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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()}. |
| |
| */ |
| |