Symbian OS: Active Objects
And The Active Scheduler
SYMBIAN OS
Version 1.0
August 26, 2004
Symbian OS: Active Objects And The Active Scheduler | 2
Legal Notice
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Contents
1. Introduction ......................................................................................................... 5
1.1 Overview .........................................................................................................................5
1.2 Scope ..............................................................................................................................5
2. Understanding Multitasking in General ............................................................ 6
2.1 Multitasking Definitions ...................................................................................................6
2.2 Multitasking in Symbian OS ............................................................................................6
2.3 Client/Server Architecture and Asynchronous Methods .................................................7
3. Active Objects and the Active Scheduler ....................................................... 10
3.1 Motivation......................................................................................................................10
3.2 Active Objects ...............................................................................................................10
3.3 Active Scheduler ...........................................................................................................13
3.3.1 Using an own active scheduler.............................................................................13
3.3.2 How active objects are tracked ............................................................................14
3.4 Miscellaneous Issues ....................................................................................................15
3.4.1 Nested scheduler loop..........................................................................................16
3.4.2 Long-running tasks ...............................................................................................17
4. Active Object Example...................................................................................... 18
4.1 Constructing the Engine................................................................................................18
4.2 Using the Engine...........................................................................................................19
4.2.1 Looking up the IP..................................................................................................19
4.2.2 Connecting the socket ..........................................................................................21
4.2.3 Requesting the document.....................................................................................21
4.2.4 Reading the document .........................................................................................22
4.3 Engine Destruction........................................................................................................24
5. Design Issues .................................................................................................... 26
5.1 Strict Error Handling......................................................................................................26
5.2 Avoid Forcing Asynchronous Calls to Synchronous Calls ............................................27
6. Where To Go Next ............................................................................................. 28
7. References ......................................................................................................... 29
Appendix A. CAOExampleEngine Sources........................................................... 30
A.1 AOExampleEngine.h.....................................................................................................30
A.2 AOExampleEngine.cpp.................................................................................................32
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Change History
August 26, 2004 Version 1.0 Initial document release
Version 1.0 | August 26, 2004
Symbian OS: Active Objects And The Active Scheduler | 5
1. Introduction
1.1 Overview
Together, active objects and an active scheduler provide nonpreemptive multitasking,
which can be used instead of multithreading in Symbian OS. The benefits obtained with
active objects are programming simplicity and a more efficient CPU when comparing
usage of threads.
This document provides a general introduction to multitasking in Chapter 2. Chapter 3
briefly explains the Symbian OS approach to effective multitasking using active objects
and the active scheduler. An example active object implementation is introduced in
Chapter 4. Design issues related to active objects are discussed in Chapter 5, and
Chapter 6 lists references for further reading.
Note that there are two new mechanisms introduced in Symbian OS v8.0, Publish-and-
Subscribe and Message Queues. These allow information to be shared between threads
and processes more conveniently. They don't provide alternatives to threads and active
objects but may impact on architectural decisions in relation to client code running in a
multi-threaded environment. These two new mechanisms are not discussed further in
this document.
1.2 Scope
This document is intended for developers who are new to Symbian OS. However,
essential idioms in Symbian OS (such as naming and exception handling) should be
understood. The document also covers several issues that some professionals may find
helpful.
Version 1.0 | August 26, 2004
