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/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
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** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
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****************************************************************************/
/*!
\example draganddrop/delayedencoding
\title Delayed Encoding Example
The Delayed Encoding example shows how to delay preparing of data
for drag and drop operations until a drop target is found.
\image delayedecoding-example.png
The \gui Export push button is pressed down to start the drag.
The data for the drag and drop operation is not processed until
the user of the application has found a valid drop target. This
removes redundant processing if the operation is aborted. In our
case, we have an SVG image that we wish to send as the \c {
"image/png" } MIME type. It is the conversion from SVG to PNG we
wish to delay - it can be quite expensive.
The example is implemented in two classes: \c SourceWidget and \c
MimeData. The \c SourceWidget class sets up the GUI and starts the
drag operation on request. The \c MimeData class, which inherits
QMimeData, sends a signal when a drop target is found. This signal
is connected to a slot in \c SourceWidget, which does the
conversion from SVG to PNG.
\section1 SourceWidget Class Definition
The \c SourceWidget class starts drag and drop operations and also
does the image conversion.
\snippet examples/draganddrop/delayedencoding/sourcewidget.h 0
The \gui Export push button is connected to the \c startDrag()
slot. The \c createData() slot will be invoked when data for the
drag and drop operation is to be created.
\section1 SourceWidget Class Implementation
Let's start our code tour with a look at the \c startDrag() slot.
\snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 0
We emit \c dataRequested() from \c MimeData when the operation has
found a valid drop target.
We gallop along to \c createData():
\snippet examples/draganddrop/delayedencoding/sourcewidget.cpp 1
Fortunately, Qt provides QSvgRenderer, which can render the SVG
image to any QPaintDevice. Also, QImage has no problems saving to
the PNG format.
Finally, we can give the data to \c MimeData.
\section1 MimeData Class Definition
The \c MimeData class inherits QMimeData and makes it possible to
delay preparing of the data for a drag and drop operation.
\snippet examples/draganddrop/delayedencoding/mimedata.h 0
We will look closer at \c retrieveData() and \c formats() in the
next section.
\section1 MimeData Class Implementation
\snippet examples/draganddrop/delayedencoding/mimedata.cpp 0
In the \c formats() function, we return the format of the
data we provide. This is the \c { image/png } MIME type.
\snippet examples/draganddrop/delayedencoding/mimedata.cpp 1
\c retrieveData() is reimplemented from QMimeData and is
called when the data is requested by the drag and drop
operation. Fortunately for us, this happens when the operation
is finishing, i.e., when a drop target has been found.
We emit the \c dataRequested() signal, which is picked up by
\c SourceWidget. The \c SourceWidget (as already explained)
sets the data on \c MimeData with \l{QMimeData::}{setData()}.
*/
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