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NAME
Command - base class for command objects
SYNOPSIS
#include <Unidraw/Commands/command.h>
DESCRIPTION
Command is an abstract base class for objects that define operations on components and
other objects. Commands are similar to messages in traditional object-oriented systems in
that components can receive and respond to them. Commands can also be executed in
isolation to perform arbitrary computation, and they can reverse the effects of such
execution to support undo.
Some commands (termed purely interpretive commands) are interpreted by components but
never executed in isolation, some are executed but never interpreted (non-interpretive
commands), and some are used in both ways. Some commands may be directly accessible to
the user through menus, while others are internal to the application. In general, an
undoable operation should be carried out by a command object.
PUBLIC OPERATIONS
virtual void Execute()
virtual void Unexecute()
virtual boolean Reversible()
Execute performs computation to carry out the command's semantics. Unexecute
performs computation to reverse the effects of a previous Execute, based on
whatever internal state the command maintains. A command is responsible for
maintaining enough state to reverse one Execute operation; repeated Unexecute
operations will not undo the effects of more than one Execute.
By default, Execute examines the current selection maintained in the command's
editor (see below). Execute will do nothing if the nothing is selected and the
command's clipboard (see below) is empty or non-existent. Otherwise, if something
is selected and the command's clipboard is non-existent, then Execute creates a
clipboard containing the selected component subjects. Finally, Execute will tell
each component on the command's clipboard to interpret this. Conversely, Unexecute
tells each component on the clipboard (if any) to Uninterpret this this. Thus,
commands are purely interpretive by default.
It may not be meaningful or appropriate, however, for some commands to reverse
their effect. For example, it is probably not feasible to undo a command that
generates an external representation. The Reversible operation returns whether or
not the command is unexecutable and uninterpretable. If the command is
irreversible, then it is ignored during the undo process. By default, Reversible
examines the command's clipboard; if the command doesn't have one, then the command
is reversible if the user has at least one component view selected. If the command
has a non-empty clipboard, then it is deemed reversible. Subclasses may redefine
Reversible to base the command's reversibility on their own criteria.
virtual void Store(Component*, Data* = nil)
virtual Data* Recall(Component*)
Since a command can affect more than one component, commands must let components
that interpret them store undo information somewhere. The commands must also let
components recall the information during uninterpretation. Store lets a component
store a Data object in the command itself as part of the component's interpretation
process. The component can later retrieve the data with the Recall operation.
Store and Recall require a Component argument to distinguish requests from
potentially many components.
virtual void Log()
A convenience function for calling the equivalent function on the global unidraw
object.
virtual void SetControlInfo(ControlInfo*)
virtual void SetEditor(Editor*)
virtual void SetClipboard(Clipboard*)
virtual ControlInfo* GetControlInfo()
virtual Editor* GetEditor()
virtual Clipboard* GetClipboard()
Operations for setting and getting constructor-specified state stored in the
command. Commands use a ControlInfo object to store information from which to
build a user interface for executing the command. Commands keep a pointer to the
Editor instance they potentially affect. Finally, a command uses a clipboard to
store the components it affects or affected. A clipboard containing the components
to be affected can be supplied in the constructor, but more often the command
itself creates and primes a clipboard based on the editor's selected components.
The Set... operations do not delete the object being replaced.
virtual void First(Iterator&)
virtual void Last(Iterator&)
virtual void Next(Iterator&)
virtual void Prev(Iterator&)
virtual boolean Done(Iterator)
Operations for iterating over the command's children, if any. These operations do
nothing by default. Commands that compose other commands should redefine these
operations to permit traversal of their children. First and Last initialize an
iterator to point to the beginning and end of the list of children, respectively.
Next increments the iterator to point to the following child, while Prev decrements
the iterator to point to the preceding child. Done returns whether or not the
iterator points beyond the first or last child in the list.
virtual Component* Copy()
Return a copy of the command. Subclasses should redefine this operation to return
an instance of their type.
virtual void Read(istream&)
virtual void Write(ostream&)
Read and write the command's contents to a stream to support catalog operations.
Read and write typically call first the corresponding operations defined by their
parent class, and then they read or write their class-specific state. Note that
you must ensure that the objects are read in the same order they are written.
virtual ClassId GetClassId()
virtual boolean IsA(ClassId)
GetClassId returns the unique class identifier for the Command subclass, while IsA
returns whether the instance is of a class or subclass corresponding to the given
identifier. IsA typically checks the given identifier against the instance's own
(as defined by its GetClassId operation) and, failing that, calls its parent
classes' IsA operation. All subclasses must redefine GetClassId and IsA to ensure
that their identifiers are unique and that instances are written and read properly.
virtual ClassId GetSubstId(const char*& delim)
A Command subclasses can redefine GetSubstId to specify a command that can
substitute for it. This lets applications that do not define a particular command
subclass still read in a substitute that is compatible with the subclass. The
substitute class should be one of the predefined commands in the Unidraw library.
This guarantees that all applications can instantiate the substitute.
GetSubstId returns the class identifier for the substitute. When an another
Unidraw application's catalog reads this object, it can create an instance of the
substitute (which it is guaranteed to define) instead of the original (which it may
not define).
The original should read and write a superset of the substitute's data. The
catalog will read to the end of the substitute's data and then advance the stream
to the point following the sequence of characters specified by delim, saving the
extra characters between the two points in the stream. When the substitute object
is saved subsequently, the original class identifier will be written along with the
substitute's data plus the extra characters saved previously. Thus information
needn't be lost as a result of being read and written by an unsophisticated
application.
PROTECTED OPERATIONS
Command(ControlInfo*, Clipboard* = nil)
Command(Editor* = nil, Clipboard* = nil)
The constructors are protected to prevent instatiation of the abstract base class.
You can supply a ControlInfo or Editor and an optional clipboard. Only commands
that will be accessible to the user need a ControlInfo object. All commands should
be given an editor object before they are used.
void InitCopy(Command*)
ControlInfo* CopyControlInfo()
Clipboard* CopyClipboard()
Clipboard* DeepCopyClipboard()
Convenience functions for use by subclasses in their Copy operation. InitCopy
gives this a copy of the given command's editor, clipboard, and interpretation data
stored by the components that interpreted it, if any. CopyControlInfo and
CopyClipboard return copies of the command's ControlInfo and Clipboard objects,
respectively, if they are non-nil. Both CopyControlInfo and CopyClipboard simply
check for a nil argument before calling the corresponding Copy operations on the
objects. DeepCopyClipboard checks for a nil clipboard before calling DeepCopy on
the command's clipboard and returning the result.
GraphicComp* GetGraphicComp()
A convenience function that returns the GraphicComp in the editor that the command
affects.
SEE ALSO
Catalog(3U), Clipboard(3U), ControlInfo(3U), Data(3U), Editor(3U), GraphicComp(3U),
Iterator(3U), Unidraw(3U), classes(3U), globals(3U), istream(3C++), ostream(3C++)