Deep Within event-driven interfaces programming styles under the bonnet … the bits leak out modal...

Deep Withinevent-driven interfaces

• programming styles

• under the bonnet … the bits leak out

• modal and non-modal interfaces

• maintaining state between events

• other events: networks, timers, devices

edit delete

STOCK SYSTEM

deletedItem: 0763612

in stock: 53

on order: 500

confirm cancel

Major System Errorapplication aborted

invalid data key access#37683 in ac_ssdb

programming styles

• on demand

• event loop

• event-driven

on demand

• fascist programs!

• program in control (internal control)

• users do what they’re told

print “what is your name”name = read()print “hello ”+name+“ how old are you”age = readInt();print “you will be ”+(age+1)+“ next year”

event loop

• program requests events

• user controls what events it gets

while (not the end){

event = getNextEvent();if (event.type == mouseClick){ … }else if ( … …

}

event-driven

• external control:• when an event (often from the user) happens

• program function/method is called from the outside

• many (e.g. VB) or single entry point

on mouseClick(pos) … …endon keyPress(key)

… …end

handleEvent(event){ if (event.type … … … …}

under the bonnet what windows does!

• when an event happens - added to a queue

• when your program is ‘active’• Windows takes event from queue

• then calls appropriate bit of your program

• when it returns, if there is enough time - repeat

– N.B. slightly different for different event typese.g. TCP/IP

the bits leak out how it affects the interface

edit delete

0763612ref no:

STOCK SYSTEM

the bits leak out how it affects the interface

edit delete

0763612ref no:

STOCK SYSTEM

Item: 0763612

in stock: 53

on order: 350

confirm cancel

the bits leak out how it affects the interface

edit delete

ref no:

STOCK SYSTEM

Item: 0763612

in stock: 53

on order: 350

confirm cancel

deleted

the bits leak out how it affects the interface

edit delete

ref no:

STOCK SYSTEM

Item: 0763612

in stock: 53

on order: 500

confirm cancel

deleted

the bits leak out how it affects the interface

edit delete

ref no:

STOCK SYSTEM

deletedItem: 0763612

in stock: 53

on order: 500

confirm cancel

Major System Errorapplication aborted

invalid data key access#37683 in ac_ssdb

the bits leak out how it affects the interface

• what goes wrong?– user does anything – unexpected events

• no sequence control

• so if not careful ...

inconsistency!!

edit delete

ref no:

STOCK SYSTEM

deletedItem: 0763612 in stock: 53

on order: 500

confirm cancel

Major System Errorapplication aborted

invalid data key access#37683 in ac_ssdb

modal and non-modal interfaces

• modal dialogue box– can’t do anything else until it is complete

• non-modal dialogue box– can do other things while it is there

a design decision … but ...

modal and non-modal interfaces

• modal dialogue box• easy with internal control• hard with external control - event-driven

• non-modal dialogue box• hard with internal control• easy with external control - event-driven

beware!implementation should not drive design

maintaining state between events

• internal control easy!

• external control– data in different modules– data in interface– … and no ‘program counter’

print “what is your name”name = read()print “hello ”+name+“ how old are you”age = readInt();print “you will be ”+(age+1)+“ next year”

other events

not just user events:

• timers• when 2 minutes since last action, save data

• network• when web page arrives show it on screen

• devices• when IR sensor shows activity contact police

who it was

Alan Dix

alan@hcibook.com

http://www.hcibook.com/

http://www.hiraeth.com/alan/teaching/bigui/

http://www.aqtive.com/

and crackers at:

http://www.websharer.com/crackers/