doc/src/examples/delayedencoding.qdoc
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/****************************************************************************
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**
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** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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** All rights reserved.
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** Contact: Nokia Corporation (qt-info@nokia.com)
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:LGPL$
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** No Commercial Usage
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** This file contains pre-release code and may not be distributed.
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** You may use this file in accordance with the terms and conditions
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** contained in the Technology Preview License Agreement accompanying
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** this package.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 2.1 as published by the Free Software
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** Foundation and appearing in the file LICENSE.LGPL included in the
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** packaging of this file.  Please review the following information to
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** ensure the GNU Lesser General Public License version 2.1 requirements
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** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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**
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** In addition, as a special exception, Nokia gives you certain additional
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** rights.  These rights are described in the Nokia Qt LGPL Exception
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** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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**
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** If you have questions regarding the use of this file, please contact
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** Nokia at qt-info@nokia.com.
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**
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**
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**
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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    \example draganddrop/delayedencoding
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    \title Delayed Encoding Example
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    The Delayed Encoding example shows how to delay preparing of data
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    for drag and drop operations until a drop target is found.
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    \image delayedecoding-example.png
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    The \gui Export push button is pressed down to start the drag.
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    The data for the drag and drop operation is not processed until
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    the user of the application has found a valid drop target. This
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    removes redundant processing if the operation is aborted. In our
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    case, we have an SVG image that we wish to send as the \c {
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    "image/png" } MIME type. It is the conversion from SVG to PNG we
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    wish to delay - it can be quite expensive.
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    The example is implemented in two classes: \c SourceWidget and \c
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    MimeData. The \c SourceWidget class sets up the GUI and starts the
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    drag operation on request. The \c MimeData class, which inherits
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    QMimeData, sends a signal when a drop target is found. This signal
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    is connected to a slot in \c SourceWidget, which does the
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    conversion from SVG to PNG. 
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    \section1 SourceWidget Class Definition
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    The \c SourceWidget class starts drag and drop operations and also
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    does the image conversion.
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    \snippet examples/draganddrop/delayedencoding/sourcewidget.h 0
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    The \gui Export push button is connected to the \c startDrag()
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    slot. The \c createData() slot will be invoked when data for the
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    drag and drop operation is to be created.
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    \section1 SourceWidget Class Implementation
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    Let's start our code tour with a look at the \c startDrag() slot.
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    \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 0
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    We emit \c dataRequested() from \c MimeData when the operation has
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    found a valid drop target.
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    We gallop along to \c createData():
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    \snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 1
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    Fortunately, Qt provides QSvgRenderer, which can render the SVG
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    image to any QPaintDevice. Also, QImage has no problems saving to
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    the PNG format.
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    Finally, we can give the data to \c MimeData.
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    \section1 MimeData Class Definition
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    The \c MimeData class inherits QMimeData and makes it possible to
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    delay preparing of the data for a drag and drop operation.
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    \snippet examples/draganddrop/delayedencoding/mimedata.h 0
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    We will look closer at \c retrieveData() and \c formats() in the
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    next section.
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    \section1 MimeData Class Implementation
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    \snippet examples/draganddrop/delayedencoding/mimedata.cpp 0
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    In the \c formats() function, we return the format of the
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    data we provide. This is the \c { image/png } MIME type.
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    \snippet examples/draganddrop/delayedencoding/mimedata.cpp 1
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    \c retrieveData() is reimplemented from QMimeData and is
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    called when the data is requested by the drag and drop
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    operation. Fortunately for us, this happens when the operation
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    is finishing, i.e., when a drop target has been found.
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    We emit the \c dataRequested() signal, which is picked up by
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    \c SourceWidget. The \c SourceWidget (as already explained)
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    sets the data on \c MimeData with \l{QMimeData::}{setData()}.
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*/
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