1 Introduction to Programming Bertrand Meyer Lecture 20 An example: undo-redo Last revised 20 January 2006 Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 The problem 2 � Enabling users of an interactive system to cancel the effect of the last command � Often implemented as “Control-Z” � Should support multi-level undo-redo, with no limitation other than a possible maximum set by the user � A good review of O-O techniques Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Working example: text editor 3 Notion of “current line”. Assume commands such as: � Insert line after current position � Insert line before current position � Delete current line � Replace current line � Swap current line with next if any � ... This is a line-oriented view for simplicity, but the discussion applies to more sophisticated views Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 1
Underlying class (from “business model”) 4 class EDIT_CONTROLLER feature text : LINKED_LIST [ STRING ] remove is -- Remove line at current position. require not off do text . remove end put_right ( line : STRING ) is -- Insert line after current position. require not after do text . put_right ( line ) end ... end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Key step in devising a software architecture 5 Finding the right abstractions (Interesting object types) Here: � The notion of “command” Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Keeping the history of the session 6 The history list: Insert Insert Remove Insert Swap Oldest Most recent history : LINKED_LIST [ COMMAND ] Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 2
What’s a “command” object? 7 A command object (instance of the class COMMAND ) includes information about one execution of a command by the user, sufficient to: � Execute the command � Cancel the command if requested later For example, in a delete command (as implemented by remove ), we need: • The position of the line being deleted • The content of that line! Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 A general notion of command 8 deferred class COMMAND feature execute is -- Carry out one execution of this command. deferred end undo is -- Cancel an earlier execution of this command. deferred end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 A general notion of command 9 deferred class COMMAND feature done : BOOLEAN is -- Has this command been executed? execute is -- Carry out one execution of this command. deferred ensure done: done end undo is -- Cancel an earlier execution of this command. require already: done deferred end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 3
Command class hierarchy 10 execute * ∗ COMMAND ∗ deferred undo * effective + + + … DELETION INSERTION execute + execute + undo + undo + line index index ... ... Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 A command class (sketch, no contracts) 11 class DELETION inherit COMMAND feature controller : EDIT_CONTROLLER -- Access to business model line : STRING -- The line being deleted index : INTEGER -- Position of line being deleted execute is -- Remove current line and remember it. do line := controller . item ; index := controller . index controller . remove ; done := True end undo is -- Re-insert previously removed line. do controller . go_ith ( index) controller . put_left ( line ) end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Underlying class (from “business model”) 12 class EDIT_CONTROLLER feature text : LINKED_LIST [ STRING ] remove is -- Remove line at current position. require not off do text . remove end put_right ( line : STRING ) is -- Insert line after current position. require not after do text . put_right (line) end ... also item, index, go_ith, put_left ... end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 4
Executing a user command 13 The history list: Insert Insert Remove Insert Swap Oldest Most recent history : LINKED_LIST [ COMMAND ] Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Executing a user command 14 decode_user_request if “Request is normal command” then -- “Create command object c corresponding to user request” history . extend ( c ) c . execute Insert Insert Remove Insert elseif “Request is UNDO” then if not history . before then history . item . undo item history . back -- We ignore excessive requests end elseif “Request is REDO” then if not history . is_last then history . forth history . item . undo -- We ignore excessive requests end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Conditional creation (1) 15 A a1 : A if condition_1 then -- “Create a1 as an instance of B” … elseif condition_2 then -- “Create a1 as an instance of C” B C ... etc... a1 : A ; b1 : B ; c1 : C ; d1 : D ; ... … if condition_1 then create b1 . make (...) a1 := b1 D elseif condition_2 then create c1 . make (...) a1 := c1 ... etc... Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 5
Conditional creation (2) 16 A a1 : A if condition_1 then -- “Create a1 as an instance of B” … elseif condition_2 then -- “Create a1 as an instance of C” B C ... etc... a1 : A … if condition_1 then create { B } a1 . make (...) elseif condition_2 then D create { C } a1 . make (...) ... etc... Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Executing a user command 17 decode_user_request if “Request is normal command” then -- “Create command object c corresponding to user request” history . extend ( c ) c . execute elseif “Request is UNDO” then Insert Insert Remove Insert if not history . before then history . item . undo item history . back -- Ignore excessive requests end elseif “Request is REDO” then if not history . is_last then history . forth history . item . undo -- Ignore excessive requests end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Creating command objects (1) 18 c : COMMAND ... decode_user_request if “Request is delete ” then create { DELETION } c elseif “Request is insert ” then create { INSERTION } c ... etc... Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 6
Command class hierarchy 19 * execute * * deferred COMMAND undo * + effective + + … DELETION INSERTION execute + execute + undo + undo + line index index ... ... Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 Creating command objects (2) 20 � Give each command type a number (or other key) � Initially, fill in a table (e.g. an array), with one instance of each command type. ... ... Swap Insertion � To get a new command object: Deletion “Determine command_type ” c := clone ( COMMAND_TABLE . item ( command_type )) Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 The undo-redo pattern 21 � Has been extensively used (e.g. in Eiffel tools) � Fairly easy to implement � Details must be handled carefully (e.g. some commands may not be undoable) � Elegant use of O-O techniques � Disadvantage: explosion of small classes � In Java, you can use “inner” classes. Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 7
Using agents 22 For each user command, have two routines: � The routine to do it � The routine to undo it! Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 The history list in the undo-redo pattern 23 history : LINKED_LIST [ COMMAND ] Insert Insert Remove Insert Swap Oldest Most recent Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 The history list using agents 24 The history list simply becomes a list of agents pairs: history : LINKED_LIST [ TUPLE [ PROCEDURE [ ANY, TUPLE ], PROCEDURE [ ANY, TUPLE ]] Insert Insert Remove Insert Swap De-insert De-insert Re-insert De-insert Swap Basic scheme remains the same, but no need for command objects any more; the history list simply contains agents. Chair of Softw are Engineering I ntroduction to Programming – Lecture 20 8
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