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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). |
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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 \example draganddrop/delayedencoding |
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44 \title Delayed Encoding Example |
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45 |
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46 The Delayed Encoding example shows how to delay preparing of data |
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47 for drag and drop operations until a drop target is found. |
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48 |
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49 \image delayedecoding-example.png |
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50 |
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51 The \gui Export push button is pressed down to start the drag. |
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52 The data for the drag and drop operation is not processed until |
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53 the user of the application has found a valid drop target. This |
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54 removes redundant processing if the operation is aborted. In our |
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55 case, we have an SVG image that we wish to send as the \c { |
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56 "image/png" } MIME type. It is the conversion from SVG to PNG we |
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57 wish to delay - it can be quite expensive. |
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58 |
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59 The example is implemented in two classes: \c SourceWidget and \c |
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60 MimeData. The \c SourceWidget class sets up the GUI and starts the |
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61 drag operation on request. The \c MimeData class, which inherits |
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62 QMimeData, sends a signal when a drop target is found. This signal |
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63 is connected to a slot in \c SourceWidget, which does the |
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64 conversion from SVG to PNG. |
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65 |
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66 \section1 SourceWidget Class Definition |
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67 |
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68 The \c SourceWidget class starts drag and drop operations and also |
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69 does the image conversion. |
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70 |
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71 \snippet examples/draganddrop/delayedencoding/sourcewidget.h 0 |
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72 |
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73 The \gui Export push button is connected to the \c startDrag() |
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74 slot. The \c createData() slot will be invoked when data for the |
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75 drag and drop operation is to be created. |
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76 |
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77 \section1 SourceWidget Class Implementation |
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78 |
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79 Let's start our code tour with a look at the \c startDrag() slot. |
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80 |
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81 \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 0 |
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82 |
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83 We emit \c dataRequested() from \c MimeData when the operation has |
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84 found a valid drop target. |
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85 |
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86 We gallop along to \c createData(): |
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87 |
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88 \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 1 |
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89 |
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90 Fortunately, Qt provides QSvgRenderer, which can render the SVG |
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91 image to any QPaintDevice. Also, QImage has no problems saving to |
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92 the PNG format. |
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93 |
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94 Finally, we can give the data to \c MimeData. |
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95 |
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96 \section1 MimeData Class Definition |
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97 |
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98 The \c MimeData class inherits QMimeData and makes it possible to |
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99 delay preparing of the data for a drag and drop operation. |
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100 |
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101 \snippet examples/draganddrop/delayedencoding/mimedata.h 0 |
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102 |
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103 We will look closer at \c retrieveData() and \c formats() in the |
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104 next section. |
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105 |
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106 \section1 MimeData Class Implementation |
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107 |
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108 \snippet examples/draganddrop/delayedencoding/mimedata.cpp 0 |
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109 |
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110 In the \c formats() function, we return the format of the |
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111 data we provide. This is the \c { image/png } MIME type. |
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112 |
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113 \snippet examples/draganddrop/delayedencoding/mimedata.cpp 1 |
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114 |
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115 \c retrieveData() is reimplemented from QMimeData and is |
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116 called when the data is requested by the drag and drop |
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117 operation. Fortunately for us, this happens when the operation |
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118 is finishing, i.e., when a drop target has been found. |
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119 |
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120 We emit the \c dataRequested() signal, which is picked up by |
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121 \c SourceWidget. The \c SourceWidget (as already explained) |
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122 sets the data on \c MimeData with \l{QMimeData::}{setData()}. |
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123 |
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124 */ |
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125 |