qt-everywhere-src-5.15.1/qtsvg/doc/src/examples/delayedencoding.qdoc

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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()}.

*/