88 lines
4.2 KiB
TeX
88 lines
4.2 KiB
TeX
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\chapter{OGLEdit: a sample OGL application}\label{ogledit}%
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\setheader{{\it CHAPTER \thechapter}}{}{}{}{}{{\it CHAPTER \thechapter}}%
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\setfooter{\thepage}{}{}{}{}{\thepage}
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OGLEdit is a sample OGL application that allows the user to draw, edit,
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save and load a few shapes. It should clarify aspects of OGL usage, and
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can act as a template for similar applications. OGLEdit can be found in\rtfsp
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{\tt samples/ogledit} in the OGL distribution.
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$$\image{10cm;0cm}{ogledit.eps}$$\par
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The wxWindows document/view model has been used in OGL, to reduce the amount of
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housekeeping logic required to get it up and running. OGLEdit also provides
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a demonstration of the Undo/Redo capability supported by the document/view classes,
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and how a typical application might implement this feature.
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\section{OGLEdit files}
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OGLEdit comprises the following source files.
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\begin{itemize}\itemsep=0pt
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\item doc.h, doc.cpp: MyDiagram, DiagramDocument, DiagramCommand, MyEvtHandler
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classes related to diagram functionality and documents.
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\item view.h, view.cpp: MyCanvas, DiagramView classes related to visualisation of
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the diagram.
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\item ogledit.h, ogledit.cpp: MyFrame, MyApp classes related to the overall application.
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\item palette.h, palette.cpp: EditorToolPalette implementing the shape palette.
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\end{itemize}
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\section{How OGLEdit works}
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OGLEdit defines a DiagramDocument class, each of instance of which holds a MyDiagram
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member which itself contains the shapes.
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In order to implement specific mouse behaviour for shapes, a class MyEvtHandler is
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defined which is `plugged into' each shape when it is created, instead of overriding each shape class
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individually. This event handler class also holds a label string.
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The DiagramCommand class is the key to implementing Undo/Redo. Each instance of DiagramCommand
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stores enough information about an operation (create, delete, change colour etc.) to allow
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it to carry out (or undo) its command. In DiagramView::OnMenuCommand, when the user initiates the
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command, a new DiagramCommand instance is created which is then sent to the document's
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command processor (see wxWindows manual for more information about doc/view and command
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processing).
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Apart from menu commands, another way commands are initiated is by the user left-clicking on
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the canvas or right-dragging on a node. MyCanvas::OnLeftClick in view.cpp shows how
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the appropriate wxClassInfo is passed to a DiagramCommand, to allow DiagramCommand::Do
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to create a new shape given the wxClassInfo.
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The MyEvtHandler right-drag methods in doc.cpp implement drawing a line between
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two shapes, detecting where the right mouse button was released and looking for a second
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shape. Again, a new DiagramCommand instance is created and passed to the command
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processor to carry out the command.
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DiagramCommand::Do and DiagramCommand::Undo embody much of the
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interesting interaction with the OGL library. A complication of note
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when implementing undo is the problem of deleting a node shape which has
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one or more arcs attached to it. If you delete the node, the arc(s)
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should be deleted too. But multiple arc deletion represents more information
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that can be incorporated in the existing DiagramCommand scheme. OGLEdit
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copes with this by treating each arc deletion as a separate command, and
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sending Cut commands recursively, providing an undo path. Undoing such a
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Cut will only undo one command at a time - not a one to one
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correspondence with the original command - but it's a reasonable
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compromise and preserves Do/Undo whilst keeping our DiagramCommand class
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simple.
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\section{Possible enhancements}
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OGLEdit is very simplistic and does not employ the more advanced features
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of OGL, such as:
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\begin{itemize}\itemsep=0pt
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\item attachment points (arcs are drawn to particular points on a shape)
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\item metafile and bitmaps shapes
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\item divided rectangles
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\item composite shapes, and constraints
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\item creating labels in shape regions
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\item arc labels (OGL has support for three movable labels per arc)
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\item spline and multiple-segment line arcs
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\item adding annotations to node and arc shapes
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\item line-straightening (supported by OGL) and alignment (not supported directly by OGL)
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\end{itemize}
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These could be added to OGLEdit, at the risk of making it a less
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useful example for beginners.
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