## How OGLEdit works

OGLEdit defines a DiagramDocument class, each of instance of which holds a MyDiagram member which itself contains the shapes.

In order to implement specific mouse behaviour for shapes, a class MyEvtHandler is defined which is 'plugged into' each shape when it is created, instead of overriding each shape class individually. This event handler class also holds a label string.

The DiagramCommand class is the key to implementing Undo/Redo. Each instance of DiagramCommand stores enough information about an operation (create, delete, change colour etc.) to allow it to carry out (or undo) its command. In DiagramView::OnMenuCommand, when the user initiates the command, a new DiagramCommand instance is created which is then sent to the document's command processor (see wxWindows manual for more information about doc/view and command processing).

Apart from menu commands, another way commands are initiated is by the user left-clicking on the canvas or right-dragging on a node. MyCanvas::OnLeftClick in view.cpp shows how the appropriate wxClassInfo is passed to a DiagramCommand, to allow DiagramCommand::Do to create a new shape given the wxClassInfo.

The MyEvtHandler right-drag methods in doc.cpp implement drawing a line between two shapes, detecting where the right mouse button was released and looking for a second shape. Again, a new DiagramCommand instance is created and passed to the command processor to carry out the command.

DiagramCommand::Do and DiagramCommand::Undo embody much of the interesting interaction with the OGL library. A complication of note when implementing undo is the problem of deleting a node shape which has one or more arcs attached to it. If you delete the node, the arc(s) should be deleted too. But multiple arc deletion represents more information that can be incorporated in the existing DiagramCommand scheme. OGLEdit copes with this by treating each arc deletion as a separate command, and sending Cut commands recursively, providing an undo path. Undoing such a Cut will only undo one command at a time - not a one to one correspondence with the original command - but it's a reasonable compromise and preserves Do/Undo whilst keeping our DiagramCommand class simple.