Threaded Applications

Introducing additional threads in a Delphi application is <Relatively>

easy

Threaded Applications

• A Thread is?

• Why would you introduce additional threads?

• Basic Thread Code

• Variables and Thread Safe Code

• Some Thread Models - Communication

What is a Thread ?

• Simple Program – Sequential Steps– Waits to read data– Single “Thread”

• Windows OS runs programs “at the same time”– Time allocation, System Waits

• The OS also allows sections of a program to run independently in the same way.

• Each TThread Object introduces an additional independent set of processor steps (thread) within the application.

Why Introduce Multiple Threads?

• Users are no longer prepared to wait– If more than (?)<1-2 Seconds> then Thread

• More efficient– Actions happen in other Thread wait cycles

• Many things lend themselves to “Background” running– Large database reports– Internet access

• Can make things simpler– Less need for state engines

Basic Thread Code

• Inherit from TThread Object– Override the Execute Routine.– Handle exceptions within Execute– Keep Execute Thread safe

• Must free TThread Object or the Application will not close– Must allow Execute to terminate

Thread Safety

• Code is unsafe when a variable can be changed by separate threads in an uncontrolled manner and the change affects the program flow.

• Procedure Local Variables are safe• Object Private and Protected variables

and objects can be managed• Thread Local >> threadvar X: Integer;

– Global Variable Equivalent ???

Synchronization Objects

• Used to manage access to variables

• TCriticalSection– Protect blocks of code

• Acquire• Release

• TSimpleEvent– One Thread initiates action then waits until

other thread has set the event

Synchronize Routine

• Only works with VCL thread

• You get to pass a procedure of object without parameters

• The procedure needs to be able to establish its context to access variables

• The calling thread is suspended until the procedure returns

Thread Communication Models

• Separate (console) process• Single Run

– Recover Results in OnTerminate

• Server with Post Messages– Post results home

• Server with Synchronized callback– Call VCL Thread with results

• Multiplexed Server– Wait for thread to complete action

Threads to Multiplex

Listen on Port

IPSession

DatabaseAccess

Sql Query

Sql Results

QueyAndWait

Spawn session

HandleQueries

Inorder

• Slides and demonstration code– http://www.innovasolutions.com.au/delphistuf/ADUGJanuary2006.htm