ISO/IEC 14478-2:1998

Information technology — Computer graphics and image processing — Presentation Environment for Multimedia Objects (PREMO) — Part 2: Foundation Component

Technologies de l'information — Infographie et traitement d'images — Environnement de présentation d'objets multimédia (PREMO) — Partie 2: Composant fondamental

General Information

• Status: Published
• Publication Date: 19-Dec-1998
• ICS: 35.140 - Computer graphics
• Technical Committee: ISO/IEC JTC 1/SC 24 - Computer graphics, image processing and environmental data representation
• Drafting Committee: ISO/IEC JTC 1/SC 24/WG 6 - Computer Graphics and Virtual Reality
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 08-Dec-2021

Relations

• Consolidated By: ISO 13385-1:2011 - Geometrical product specifications (GPS) — Dimensional measuring equipment — Part 1: Callipers; Design and metrological characteristics
• Effective Date: 06-Jun-2022

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 14478-2
First edition
1998-12-15
Corrected and reprinted
2000-09-15
Information technology — Computer
graphics and image processing —
Presentation Environment for Multimedia
Objects (PREMO) —
Part 2:
Foundation Component
Technologies de l'information — Infographie et traitement d'images —
Environnement de présentation d'objects multimédia (PREMO) —
Partie 2: Composant fondamental
Reference number
ISO/IEC 14478-2:1998(E)
©
ISO/IEC 1998

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ISO/IEC 14478-2:1998(E)
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© ISO/IEC ISO/IEC 14478-2:1998(E)
Contents
Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
1 Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Normative references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3 Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3.1 PREMO Part 1 definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3.2 Additional definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
4 Symbols and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5 Conformance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
6 Foundation non-object types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
7 Foundation object types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.2 PREMO objects and fundamental object behaviour . . . . . . . . . . . . . . . . . 5
7.2.1 Creation and destruction of objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.2.2 Inquiries on types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.3 Simple PREMO objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.3.1 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.4 Callback objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.5 Enhanced PREMO Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.5.1 Object properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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7.6 Controller objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.7 Event handler objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.7.1 Basic Event Handler objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.7.2 Synchronization Points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.8 Time objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.8.1 Clock object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.8.2 System clock object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.8.3 Timer object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.9 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.9.1 Event Synchronizable objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.9.2 Time synchronizable objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.9.3 Time slave objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.9.4 Time line objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8 Enhanced property management and factories. . . . . . . . . . . . . . . . . . . . . . . 19
8.1 Enhanced Property management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1.1 Motivation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1.2 Capabilities and native property values: the PropertyInquiry type . . . . . . . . 20
8.1.3 Property constraint and selection: the PropertyConstraint type. . . . . . . . . . . 21
8.2 Creating PREMO objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.2.1 Generic Factory objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9 Functional specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.2 Common non–object data types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.3 Exceptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.4 PREMOObject and fundamental object behaviour. . . . . . . . . . . . . . . . . 30
9.5 Simple PREMO object and structures. . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9.5.1 SimplePREMOObject. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9.5.2 Event structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9.5.3 Constraint structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9.5.4 Action Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.5.5 Synchronization Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.6 Callback objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.7 Enhanced PREMO object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.8 Controller object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.9 EventHandler objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.9.1 Basic event handler objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.9.2 SynchronizationPoint object. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
9.9.3 ANDSynchronizationPoint object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
9.10 Timing objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.10.1 Clock object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.10.2 SysClock object. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.10.3 Timer object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
9.11 Synchronization objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
9.11.1 Synchronizable object. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
9.11.2 TimeSynchronizable object. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
9.11.3 TimeLine object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
9.11.4 TimeSlave object. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
9.12 Enhanced Property management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
9.12.1 PropertyInquiry object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
9.12.2 PropertyConstraint object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
9.13 Creating PREMO objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
9.13.1 GenericFactory object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
9.13.2 FactoryFinder object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
10 Component specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
A Overview of PREMO Foundation Object Types. . . . . . . . . . . . . . . . . . . . . . 66
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© ISO/IEC ISO/IEC 14478-2:1998(E)
B Extensibility for PREMO objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70
C An example for event–based synchronization. . . . . . . . . . . . . . . . . . . . . . . . .71
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ISO/IEC 14478-2:1998(E) © ISO/IEC
Foreword
ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide standardi-
zation. National bodies that are members of ISO or IEC participate in the development of
International Standards through technical committees established by the respective
organization to deal with particular fields of technical activity. ISO and IEC technical
committees collaborate in fields of mutual interest. Other international organizations,
governmental and non-governmental, in liaison with ISO and IEC, also take part in the
work.
International Standards are drafted in accordance with the rules given in the ISO/IEC
Directives, Part 3.
In the field of information technology, ISO and IEC have established a joint technical
committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical
committee are circulated to national bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this part of
ISO/IEC 14478 may be the subject of patent rights. ISO and IEC shall not be held
responsible for identifying any or all such patent rights.
International Standard ISO/IEC 14478-2 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 24, Computer graphics and
image processing.
ISO/IEC 14478 consists of the following parts, under the general title Information
technology — Computer graphics and image processing — Presentation Environment
for Multimedia Objects (PREMO):
� Part 1: Fundamentals of PREMO
� Part 2: Foundation Component
� Part 3: Multimedia Systems Services
� Part 4: Modelling, rendering and interaction component
Annexes A and B form a normative part of this part of ISO/IEC 14478. Annex C is for
information only.
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© ISO/IEC ISO/IEC 14478-2:1998(E)
Introduction
This part of ISO/IEC 14478 defines those object types and non–object types which be-
long to the Foundation Component. Any conforming PREMO implementation shall
support these object types. The description of object types categories are given first
and then the foundation object types in each category are described.
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INTERNATIONAL STANDARD © ISO/IEC ISO/IEC 14478-2:1998(E)
Information technology — Computer graphics and image
processing — Presentation Environment for Multimedia Objects
(PREMO) —
Part 2: Foundation Component
1 Scope
This part of ISO/IEC 14478 lists an initial set of object types and non–object types useful for the construction of, presentation of,
and interaction with multimedia information. This part is dependent on the PREMO object model defined in clause 8 of ISO/IEC
14478-1. The foundation component does not depend on any other components.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this part of ISO/IEC
14478. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agree-
ments based on this part of ISO/IEC 14478 are encouraged to investigate the possibility of applying the most recent editions of
the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
ISO/IEC 14478-1:1998, Information technology — Computer graphics and image processing — Presentation Environment for
Multimedia Objects (PREMO) — Part 1: Fundamentals of PREMO.
ISO/IEC 11172 (all parts), Information technology — Coding of moving pictures and associated audio for digital storage
media at up to about 1,5 Mbit/s.
3 Definitions
3.1 PREMO Part 1 definitions
This part of ISO/IEC 14478 makes use of all terms defined in ISO/IEC 14478-1 (Fundamentals of PREMO).
3.2 Additional definitions
For the purposes of this part of ISO/IEC 14478, the following definitions apply.
3.2.1 basic data type: Non-object data type which cannot be expressed via other data types. Examples are integers, float-
ing point numbers.
3.2.2 constructed data type: As opposed to basic data type; non-object data type which is constructed with the help of
permitted type constructors using basic data types.
3.2.3 time: A non–object data type which is appropriate for the representation of real time in the execution environment.
It is typically realized through either float numbers or large (64 bit) integers.
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3.2.4 extended coordinates: An extension of real, integer, or time coordinates with the symbols –¥ and ,¥ and the nat-
ural comparison operators. It gives a succinct way of describing unlimited intervals on these coordinate systems.
3.2.5 key–value pair: A constructed data type, consisting of a key (described as a string) and a corresponding value.
3.2.6 foundation object type: Object types defined in the foundation component of PREMO.
3.2.7 structure: A category of object types in PREMO; these objects are characterized through attributes only.
3.2.7.1 structure tag: A synonym for an attribute for a structure.
3.2.8 property: Key with associated value or sequence of values, which can be attached to any PREMO object, and which
can be inquired, possibly created and deleted through operations defined on the object.
3.2.8.1 read–only property: A property whose value or values cannot be set by operations of the object.
3.2.9 fundamental object behaviour: Operations defined on the PREMOObject type; this type is the supertype of all
PREMO object types.
3.2.10 finite state machine: Implementation of an abstract finite state automaton.
3.2.11 constraint: A constructed data type, consisting of a key-value pair and an associated constraint operation; this latter
is used to compare the values, in case the keys are identical.
3.2.12 event: A constructed data type, serving as a basic building block for the PREMO Event Model.
3.2.12.1 event source: Object (instance) which creates events. This is a structure tag of an event.
3.2.12.2 event client: Object (instance) which consumes events.
3.2.12.3 event name: A means to denote and/or to refer to a specific event. This name is also referred to as event type.
This is a structure tag of an event.
3.2.12.4 event data: List of non-object types in the form of key–value pairs attached to an event. This is a structure tag
of an event.
3.2.13 event handler: An object which provides event processing services to other objects.
3.2.14 era: The base date for all PREMO systems to measure the amount of elapsed time. This value is set to 00:00am, 1st
January 1970, UTC.
3.2.15 reference point: A point in the internal coordinate system of a synchronizable objects, to which a synchronization
element is attached.
3.2.16 synchronization element: Synchronization information for a synchronizable object; it contains information on an-
other object and its operation which shall be invoked if synchronization is set up.
3.3.17 capability: Description of the property values an object type can take for a specific key.
3.3.18 native property value: Description of the property value an object instance can take for a specific key.
3.3.19 private properties: Properties of the object which are not defined as part of the functional specification of the object.
The following alphabetical list gives the sub-clause of each definition.
basic data type 3.2.1
capability 3.3.17
constraint 3.2.11
constructed data type 3.2.2
event 3.2.12
event client 3.2.12.2
event data 3.2.12.4
event handler 3.2.13
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© ISO/IEC ISO/IEC 14478-2:1998(E)
event name 3.2.12.3
event source 3.2.12.1
event type 3.2.12.3
era 3.2.14
extended coordinates 3.2.4
finite state machine 3.2.10
foundation object type 3.2.6
fundamental object behaviour 3.2.9
key–value pair 3.2.5
native property value 3.3.18
private properties 3.3.19
property 3.2.8
read–only property 3.2.8.1
reference point 3.2.15
structure 3.2.7
structure tag 3.2.7.1
synchronization element 3.2.16
time 3.2.3
4 Symbols and abbreviations
AIFF: Audio Interchange File Format.
FSM: Finite State Machine.
IEC: International Electrotechnical Commission.
IS: International Standard.
ISO: International Organization for Standardization.
MPEG: Moving Picture Experts Group.
PREMO: Presentation Environments for Multimedia Objects.
2D: Two-dimensional.
3D: Three-dimensional.
5 Conformance
A conforming implementation of the PREMO Foundation Component shall comply with the general conformance rules defined
in clause 5 of ISO/IEC 14478-1 and the component specification in clause 10.
6 Foundation non-object types
The foundation non–object types in PREMO are defined in two categories: basic data types, and data types directly defined from
these basic data types in terms of the notations described in clause A.2 of ISO/IEC 14478-1.
The basic data types are (with their type names):
a) N: non-negative integer.
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ISO/IEC 14478-2:1998(E) © ISO/IEC
b) Z: integer.
c) R: real number.
d) ObjectType: a data type uniquely identifying an object type.
e) EventId: a data type uniquely identifying an event registration for a PREMO event handler.
f) Time: a data type to measure progression of real world time. This type is either a real number or a (possibly large) inte-
ger. The choice among these is implementation dependent.
g) As described in 8.5 of ISO/IEC 14478-1, for each object of type T an object reference type, which is a non–object type,
referring to object instances of type T, automatically exists in PREMO. As a notational convention, RefT denotes the non–
object type of object reference referring to object instances of type T.
The environment shall provide comparison facilities for each basic data type which unambiguously decide whether two data val-
ues are identical or not. In the case of object references the environment shall also include a facility to test whether two references
refer to the same object instance or not, or whether the value of the object reference is NULLObject. How these facilities are re-
alized depends on the programming language and the execution environment in which PREMO is implemented.
Coordinate spaces can be “extended” to include positive and negative “infinity”. Although the underlying implementation may
not have a direct representation of these types, the obvious extension of the notion of “greater than”, “smaller than”, etc., on these
types allows the behaviour of objects to be defined more succinctly. The following extended coordinate space definitions are
used:
h) Extended real numbers:
R == R È{–¥,}¥
¥
i) Extended integers:
Z == Z È{–¥,}¥
¥
j) Extended time:
Time == Time È{–¥,}¥
¥
The foundation object types, described in this part, make also use of a number of (constructed) non–object types, defined formally
in 9.2 (page 27). Some of these non–object types play a key role in the behavioural description of several object types; they are
therefore also listed here, to make the semantic description in clause 7 easier to follow.
— Boolean:
Boolean ::= TRUE ½ FALSE
— Character String:
String ::= seq Char
— Constraint specification for key–value pairs (used, for example, by property management, event handlers, and aggregate
object types):
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© ISO/IEC ISO/IEC 14478-2:1998(E)
ConstraintOp ::= Equal ½ NotEqual
½GreaterThan½ GreaterThanOrEqual½½LessThan LessThanOrEqual
½Prefix½ Suffix½½NotPrefix NotSuffix
½Includes ½ Excludes
Values in an operation request are constrained to values which satisfy these type constraints and the constructions defined in
clause A.2 of ISO/IEC 14478-1 (see also 8.6 of ISO/IEC 14478-1). No particular representation for these values is mandated by
the PREMO functional specification, although bindings of PREMO to programming languages or to distributed programming
paradigms may specify such formats.
7 Foundation object types
7.1 Introduction
Foundation objects types are those which support a fundamental set of services suitable for use by a wide variety of higher level
components. PREMO conformance rules require that, whenever a PREMO implementation includes these objects, they be in-
cluded in the manner specified in this clause. This is the basis for interoperability. The following criteria are used to identify foun-
dation objects:
a) they are used by a majority of higher level components;
b) together they provide an adequate minimal functional set;
c) they are needed to support output on widely available presentation resources;
d) algorithms exist for decomposing more complex functionality into the foundation object types.
In this clause, foundation object types are identified. By means of subtyping the application developer or component supplier
may create objects and object types for their own specific needs. Clause 9 of this part gives the detailed definitions of each of
these object types; clause A gives an pictorial overview of all object types defined in this clause.
7.2 PREMO objects and fundamental object behaviour
All PREMO objects are assumed to be subtyped from a type called PREMOObject. PREMOObject is an abstract type, i.e., it is
not instantiable.
Operations on PREMOObject type fall into two categories described below.
7.2.1 Creation and destruction of objects
These operations are used by the object and object reference life cycle facilities when object instances are created and destroyed
(see 8.11 of ISO/IEC 14478-1 for a detailed description of these facilities). The initialize, initializeOnCopy, and destruct opera-
tions are defined to be protected, i.e., no other PREMO object can re–initialize a PREMO object or directly call the destruct op-
eration, only through the facilities provided by the environment.
7.2.2 Inquiries on types
These operations return information on the object type, the sequence of supertypes, or the complete type graph of the object. Us-
ing the information returned by these operations, complex negotiations are possible to optimize the behaviour of various other
PREMO objects.
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7.3 Simple PREMO objects
SimplePREMOObject is an abstract subtype of PREMOObject. SimplePREMOObject does not extend the behaviour of PRE-
MOObject, but serves as a common supertype for a family of PREMO objects, called structures. Using such a supertype allows
operation specifications to impose type constraints on their arguments.
7.3.1 Structures
The term “structure” does not denote a specific object type in PREMO but, instead, a category of types. These object types are
characterized by:
a) they are the subtypes of SimplePREMOObject but are not subtypes of EnhancedPREMOObject;
b) they are not abstract types, although they may be generic types;
c) their behaviour in PREMO is expressed in terms of attributes rather than explicit operations (apart from the operations
inherited from the supertype PREMOObject).
The attributes of a structure are also referred to as “structure tags”.
NOTE — Implementations, or further components, may define subtypes of structures by adding operations to the type specification. Item c
above does not preclude this. However, as a use of terminology, such types are not labelled as “structures” any more.
Structures can be used as tools to encapsulate various non–object data into the object hierarchy. As an example, the following
object type is used to describe constraints on key–value pairs:
Constraint
SimplePREMOObject
constraintOp: ConstraintOp
kv: Key ´ Value
Constraint
(This structure, formally defined in 9.5, plays an important role in the behavioural description of various objects in PREMO.)
NOTE — To increase the efficiency of the implementations, some programming languages may choose to implement structures as special
data types and not as objects.
One of the most important structures used in PREMO is the event structure. Events structures consist of the following structure
tags (see 9.5 for the precise specifications): an event name that provides a means to denote or refer to the event, also referred to
as event type, an event data which is a sequence of key–value pairs, and the event source, which is the reference to the object
instance which has created this event.
7.4 Callback objects
Very often object instances have to be notified by other objects on some status change, event occurrences, etc. This is done by
‘registering interest’ in some events. PREMO defines an abstract type, called Callback, to facilitate such mechanisms.
The Callback object type defines one single asynchronous operation, called callback. The signature of this operation consists of
one input argument, which is a reference to an Event structure (see 9.5.2 for a detailed specification of this structure). Various
PREMO objects, which may have to be notified under various circumstances, are defined to be subtypes of Callback, defining a
type–specific behaviour to the callback operation.
NOTE — A typical example for the usage of the callback mechanism is the PREMO Event Model, described in detail in 7.7.1.
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Whereas, in simple cases, the semantics of the callback operation may be defined to affect the state of the object directly, it is
very often the case that this operation acts only as an entry point to call other operations on the object. To facilitate this second
case, PREMO also defines a subtype of Callback, called CallbackByName. The (inherited) asynchronous callback operation of
CallbackByName has the following behaviour: the eventName structure tag of the Event structure (appearing as the input argu-
ment of callback) is interpreted to be the name of a local operation which is then internally invoked by the callback operation.
By default, all other structure tags of the Event structure are disregarded by the callback operation; subtypes of CallbackByName
may add an additional behaviour to the operation which also takes these tags into consideration.
7.5 Enhanced PREMO Objects
EnhancedPREMOObject is an abstract type, i.e., is not instantiable. This type describes a set of behaviour, referred to as the en-
hanced object behaviour. The operations on EnhancedPREMOObject are related to object properties.
Enhanced PREMOObject represents the common, abstract supertype for PREMO objects with a more complex behaviour than,
for example, structures. An important restriction in PREMO, which also reflects this characterization, is that only subtypes of
EnhancedPREMOObject can appear in the provides service sub–schemas of profile specification (see clause 9 of ISO/IEC
14478-1).
7.5.1 Object properties
Properties are used to store values with an object that may b
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