source: trunk/doc/src/examples/delayedencoding.qdoc@ 268

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41
42/*!
43 \example draganddrop/delayedencoding
44 \title Delayed Encoding Example
45
46 The Delayed Encoding example shows how to delay preparing of data
47 for drag and drop operations until a drop target is found.
48
49 \image delayedecoding-example.png
50
51 The \gui Export push button is pressed down to start the drag.
52 The data for the drag and drop operation is not processed until
53 the user of the application has found a valid drop target. This
54 removes redundant processing if the operation is aborted. In our
55 case, we have an SVG image that we wish to send as the \c {
56 "image/png" } MIME type. It is the conversion from SVG to PNG we
57 wish to delay - it can be quite expensive.
58
59 The example is implemented in two classes: \c SourceWidget and \c
60 MimeData. The \c SourceWidget class sets up the GUI and starts the
61 drag operation on request. The \c MimeData class, which inherits
62 QMimeData, sends a signal when a drop target is found. This signal
63 is connected to a slot in \c SourceWidget, which does the
64 conversion from SVG to PNG.
65
66 \section1 SourceWidget Class Definition
67
68 The \c SourceWidget class starts drag and drop operations and also
69 does the image conversion.
70
71 \snippet examples/draganddrop/delayedencoding/sourcewidget.h 0
72
73 The \gui Export push button is connected to the \c startDrag()
74 slot. The \c createData() slot will be invoked when data for the
75 drag and drop operation is to be created.
76
77 \section1 SourceWidget Class Implementation
78
79 Let's start our code tour with a look at the \c startDrag() slot.
80
81 \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 0
82
83 We emit \c dataRequested() from \c MimeData when the operation has
84 found a valid drop target.
85
86 We gallop along to \c createData():
87
88 \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 1
89
90 Fortunately, Qt provides QSvgRenderer, which can render the SVG
91 image to any QPaintDevice. Also, QImage has no problems saving to
92 the PNG format.
93
94 Finally, we can give the data to \c MimeData.
95
96 \section1 MimeData Class Definition
97
98 The \c MimeData class inherits QMimeData and makes it possible to
99 delay preparing of the data for a drag and drop operation.
100
101 \snippet examples/draganddrop/delayedencoding/mimedata.h 0
102
103 We will look closer at \c retrieveData() and \c formats() in the
104 next section.
105
106 \section1 MimeData Class Implementation
107
108 \snippet examples/draganddrop/delayedencoding/mimedata.cpp 0
109
110 In the \c formats() function, we return the format of the
111 data we provide. This is the \c { image/png } MIME type.
112
113 \snippet examples/draganddrop/delayedencoding/mimedata.cpp 1
114
115 \c retrieveData() is reimplemented from QMimeData and is
116 called when the data is requested by the drag and drop
117 operation. Fortunately for us, this happens when the operation
118 is finishing, i.e., when a drop target has been found.
119
120 We emit the \c dataRequested() signal, which is picked up by
121 \c SourceWidget. The \c SourceWidget (as already explained)
122 sets the data on \c MimeData with \l{QMimeData::}{setData()}.
123
124*/
125
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