91 lines
3.8 KiB
Plaintext
91 lines
3.8 KiB
Plaintext
/****************************************************************************
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**
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** Copyright (C) 2016 The Qt Company Ltd.
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** Contact: https://www.qt.io/licensing/
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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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** $QT_BEGIN_LICENSE:FDL$
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** Commercial License Usage
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** accordance with the commercial license agreement provided with the
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** Software or, alternatively, in accordance with the terms contained in
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**
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** GNU Free Documentation License Usage
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** Alternatively, this file may be used under the terms of the GNU Free
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**
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****************************************************************************/
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/*!
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\example draganddrop/draggableicons
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\title Draggable Icons Example
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\brief The Draggable Icons example shows how to drag and drop image data between widgets
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in the same application, and between different applications.
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\image draggableicons-example.png
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In many situations where drag and drop is used, the user starts dragging from
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a particular widget and drops the payload onto another widget. In this example,
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we subclass QLabel to create labels that we use as drag sources, and place them
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inside \l{QWidget}s that serve as both containers and drop sites.
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In addition, when a drag and drop operation occurs, we want to send more than
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just an image. We also want to send information about where the user clicked in
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the image so that the user can place it precisely on the drop target. This level
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of detail means that we must create a custom MIME type for our data.
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\section1 DragWidget Class Definition
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The icon widgets that we use to display icons are subclassed from QLabel:
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\snippet draganddrop/draggableicons/dragwidget.h 0
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Since the QLabel class provides most of what we require for the icon, we
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only need to reimplement the \l QWidget::mousePressEvent() to provide
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drag and drop facilities.
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\section1 DragWidget Class Implementation
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The \c DragWidget constructor sets an attribute on the widget that ensures
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that it will be deleted when it is closed:
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\snippet draganddrop/draggableicons/dragwidget.cpp 0
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To enable dragging from the icon, we need to act on a mouse press event.
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We do this by reimplementing \l QWidget::mousePressEvent() and setting up
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a QDrag object.
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\snippet draganddrop/draggableicons/dragwidget.cpp 1
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Since we will be sending pixmap data for the icon and information about the
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user's click in the icon widget, we construct a QByteArray and package up the
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details using a QDataStream.
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For interoperability, drag and drop operations describe the data they contain
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using MIME types. In Qt, we describe this data using a QMimeData object:
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\snippet draganddrop/draggableicons/dragwidget.cpp 2
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We choose an unofficial MIME type for this purpose, and supply the QByteArray
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to the MIME data object.
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The drag and drop operation itself is handled by a QDrag object:
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\snippet draganddrop/draggableicons/dragwidget.cpp 3
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Here, we pass the data to the drag object, set a pixmap that will be shown
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alongside the cursor during the operation, and define the position of a hot
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spot that places the position of this pixmap under the cursor.
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*/
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