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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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4 ** All rights reserved. |
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5 ** Contact: Nokia Corporation (qt-info@nokia.com) |
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7 ** This file is part of the documentation of the Qt Toolkit. |
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9 ** $QT_BEGIN_LICENSE:LGPL$ |
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14 ** this package. |
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16 ** GNU Lesser General Public License Usage |
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24 ** In addition, as a special exception, Nokia gives you certain additional |
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35 ** |
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36 ** |
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37 ** |
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38 ** $QT_END_LICENSE$ |
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39 ** |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \page qt4-styles.html |
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44 \title The Qt 4 Style API |
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45 |
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46 \contentspage {What's New in Qt 4}{Home} |
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47 \previouspage The Network Module in Qt 4 |
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48 \nextpage Thread Support in Qt 4 |
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49 |
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50 Qt's style API is responsible for performing the widget drawing |
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51 for built-in widgets. The Qt 4 style API has been revised to make |
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52 it possible for a style to draw widgets without calling any |
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53 functions on the widget. |
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54 |
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55 Because Qt 4 is split across multiple libraries, Qt needed this |
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56 update to be able to draw widgets from other libraries than |
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57 QtGui. For application developers, this has other benefits, such |
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58 as more managable parameter lists and the possibility of drawing |
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59 any graphical element without having a widget of a specific |
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60 type. |
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61 |
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62 \section1 General Overview |
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63 |
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64 The QStyle class is an abstract base class that encapsulates |
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65 the look and feel of a GUI. Qt's built-in widgets use it to |
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66 perform nearly all of their drawing, ensuring that they look |
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67 exactly like the equivalent native widgets. |
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68 |
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69 Most draw functions now take four arguments: |
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70 |
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71 \list |
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72 \o an enum value specifying which graphical element to draw |
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73 \o a QStyleOption specifying how and where to render that element |
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74 \o a QPainter that should be used to draw the element |
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75 \o a QWidget on which the drawing is performed (optional) |
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76 \endlist |
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77 |
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78 The style gets all the information it needs to render the |
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79 graphical element from QStyleOption. The widget is passed as the |
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80 last argument in case the style needs it to perform special |
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81 effects (such as animated default buttons on Mac OS X), but it |
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82 isn't mandatory. In fact, QStyle can be used to draw on any |
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83 paint device, not just widgets, by setting the QPainter properly. |
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84 |
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85 Thanks to QStyleOption, it is now possible to make QStyle draw |
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86 widgets without linking in any code for the widget. This is how |
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87 Qt's built-in styles can draw Qt 3 widgets such as |
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88 Q3ListView without necessarily linking against the Qt3Support |
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89 library. Another significant benefit of the new approach is that |
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90 it's now possible to use \l{QStyle}'s draw functions on other |
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91 widgets than the built-in widgets; for example, you can draw a |
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92 combobox on any widget, not just on a QComboBox. |
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93 |
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94 QStyleOption has various subclasses for the various types of |
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95 graphical elements that can be drawn, and it's possible to create |
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96 custom subclasses. For example, the QStyle::PE_FrameFocusRect |
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97 element expects a QStyleOptionFocusRect argument. This is |
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98 documented for each enum value. |
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99 |
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100 When reimplementing QStyle functions that take a |
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101 QStyleOption parameter, you often need to cast the |
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102 QStyleOption to a subclass (e.g., QStyleOptionFocusRect). For |
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103 safety, you can use qstyleoption_cast() to ensure that the |
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104 pointer type is correct. If the object isn't of the right type, |
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105 qstyleoption_cast() returns 0. For example: |
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106 |
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107 \snippet doc/src/snippets/code/doc_src_qt4-styles.qdoc 0 |
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108 |
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109 For performance reasons, there are few member functions and the |
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110 access to the variables is direct. This "low-level" feel makes |
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111 the structures use straightforward and emphasizes that these are |
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112 simply parameters used by the style functions. In addition, the |
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113 caller of a QStyle function usually creates QStyleOption |
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114 objects on the stack. This combined with Qt's extensive use of |
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115 \l{implicit sharing} for types such as QString, QPalette, and |
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116 QColor ensures that no memory allocation needlessly takes place. |
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117 (Dynamic memory allocation can be an expensive operation, |
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118 especially when drawing very often in a short time.) |
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119 |
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120 \section1 Example Code |
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121 |
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122 The following code snippet illustrates how to use QStyle to |
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123 draw the focus rectangle from a custom widget's paintEvent(): |
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124 |
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125 \snippet doc/src/snippets/code/doc_src_qt4-styles.qdoc 1 |
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126 |
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127 The next example shows how to derive from an existing style to |
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128 customize the look of a graphical element: |
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129 |
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130 \snippet doc/src/snippets/customstyle/customstyle.h 0 |
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131 \codeline |
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132 \snippet doc/src/snippets/customstyle/customstyle.cpp 2 |
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133 \snippet doc/src/snippets/customstyle/customstyle.cpp 3 |
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134 \snippet doc/src/snippets/customstyle/customstyle.cpp 4 |
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135 |
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136 See also the \l{Styles Example} for a more detailed description of |
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137 how custom styles can be created. |
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138 |
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139 \section1 Comparison with Qt 3 |
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140 |
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141 The QStyle class has a similar API in Qt 4 as in Qt 3, with |
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142 more or less the same functions. What has changed is the |
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143 signature of the functions and the role played by QStyleOption. |
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144 For example, here's the signature of the QStyle::drawControl() |
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145 function in Qt 3: |
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146 |
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147 \snippet doc/src/snippets/code/doc_src_qt4-styles.qdoc 2 |
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148 |
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149 Here's the signature of the same function in Qt 4: |
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150 |
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151 \snippet doc/src/snippets/code/doc_src_qt4-styles.qdoc 3 |
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152 |
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153 In Qt 3, some of the information required to draw a graphical |
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154 element was stored in a QStyleOption parameter, while the rest |
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155 was deduced by querying the widget. In Qt 4, everything is stored |
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156 in the QStyleOption parameter. |
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157 */ |