Stephen Torri wrote:
I believe I have a good understanding of the first pass. Right now the
second pass that stumps me. So I have a few questions, which I will
research myself tomorrow, are:
1. Where is the function execute() called on my
FingeringInsertionCommand?
In MultiViewCommandHistory::addCommand()
2. How does the Event object passed to SegmentNotationHelper for
insertText, for example, get displayed?
Executing any command will trigger a refresh of the view.
The design behind this stems from two needs :
- an unlimited undo/redo stack
- abstracting the graphic representation of the data (notation, matrix,
event list...) from the data itself (events in segments), that is,
changes are performed on the data itself, and the view takes care of
updating itself.
(Note that this 2nd need isn't as well implemented as it should, in part
due to the QCanvas model, and in part due to my ignorance of the
Model/View/Controller (or at least Model/View) pattern when I first laid
this out back in 2000 - shame, which is one of the reaons behind my
infamous rewrite of the segment canvas).
Anyway, the 2nd need explains why your commands should alter the
Composition directly and ignore the existence of EditView. The 1st need
explains why you need to have commands in the first place. This is a
straightforward implementation of the Command design pattern :
http://www.dofactory.com/Patterns/PatternCommand.aspx#_self1
(btw, if you don't have it yet, "Design Patterns" by the "gang of four",
Gamma, Helm, Johnson & Vlissides is an absolute must-have)
This pattern is very simple : all user commands are represented by
instances of an abstract Command class, which has only two methods :
execute() and, obviously, unexecute(). The undo/redo stack is simply a
stack of these objects. When the user performs a new action, the
corresponding command is created, added on the stack, its execute()
method is called, and the stack pointer is moved to this command. When
the user calls 'undo', the 'unexecute()' method of the command pointed
by the stack pointer is called, and the stack pointer is moved back one
step.
--
Guillaume
http://telegraph-road.org
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