Smalltalk and Object Orientation: an Introduction - Free

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26.4 Views and subviews In the example in the previous section, it is unlikely that the button would actually have been handled by the window controller or that the window view would have determined how it was displayed. Rather the button would have had its own ButtonView and associated ButtonController (indeed Figure 26.4 and Figure 26.5 suggest this). The ButtonView would have been a subview of the main window and would have been responsible for displaying the button and checking whether the mouse was over the button. In fact views are designed to be nested. Most windows actually involve at least two views, one nested inside the other. The outermost view, known as the top component, manages the familiar window features (for example, the window menu bar etc.). It has an associated controller which manages the familiar moving, framing, collapsing and closing operations available from windowing environments. Inside the top component are one or more subviews, known as components, and their controllers (remember almost all views have an associated controller) which manage specific elements of the view (e.g. buttons, scroll bars, selection boxes etc.). The subview (component) refers to the view it is an element of, as its container. Thus a subview is held within a c ontainer and a container possesses components (which are its subviews). A component may, in turn, have additional components (although this is often not required). The container/component relationships are recorded in instance variables within the views. Each component has a link to its container and a container possesses an ordered collection of components. Thus each window’s top component is the top of a hierarchy of components. Typically, this means that when creating a window you first instantiate the top most view and then create and place subviews within the top level view. Figure 26.9 illustrates exactly this for a very simple window. This is a working example which you can type into a Workspace and evaluate. The actual source code can be broken down into three sections. The first section creates a new ScheduledWindow. Scheduled windows represent the top connection to the host window system 13 . They are always the top of the view hierarchy (that is they are always t he top component). Scheduled windows can have a label, a minimum and maximum size, can possess a single component (which can be made up of multiple views), possess a StandardSystemController (a controller specifically designed to work with scheduled window s) and of course a model. Having created the scheduled window, the code sets its controller, gives the window a label and specifies its minimum size. Figure 26.9: Creating a simple window The second part of the code creates a composite part. A composite part is essentially a view which is made up of one or more subviews. That is, it is a collection of subviews and appropriate behaviour for handling these subviews 14 . In the example in Figure 26.9, two components are added to the compositePart. One is a very simple view which is a composed text. Composed texts allow strings to be displayed as a view. Note that it is the compositePart which determines where the ‘Hello’ will be 13 “Scheduled” really relates to the fact that all ScheduledWindows are held by a control manager which determines (schedules) which of them is active at any one moment in time. Note only one window can be active for user input at any particular moment. 14 There is of course more to it than that, involving Wrappers and VisualComponents, however as these are essentially modifications of the basic MVC we shall leave an investigation of this issue to the reader. 220
displayed. The second component added to the compositePart is another composed text (displaying the string Goodbye). However this time, the string is being bordered. That is, a box is being drawn around that string as a border. This border is actually handled by a Wrapper (Wrappers provide needed bookkeeping information such as translation, clipping, borders, etc.). Note that the border is specified relative to the size of the encapsulating window (this is why the rectangle specifying the border is specified as 0.2 by 0.3 to 0.5 by 0.6. This means that the top right hand corner of the border should start at a position which is 20% of the width of the window by 30% of its depth and should extend to a position which is 50% of its width and 60% of its depth (note windows have their zero, zero position in the top left hand corner of the window). The third part of the source code in Figure 26.9 adds the compositePart (containing the two strings) to the scheduled window and opens t he scheduled window. The result of opening the window is illustrated in Figure 26.10. Figure 26.10: A simple window An important point to note is that although a view can co ntain multiple subviews which it coordinates, only the views are connected; the associated controllers are not. This is illustrated in Figure 26.11. Finally, views can only have one controller and one model. Controllers can also only have one view and one model. However, models may have any number of views associated with them. 26.5 The controller hierarchy To determine which controller should respond to the user input, there is a controller hierarchy which mirrors the view hierarchy. At the root of this tree is the controller associated with the top component. This controller tries to find a sub controller which will accept control. If none will accept control then it must process the input. Controllers wil l only accept control if the cursor is within their view. Upon accepting control the first thing a controller will do is see if a subcontroller will accept control. Once a controller has control, it will retain control until the cursor leaves its view. Any input from the user will then be handled by the appropriate controller. view1 controller1 view3 view2 controller3 controller2 Figure 26.11: The view-subview / controller relationships The actual mechanism through which this is performed is slightly complicated, however, basically, whenever the cursor enters a window a control manager (see the class ControlManager for further details) passes control to that window. This is done by asking each window controller whether it wants to take control or not (via a message isControlWanted). The first controller that requests control is given control. This is done via the message startUp. It is then up to the windows controller to decide how it will handle this control. Typically it determines whic h of its subcomponents wants to gain control via the message subViewWantingControl. The window (or top component) then asks its components whether they want control (using the objectWantingControl message). The components (or views) then ask their controllers whether they want control. If the controller identifies that it should have control, then it takes over the primary control operation. 221
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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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20.2.2.5 Specify optimization crite
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are therefore concurrent and can be
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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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