contrib/docs/latex/ogl/sample.tex

   1 \chapter{OGLEdit: a sample OGL application}\label{ogledit}%
   2 \setheader{{\it CHAPTER \thechapter}}{}{}{}{}{{\it CHAPTER \thechapter}}%
   3 \setfooter{\thepage}{}{}{}{}{\thepage}
   4
   5 OGLEdit is a sample OGL application that allows the user to draw, edit,
   6 save and load a few shapes. It should clarify aspects of OGL usage, and
   7 can act as a template for similar applications. OGLEdit can be found in\rtfsp
   8 {\tt samples/ogledit} in the OGL distribution.
   9
  10 $$\image{10cm;0cm}{ogledit.eps}$$\par
  11
  12 The wxWindows document/view model has been used in OGL, to reduce the amount of
  13 housekeeping logic required to get it up and running. OGLEdit also provides
  14 a demonstration of the Undo/Redo capability supported by the document/view classes,
  15 and how a typical application might implement this feature.
  16
  17 \section{OGLEdit files}
  18
  19 OGLEdit comprises the following source files.
  20
  21 \begin{itemize}\itemsep=0pt
  22 \item doc.h, doc.cpp: MyDiagram, DiagramDocument, DiagramCommand, MyEvtHandler
  23 classes related to diagram functionality and documents.
  24 \item view.h, view.cpp: MyCanvas, DiagramView classes related to visualisation of
  25 the diagram.
  26 \item ogledit.h, ogledit.cpp: MyFrame, MyApp classes related to the overall application.
  27 \item palette.h, palette.cpp: EditorToolPalette implementing the shape palette.
  28 \end{itemize}
  29
  30 \section{How OGLEdit works}
  31
  32 OGLEdit defines a DiagramDocument class, each of instance of which holds a MyDiagram
  33 member which itself contains the shapes.
  34
  35 In order to implement specific mouse behaviour for shapes, a class MyEvtHandler is
  36 defined which is `plugged into' each shape when it is created, instead of overriding each shape class
  37 individually. This event handler class also holds a label string.
  38
  39 The DiagramCommand class is the key to implementing Undo/Redo. Each instance of DiagramCommand
  40 stores enough information about an operation (create, delete, change colour etc.) to allow
  41 it to carry out (or undo) its command. In DiagramView::OnMenuCommand, when the user initiates the
  42 command, a new DiagramCommand instance is created which is then sent to the document's
  43 command processor (see wxWindows manual for more information about doc/view and command
  44 processing).
  45
  46 Apart from menu commands, another way commands are initiated is by the user left-clicking on
  47 the canvas or right-dragging on a node. MyCanvas::OnLeftClick in view.cpp shows how
  48 the appropriate wxClassInfo is passed to a DiagramCommand, to allow DiagramCommand::Do
  49 to create a new shape given the wxClassInfo.
  50
  51 The MyEvtHandler right-drag methods in doc.cpp implement drawing a line between
  52 two shapes, detecting where the right mouse button was released and looking for a second
  53 shape. Again, a new DiagramCommand instance is created and passed to the command
  54 processor to carry out the command.
  55
  56 DiagramCommand::Do and DiagramCommand::Undo embody much of the
  57 interesting interaction with the OGL library. A complication of note
  58 when implementing undo is the problem of deleting a node shape which has
  59 one or more arcs attached to it. If you delete the node, the arc(s)
  60 should be deleted too. But multiple arc deletion represents more information
  61 that can be incorporated in the existing DiagramCommand scheme. OGLEdit
  62 copes with this by treating each arc deletion as a separate command, and
  63 sending Cut commands recursively, providing an undo path. Undoing such a
  64 Cut will only undo one command at a time - not a one to one
  65 correspondence with the original command - but it's a reasonable
  66 compromise and preserves Do/Undo whilst keeping our DiagramCommand class
  67 simple.
  68
  69 \section{Possible enhancements}
  70
  71 OGLEdit is very simplistic and does not employ the more advanced features
  72 of OGL, such as:
  73
  74 \begin{itemize}\itemsep=0pt
  75 \item attachment points (arcs are drawn to particular points on a shape)
  76 \item metafile and bitmaps shapes
  77 \item divided rectangles
  78 \item composite shapes, and constraints
  79 \item creating labels in shape regions
  80 \item arc labels (OGL has support for three movable labels per arc)
  81 \item spline and multiple-segment line arcs
  82 \item adding annotations to node and arc shapes
  83 \item line-straightening (supported by OGL) and alignment (not supported directly by OGL)
  84 \end{itemize}
  85
  86 These could be added to OGLEdit, at the risk of making it a less
  87 useful example for beginners.