Hierarchical State Machines - a Fundamentally Important Way of.pdf

Hierarchical State Machines - a Fundamentally Important Way of Design Presented by Madhukar Anand Based on slides by Miro Samek The Challenge of Event-Driven Systems The Challenge of Event-Driven Systems • Almost all computers today are event-driven systems • The main programming challenge is to quickly pick and execute the right code in reaction to an event • The reaction depends both on the nature of the event and on the current context, that is, the sequence of past events in which the system was involved • Traditional “bottom up” approaches represent the context ambiguously by a multitude of variables and flags, which results in code riddled with a disproportionate number of convoluted conditional branches (if-elseor switch- case statements in C/C++) quantum programming 4 Copyright © 2003 by Miro Samek. All Rights Reserved. The Significance of “State” The Significance of “State” • State machines make the response to an event explicitly dependent on both the nature of the event and the context of the system (state) • State captures the relevant aspects of the system’s history very efficiently EXAMPLE: a character code generated by a A keyboard depends if the Shift has been a depressed, but not on how many and which specific characters have been typed previously. A keyboard can be said to be in the “shifted” state or in the “default” state. • A state can abstract away all possible (but irrelevant) event sequences and capture only the relevant ones quantum programming 5 Copyright © 2003 by Miro Samek. All Rights Reserved. State Machines — Coding Perspective State Machines — Codin