STOCK SYSTEM Major System Error Item 0763612 Deep

STOCK SYSTEM Major System Error Item: 0763612 Deep Within event-driven interfaces • • • application aborted invalid data key access deleted in stock: #37683 53 in ac_ssdb on order: confirm edit 500 delete cancel programming styles under the bonnet … the bits leak out modal and non-modal interfaces maintaining state between events other events: networks, timers, devices

programming styles • on demand • event loop • event-driven

on demand print “what is your name” name = read() print “hello ”+name+“ how old are you” age = read. Int(); print “you will be ”+(age+1)+“ next year” • fascist programs! • program in control (internal control) • users do what they’re told

event loop while (not the end) { event = get. Next. Event(); if (event. type == mouse. Click) { … } else if ( … … } • program requests events • user controls what events it gets

event-driven on mouse. Click(pos) … … end on key. Press(key) … … end handle. Event(event) { if (event. type … … } • 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

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 types e. g. TCP/IP

the bits leak out how it affects the interface STOCK SYSTEM ref no: edit 0763612 delete

the bits leak out how it affects the interface STOCK SYSTEM Item: 0763612 ref no: in stock: 0763612 53 on order: 350 confirm cancel edit delete

the bits leak out how it affects the interface STOCK SYSTEM Item: 0763612 ref no: in stock: deleted 53 on order: 350 confirm cancel edit delete

the bits leak out how it affects the interface STOCK SYSTEM Item: 0763612 ref no: in stock: deleted 53 on order: 500 confirm cancel edit delete

the bits leak out how it affects the interface STOCK SYSTEM Major System Error Item: 0763612 ref no: deleted application aborted in stock: invalid 53 data key access #37683 in ac_ssdb on order: 500 confirm cancel edit delete

the bits leak out how it affects the interface • what goes wrong? – user does anything – unexpected events STOCK SYSTEM Major System Error application Item: 0763612 deleted ref aborted invalid data key access in stock: #37683 53 in ac_ssdb delete on order: 500 no: edit confirm • no sequence control • so if not careful. . . inconsistency!! cancel

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 print “what is your name” name = read() print “hello ”+name+“ how old are you” age = read. Int(); print “you will be ”+(age+1)+“ next year” • internal control easy! • external control – data in different modules – data in interface – … and no ‘program counter’

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

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