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2 | **
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3 | ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
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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 |
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