| 1 | /****************************************************************************
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| 2 | **
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| 3 | ** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
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| 4 | ** All rights reserved.
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| 5 | ** Contact: Nokia Corporation ([email protected])
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| 6 | **
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| 7 | ** This file is part of the documentation of the Qt Toolkit.
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| 8 | **
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| 9 | ** $QT_BEGIN_LICENSE:FDL$
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| 10 | ** Commercial Usage
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| 11 | ** Licensees holding valid Qt Commercial licenses may use this file in
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| 12 | ** accordance with the Qt Commercial License Agreement provided with the
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| 13 | ** Software or, alternatively, in accordance with the terms contained in a
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| 14 | ** written agreement between you and Nokia.
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| 15 | **
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| 16 | ** GNU Free Documentation License
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| 17 | ** Alternatively, this file may be used under the terms of the GNU Free
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| 18 | ** Documentation License version 1.3 as published by the Free Software
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| 19 | ** Foundation and appearing in the file included in the packaging of this
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| 20 | ** file.
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| 22 | ** If you have questions regarding the use of this file, please contact
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| 23 | ** Nokia at [email protected].
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| 24 | ** $QT_END_LICENSE$
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| 25 | **
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| 26 | ****************************************************************************/
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| 27 |
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| 28 | /*!
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| 29 | \example draganddrop/delayedencoding
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| 30 | \title Delayed Encoding Example
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| 31 |
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| 32 | The Delayed Encoding example shows how to delay preparing of data
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| 33 | for drag and drop operations until a drop target is found.
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| 34 |
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| 35 | \image delayedecoding-example.png
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| 36 |
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| 37 | The \gui Export push button is pressed down to start the drag.
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| 38 | The data for the drag and drop operation is not processed until
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| 39 | the user of the application has found a valid drop target. This
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| 40 | removes redundant processing if the operation is aborted. In our
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| 41 | case, we have an SVG image that we wish to send as the \c {
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| 42 | "image/png" } MIME type. It is the conversion from SVG to PNG we
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| 43 | wish to delay - it can be quite expensive.
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| 44 |
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| 45 | The example is implemented in two classes: \c SourceWidget and \c
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| 46 | MimeData. The \c SourceWidget class sets up the GUI and starts the
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| 47 | drag operation on request. The \c MimeData class, which inherits
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| 48 | QMimeData, sends a signal when a drop target is found. This signal
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| 49 | is connected to a slot in \c SourceWidget, which does the
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| 50 | conversion from SVG to PNG.
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| 51 |
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| 52 | \section1 SourceWidget Class Definition
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| 53 |
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| 54 | The \c SourceWidget class starts drag and drop operations and also
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| 55 | does the image conversion.
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| 56 |
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| 57 | \snippet examples/draganddrop/delayedencoding/sourcewidget.h 0
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| 58 |
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| 59 | The \gui Export push button is connected to the \c startDrag()
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| 60 | slot. The \c createData() slot will be invoked when data for the
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| 61 | drag and drop operation is to be created.
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| 62 |
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| 63 | \section1 SourceWidget Class Implementation
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| 64 |
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| 65 | Let's start our code tour with a look at the \c startDrag() slot.
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| 66 |
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| 67 | \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 0
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| 68 |
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| 69 | We emit \c dataRequested() from \c MimeData when the operation has
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| 70 | found a valid drop target.
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| 71 |
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| 72 | We gallop along to \c createData():
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| 73 |
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| 74 | \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 1
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| 75 |
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| 76 | Fortunately, Qt provides QSvgRenderer, which can render the SVG
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| 77 | image to any QPaintDevice. Also, QImage has no problems saving to
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| 78 | the PNG format.
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| 79 |
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| 80 | Finally, we can give the data to \c MimeData.
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| 81 |
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| 82 | \section1 MimeData Class Definition
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| 83 |
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| 84 | The \c MimeData class inherits QMimeData and makes it possible to
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| 85 | delay preparing of the data for a drag and drop operation.
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| 86 |
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| 87 | \snippet examples/draganddrop/delayedencoding/mimedata.h 0
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| 88 |
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| 89 | We will look closer at \c retrieveData() and \c formats() in the
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| 90 | next section.
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| 91 |
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| 92 | \section1 MimeData Class Implementation
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| 93 |
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| 94 | \snippet examples/draganddrop/delayedencoding/mimedata.cpp 0
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| 95 |
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| 96 | In the \c formats() function, we return the format of the
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| 97 | data we provide. This is the \c { image/png } MIME type.
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| 98 |
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| 99 | \snippet examples/draganddrop/delayedencoding/mimedata.cpp 1
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| 100 |
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| 101 | \c retrieveData() is reimplemented from QMimeData and is
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| 102 | called when the data is requested by the drag and drop
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| 103 | operation. Fortunately for us, this happens when the operation
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| 104 | is finishing, i.e., when a drop target has been found.
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| 105 |
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| 106 | We emit the \c dataRequested() signal, which is picked up by
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| 107 | \c SourceWidget. The \c SourceWidget (as already explained)
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| 108 | sets the data on \c MimeData with \l{QMimeData::}{setData()}.
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| 109 |
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| 110 | */
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| 111 |
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