Component(3U) InterViews Reference Manual Component(3U)
Component - base class for objects that model domain-specific elements
Component is an abstract base class for objects that are created, edited, and composed to form domain-specific drawings. Components repre-
sent the objects of interest in an editing domain, encapsulating their appearance and semantics. Components are partitioned into a subject
and zero or more views: the subject encapsulates the context-independent state and operations of a component, and each view supports a con-
text-dependent presentation of the subject. A subject notifies its views whenever its state is modified to allow them to change their
state or appearance to reflect the modification. The Component class defines the protocol for component subjects, while ComponentView is
the abstract base class defining the protocol for component views. See ComponentView(3U) for more information on the component view class.
virtual void Interpret(Command*)
virtual void Uninterpret(Command*)
Interpret performs a component-specific operation based on the type of command passed. Uninterpret reverses the effects of a pre-
ceding Interpret. The component is responsible for storing enough state (either in itself or in the command) to carry out the Unin-
virtual void Attach(ComponentView*)
virtual void Detach(ComponentView*)
Attach adds a component view to the component's list of views. Note that by default these operations do not check to ensure that
the view is compatible with the subject. Detach removes the argument from the component's list of views.
virtual void Notify()
Notify the component's views that its state has changed. This operation calls Update on each view in the component's list of
virtual void Update()
Other classes may use this operation to notify the component subject of a possible change in state that the component depends upon.
This operation does nothing by default.
virtual Component* GetParent()
virtual TransferFunct* GetTransferFunct()
Return the component's parent and transfer function, if any. These operations return nil by default.
virtual Component* GetRoot()
Return the root component in this hierarchy. GetRoot calls GetParent recursively and returns the last non-nil parent, if any.
virtual StateVar* GetState(const char*)
The component may maintain a string-to-state variable mapping to provide external access to any state variables it defines. The
GetState operation returns a state variable given an identifying string. Defining such a mapping lets other objects (typically com-
mands) query the component for state variables that it may define without extending the component protocol, potentially allowing
interchange of components from different applications. This operation returns nil by default.
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 component subject's children, if any. 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 component. Subclasses should redefine this operation to return an instance of their type.
virtual void Read(istream&)
virtual void Write(ostream&)
Read and write the component's contents to a stream to support catalog operations. Read and write typically call first the corre-
sponding 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 Component 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 Component subclasses can redefine GetSubstId to specify a component that can substitute for it. This lets applications that do
not define a particular component subclass still read in a substitute that is compatible with the subclass. The substitute class
should be one of the predefined components in the Unidraw library. This guarantees that all applications can instantiate the sub-
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 writ-
ten 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.
Create a view instance that is appropriate for the component subject given a view category. The relationship between subjects,
views, and view categories is encoded in their class identifiers. See classes(3U) for more information on class identifiers and
The Component class is abstract; therefore the constructor is protected.
Each component maintains a list of its views, implemented with a UList. This operation returns the component view associated with a
given entry in the UList.
virtual void SetParent(Component* child, Component* parent)
Notify a child component that it has a new or different parent. This operation does nothing by default. Composite components
should call this function in their structure-modifying operations, and components that keep information about their parents should
redefine it to update this information.
Catalog(3U), Command(3U), ComponentView(3U), Creator(3U), Iterator(3U), StatVar(3U), TransferFunct(3U), UList(3U), classes(3U),
Unidraw 20 August 1990 Component(3U)