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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). |
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38 ** $QT_END_LICENSE$ |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \page qundo.html |
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44 \title Overview of Qt's Undo Framework |
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45 \keyword Undo framework |
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46 \ingroup frameworks-technologies |
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47 |
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48 \section1 Introduction |
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49 |
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50 Qt's Undo Framework is an implementation of the Command pattern, for |
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51 implementing undo/redo functionality in applications. |
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52 |
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53 The Command pattern is based on the idea that all editing in |
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54 an application is done by creating instances of command objects. |
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55 Command objects apply changes to the document and are stored |
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56 on a command stack. Furthermore, each command knows how to undo its |
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57 changes to bring the document back to its previous state. As long |
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58 as the application only uses command objects to change the state of |
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59 the document, it is possible to undo a sequence of commands by |
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60 traversing the stack downwards and calling undo |
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61 on each command in turn. It is also possible to redo a sequence of |
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62 commands by traversing the stack upwards and calling |
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63 redo on each command. |
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64 |
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65 \section1 Classes |
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66 |
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67 The framework consists of four classes: |
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68 |
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69 \list |
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70 \i \l QUndoCommand is the base class of all commands stored on an |
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71 undo stack. It can apply (redo) or undo a single change in the document. |
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72 \i \l QUndoStack is a list of QUndoCommand objects. It contains all the |
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73 commands executed on the document and can roll the document's state |
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74 backwards or forwards by undoing or redoing them. |
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75 \i \l QUndoGroup is a group of undo stacks. It is useful when an application |
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76 contains more than one undo stack, typically one for each opened |
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77 document. QUndoGroup provides a single pair of undo/redo slots for all |
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78 the stacks in the group. It forwards undo and redo requests to |
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79 the active stack, which is the stack associated with the document that |
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80 is currently being edited by the user. |
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81 \i \l QUndoView is a widget which shows the contents of an undo stack. Clicking |
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82 on a command in the view rolls the document's state backwards or |
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83 forwards to that command. |
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84 \endlist |
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85 |
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86 \section1 Concepts |
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87 |
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88 The following concepts are supported by the framework: |
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89 |
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90 \list |
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91 \i \bold{Clean state:} Used to signal when the document enters and leaves a |
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92 state that has been saved to disk. This is typically used to disable or |
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93 enable the save actions, and to update the document's title bar. |
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94 \i \bold{Command compression:} Used to compress sequences of commands into a |
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95 single command. |
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96 For example: In a text editor, the commands that insert individual |
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97 characters into the document can be compressed into a single command that |
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98 inserts whole sections of text. These bigger changes are more convenient |
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99 for the user to undo and redo. |
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100 \i \bold{Command macros:} A sequence of commands, all of which are undone or |
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101 redone in one step. |
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102 These simplify the task of writing an application, since a set of simpler |
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103 commands can be composed into more complex commands. For example, a command |
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104 that moves a set of selected objects in a document can be created by |
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105 combining a set of commands, each of which moves a single object. |
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106 \endlist |
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107 |
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108 QUndoStack provides convenient undo and redo QAction objects that |
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109 can be inserted into a menu or a toolbar. The text properties of these |
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110 actions always reflect what command will be undone or redone when |
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111 they are triggered. Similarly, QUndoGroup provides undo and redo actions |
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112 that always behave like the undo and redo actions of the active stack. |
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113 */ |