the limitation of single inheritance enforced by Smalltalk (but not all object oriented languages). • Secondly, it allows different combinations of views and controllers. This may give different looks (views) or feels (controllers). 26.2.3 How the MVC works You may notice that in Figure 26.6 the arrows only point from the view and controller to the model. There are no arrows from the model back to the view or controller. This is because the model does not directly know anythi ng about its view or controller. This is partly because it does not need to know anything about them and also because it enforces this separation. A common mistake for those new to the MVC is to find that they have created instance variables in their model object which allow them to link the model directly to the view and/or controller. This is extremely poor Smalltalk style! So how do the view and the controller find out about changes in the model. Remember, the view bases what it displays on the data held within the information model. In terms of Smalltalk relationships this means that the view is a dependent of the model (see last chapter). The view, must therefore, record itself as a dependent of the model ( Figure 26.6 illustrates how interaction occurs between the model, view and controller). This means that it is the responsibility of the model to inform its dependents of any changes to its contents (i.e. its view). This is done by the model sendin g a message to itself stating that its contents has changed in some way. The changed method is actually defined in class Object and is thus inherited by all classes in the system. This means that any object in the system can act as the model in an MVC tria d 11 . This method causes an update: message to be sent to each of the views referenced in the dependents collection. 11 This actually means that any object, irrespective of whether it is used in the MVC, can have dependents , and that those dependents can be informed of changes to t he object is the same way that views are informed of changes in their associated information model. 217
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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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14.3 The class Date Class Date, a s
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aCharacter isAlphabetic. aCharacter
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Breakpoints may be placed in a me t
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If the file does not exist you will
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Figure 15.4: Creating a project To
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Part Three Object Oriented Design 1
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16. Object Oriented Analysis and De
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devolved amongst the objects). This
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driving force behind the whole of t
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17. The Unified Modeling Language 1
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Deployment diagrams . Deployment di
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Window abstract +size : Area = (10,
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Micro Computer 1.. * Monitor System
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connectedTo * Lamp * Weight Sensor
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18. UML: Dynamic Modeling And Deplo
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edisplay() Controller window A Wind
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18.2.3.1 A start point A start poin
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etween nodes and their stereotype.
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OMT is a prescriptive method, it pr
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confirming the users’ needs, thei
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19.3.2.2 Prepare a data dictionary
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car, truck and bus under a supercla
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20.2.1.3 Prepare sequence and colla
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Page 183 and 184: made worse, because the symbol held
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Page 221 and 222: displayed. The second component add
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Page 247 and 248: DrawingView model SmallDraw drawing
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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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