Event Based Styles

Event Based Styles

Objective

•      Instead of invoking a procedure directly, a component can announce one or more events (such as arrival of data or execution of an operation)

•               ON <EVENT> IF <CONDITION> THEN <ACTION>

•               On arrive D  if D < a or D ≥ b then print ( “out of bounds”)

•                  Such procedures are also called triggers, actors or eventcondition-action (ECA) rules

•               An advantage of event-based invocation is that it encourages reuse; a component can be introduced in a system simply by registering it for the events of that system

• A drawback is that sometimes event-based systems become quite unpredictable and hard to control.

•               A component may:

•               Announce events

•               Register a callback for events of other components

•               Connectors are the bindings between event announcements and routine calls (callbacks)

•               Publishers of events do not know which components (subscribers) will be affected by those events

•               Components cannot make assumptions about ordering of processing, or what processing will occur as a result of their events

•               Examples

•                   Programming environment tool integration

•               User interfaces (Model-View-Controller)

•               Syntax-directed editors to support incremental semantic checking

Event-Based Style: example

•               Integrating tools in a shared environment

•               Editor announces it has finished editing a module Compiler registers for such announcements and automatically re-compiles module

Editor shows syntax errors reported by compiler

Debugger announces it has reached a breakpoint Editor registers for such announcements and automatically scrolls to relevant source line

Advantages and Disadvantages

•               Strengths:

Strong support for reuse: plug in new component by registering it for events

 Maintenance: add and replace components with minimum effect on other components in the system

•  Weaknesses:

Loss of control:

What components will respond to an event?

In which order will components be invoked?

Are invoked components finished?

Correctness hard to ensure: depends on context and order of invocation