Introduction to Programming. Bertrand Meyer. Lecture 23: An example: undo-redo. The problem

Transcription

1 1 Introduction to Programming Bertrand Meyer Last revised 20 January Lecture 23: An example: un-re The problem 3 Enabling users of an interactive system to cancel the effect of the last command. Often implemented as Control-Z. Should support multi-level un-re, with no limitation other than a possible maximum set by the user 1

2 A text editor 4 Notion of current line. Assume commands such as: Insert line after current position Insert line before current position Delete current line Replace current line current line with next if any This is a line-oriented view for simplicity, but the discussion applies to more sophisticated views Underlying class (from business model ) 5 class EDIT_CONTROLLER feature text: LINKED_LIST [STRING] remove is -- Remove line at current position. not off text.remove put_right (line: STRING) is -- Insert line after current position. not after text.put_right (line) Key step in devising a software architecture 6 Finding the right abstractions (The types of objects involved.) Here: The notion of command 2

3 Keeping the history of the session 7 The history list: Oldest Most recent history: LINKED_LIST [COMMAND] What s a command object? 8 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! A general notion of command 9 deferred class COMMAND feature execute is -- Carry out one execution of this command. deferred ensure ne: ne un is -- Cancel an earlier execution of this command. already: ne deferred 3

4 A general notion of command 10 deferred class COMMAND feature ne: BOOLEAN is -- Has this command been executed? execute is -- Carry out one execution of this command. un is -- Cancel an earlier execution of this command. deferred ensure ne: ne already: ne deferred Command class hierarchy 11 execute un* COMMAND deferred + effective + DELETION execute + un + line index + INSERTION execute + un + index A command class (sketch, no contracts) 12 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. line := controller.item ; index := controller.index controller.remove ; ne := True un is -- Re-insert previously removed line. controller.go_ith (index) controller.put_left (line) 4

5 Underlying class (from business model ) 13 class EDIT_CONTROLLER feature text: LINKED_LIST [STRING] remove is -- Remove line at current position. not off text.remove put_right (line: STRING) is -- Insert line after current position. not after text.put_right (line) Also item, index, go_ith, put_left Executing a user command 14 The history list: Oldest Most recent history: LINKED_LIST [COMMAND] Executing a user command 15 decode_user_request if Request is normal command then -- Create command object c corresponding to user request history.ext (c) c.execute elseif Request is UNDO then if not history.before then history.item.un history.back -- We ignore excessive requests elseif Request is REDO then if not history.is_last then history.forth history. item.un -- We ignore excessive requests 5

6 Conditional creation (1) 16 B A C a1: A if condition_1 then -- Create a1 as an instance of B elseif condition_2 then -- Create a1 as an instance of C etc D a1: A; b1: B; c1: C; d1: D; if condition_1 then create b1.make () a1 := b1 elseif condition_2 then create c1.make () a1 := c1 etc Conditional creation (2) 17 B A C a1: A if condition_1 then -- Create a1 as an instance of B elseif condition_2 then -- Create a1 as an instance of C etc D a1: A if condition_1 then create {B} a1.make () elseif condition_2 then create {C} a1.make () etc Executing a user command 18 decode_user_request if Request is normal command then -- Create command object c corresponding to user request history.ext (c) c.execute elseif Request is UNDO then if not history.before then history.item.un history.back -- Ignore excessive requests elseif Request is REDO then if not history.is_last then history.forth history. item.un -- Ignore excessive requests 6

7 Creating command objects (1) 19 c: COMMAND decode_user_request if Request is delete then create {DELETION} c elseif Request is insert then create {INSERTION} c etc Command class hierarchy 20 execute un* COMMAND deferred + effective + DELETION execute + un + line index + INSERTION execute + un + index Creating command objects (2) 21 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. To get a new command object: Insertion Deletion Determine command_type c := clone (COMMAND_TABLE.item (command_type)) 7

8 The un-re pattern 22 Has been extensively used (e.g. in Eiffel tools) Fairly easy to implement Elegant use of O-O techniques Disadvantage: explosion of small classes In Java, can use inner classes. Using agents 23 For each user command, have two routines: The routine to it The routine to un it! The history list in the un-re pattern 24 history: LINKED_LIST [COMMAND] Oldest Most recent 8

9 The history list using agents The history list simply becomes a list of agents pairs: history: LINKED_LIST [TUPLE [PROCEDURE [ANY, TUPLE], PROCEDURE [ANY, TUPLE]] 25 De-Insert De-Insert Re-insert De-Insert Basic scheme remains the same, but no need for command objects any more; we simply store agents. This is the technique used in all current Eiffel tools Executing a user command (before) 26 decode_user_request if Request is normal command then -- Create command object c corresponding to user request history.ext (c) c.execute elseif Request is UNDO then if not history.before then history.item.un history.back -- Ignore excessive requests elseif Request is REDO then if not history.is_last then history.forth history. item.un -- Ignore excessive requests Executing a user command (now) 27 Decode user_request giving two agents _it and un_it if Request is normal command then history.ext ([_it, un_it]) _it.call ([]) elseif Request is UNDO then if not history.before then history.item.item (2).call ([]) history.back De- De- Reinsert Insert Insert elseif Request is REDO then if not history.is_last then history.forth history.item.item (1).call ([]) Insert Insert Remove 9

10 28 End of lecture 23 10