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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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Design pattern: Observer

Description: "Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically."

This pattern can be used with both classes and prototypes.

Given Logtalk support for events, this design pattern is trivial to implement. Any number of objects can play the role of observer and any number of objects can be subjects. Events are managed by the runtime, avoiding the coupling typical of solutions using a Smalltalk-like dependents mechanism (which is the most typical implementation in other programming languages): subjects don't need to be aware of observers. The observer registry is global and subject predicates don't need to be modified to notify observers. A possible limitation is that events are only generated for public messages, i.e. for messages using the (::)/2 control construct. Messages to self, which use the ::/1 control construct, don't generate events. This restriction exists to ensure that events cannot be used to break encapsulation. When this limitation is an issue, note that the Logtalk standard library includes an implementation of the Smalltalk dependents mechanism that can be used in alternative.