Common Design Patterns for Symbian OS: The Foundations of Smartphone Software

4.3. Publish and Subscribe

Intent

Securely broadcast changes to other threads or processes listening for such notifications as a way of saving power and decoupling components.

AKA

P&S

4.3.1. Problem

4.3.1.1. Context

An event generator needs to broadcast the latest content of a data structure to zero or more event consumers located in other threads or processes that do not need to track each intermediate state of the data structure.

4.3.1.2. Summary

• You wish to reduce the power usage of your component.

• You want to promote encapsulation of and loose coupling between your event generators and event consumers.

• You would like to perform security checks on either the event generators or the event consumers so that you can control who sends and receives event signals.

4.3.1.3. Description

The problem that we wish to overcome here is an interesting one because of the large amount of flexibility that is required. First of all, there needs to be no restriction on the number of event generators or event consumers. Secondly, the event generators and the event consumers can be anywhere in the system: in different threads or processes or even in the kernel itself. Clearly Event Mixin (see page 93) can't be used to solve this problem by itself!

A natural consequence of this flexibility is that it is not easy to guarantee that each event consumer receives each and every event signal, which is known as Reliable event signals. The only completely reliable way to do this is for the event generator ...