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<title>Qt 4.6: Drop Site Example</title>
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<td align="left" valign="top" width="32"><a href="http://qt.nokia.com/"><img src="images/qt-logo.png" align="left" border="0" /></a></td>
<td width="1"> </td><td class="postheader" valign="center"><a href="index.html"><font color="#004faf">Home</font></a> · <a href="classes.html"><font color="#004faf">All Classes</font></a> · <a href="functions.html"><font color="#004faf">All Functions</font></a> · <a href="overviews.html"><font color="#004faf">Overviews</font></a></td></tr></table><h1 class="title">Drop Site Example<br /><span class="subtitle"></span>
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<p>Files:</p>
<ul>
<li><a href="draganddrop-dropsite-droparea-cpp.html">draganddrop/dropsite/droparea.cpp</a></li>
<li><a href="draganddrop-dropsite-droparea-h.html">draganddrop/dropsite/droparea.h</a></li>
<li><a href="draganddrop-dropsite-dropsitewindow-cpp.html">draganddrop/dropsite/dropsitewindow.cpp</a></li>
<li><a href="draganddrop-dropsite-dropsitewindow-h.html">draganddrop/dropsite/dropsitewindow.h</a></li>
<li><a href="draganddrop-dropsite-main-cpp.html">draganddrop/dropsite/main.cpp</a></li>
<li><a href="draganddrop-dropsite-dropsite-pro.html">draganddrop/dropsite/dropsite.pro</a></li>
</ul>
<p>The example shows how to distinguish the various MIME formats available in a drag and drop operation.</p>
<p align="center"><img src="images/dropsite-example.png" alt="Screenshot of the Drop Site example" /></p><p>The Drop Site example accepts drops from other applications, and displays the MIME formats provided by the drag object.</p>
<p>There are two classes, <tt>DropArea</tt> and <tt>DropSiteWindow</tt>, and a <tt>main()</tt> function in this example. A <tt>DropArea</tt> object is instantiated in <tt>DropSiteWindow</tt>; a <tt>DropSiteWindow</tt> object is then invoked in the <tt>main()</tt> function.</p>
<a name="droparea-class-definition"></a>
<h2>DropArea Class Definition</h2>
<p>The <tt>DropArea</tt> class is a subclass of <a href="qlabel.html">QLabel</a> with a public slot, <tt>clear()</tt>, and a <tt>changed()</tt> signal.</p>
<pre> class DropArea : public QLabel
{
Q_OBJECT
public:
DropArea(QWidget *parent = 0);
public slots:
void clear();
signals:
void changed(const QMimeData *mimeData = 0);</pre>
<p>In addition, <tt>DropArea</tt> also contains a private instance of <a href="qlabel.html">QLabel</a> and reimplementations of four <a href="qwidget.html">QWidget</a> event handlers:</p>
<ol type="1">
<li><a href="qwidget.html#dragEnterEvent">dragEnterEvent()</a></li>
<li><a href="qwidget.html#dragMoveEvent">dragMoveEvent()</a></li>
<li><a href="qwidget.html#dragLeaveEvent">dragLeaveEvent()</a></li>
<li><a href="qwidget.html#dropEvent">dropEvent()</a></li>
</ol>
<p>These event handlers are further explained in the implementation of the <tt>DropArea</tt> class.</p>
<pre> protected:
void dragEnterEvent(QDragEnterEvent *event);
void dragMoveEvent(QDragMoveEvent *event);
void dragLeaveEvent(QDragLeaveEvent *event);
void dropEvent(QDropEvent *event);
private:
QLabel *label;
};</pre>
<a name="droparea-class-implementation"></a>
<h2>DropArea Class Implementation</h2>
<p>In the <tt>DropArea</tt> constructor, we set the <a href="qwidget.html#minimumSize-prop">minimum size</a> to 200x200 pixels, the <a href="qframe.html#setFrameStyle">frame style</a> to both <a href="qframe.html#Shadow-enum">QFrame::Sunken</a> and <a href="qframe.html#Shape-enum">QFrame::StyledPanel</a>, and we align its contents to the center.</p>
<pre> DropArea::DropArea(QWidget *parent)
: QLabel(parent)
{
setMinimumSize(200, 200);
setFrameStyle(QFrame::Sunken | QFrame::StyledPanel);
setAlignment(Qt::AlignCenter);
setAcceptDrops(true);
setAutoFillBackground(true);
clear();
}</pre>
<p>Also, we enable drop events in <tt>DropArea</tt> by setting the <a href="qwidget.html#acceptDrops-prop">acceptDrops</a> property to <tt>true</tt>. Then, we enable the <a href="qwidget.html#autoFillBackground-prop">autoFillBackground</a> property and invoke the <tt>clear()</tt> function.</p>
<p>The <a href="qwidget.html#dragEnterEvent">dragEnterEvent()</a> event handler is called when a drag is in progress and the mouse enters the <tt>DropArea</tt> object. For the <tt>DropSite</tt> example, when the mouse enters <tt>DropArea</tt>, we set its text to "<drop content>" and highlight its background.</p>
<pre> void DropArea::dragEnterEvent(QDragEnterEvent *event)
{
setText(tr("<drop content>"));
setBackgroundRole(QPalette::Highlight);
event->acceptProposedAction();
emit changed(event->mimeData());
}</pre>
<p>Then, we invoke <a href="qdropevent.html#acceptProposedAction">acceptProposedAction()</a> on <i>event</i>, setting the drop action to the one proposed. Lastly, we emit the <tt>changed()</tt> signal, with the data that was dropped and its MIME type information as a parameter.</p>
<p>For <a href="qwidget.html#dragMoveEvent">dragMoveEvent()</a>, we just accept the proposed <a href="qdragmoveevent.html">QDragMoveEvent</a> object, <i>event</i>, with <a href="qdropevent.html#acceptProposedAction">acceptProposedAction()</a>.</p>
<pre> void DropArea::dragMoveEvent(QDragMoveEvent *event)
{
event->acceptProposedAction();
}</pre>
<p>The <tt>DropArea</tt> class's implementation of <a href="qwidget.html#dropEvent">dropEvent()</a> extracts the <i>event</i>'s mime data and displays it accordingly.</p>
<pre> void DropArea::dropEvent(QDropEvent *event)
{
const QMimeData *mimeData = event->mimeData();</pre>
<p>The <tt>mimeData</tt> object can contain one of the following objects: an image, HTML text, plain text, or a list of URLs.</p>
<pre> if (mimeData->hasImage()) {
setPixmap(qvariant_cast<QPixmap>(mimeData->imageData()));
} else if (mimeData->hasHtml()) {
setText(mimeData->html());
setTextFormat(Qt::RichText);
} else if (mimeData->hasText()) {
setText(mimeData->text());
setTextFormat(Qt::PlainText);
} else if (mimeData->hasUrls()) {
QList<QUrl> urlList = mimeData->urls();
QString text;
for (int i = 0; i < urlList.size() && i < 32; ++i) {
QString url = urlList.at(i).path();
text += url + QString("\n");
}
setText(text);
} else {
setText(tr("Cannot display data"));
}</pre>
<ul>
<li>If <tt>mimeData</tt> contains an image, we display it in <tt>DropArea</tt> with <a href="qlabel.html#pixmap-prop">setPixmap()</a>.</li>
<li>If <tt>mimeData</tt> contains HTML, we display it with <a href="qlabel.html#text-prop">setText()</a> and set <tt>DropArea</tt>'s text format as <a href="qt.html#TextFormat-enum">Qt::RichText</a>.</li>
<li>If <tt>mimeData</tt> contains plain text, we display it with <a href="qlabel.html#text-prop">setText()</a> and set <tt>DropArea</tt>'s text format as <a href="qt.html#TextFormat-enum">Qt::PlainText</a>. In the event that <tt>mimeData</tt> contains URLs, we iterate through the list of URLs to display them on individual lines.</li>
<li>If <tt>mimeData</tt> contains other types of objects, we set <tt>DropArea</tt>'s text, with <a href="qlabel.html#text-prop">setText()</a> to "Cannot display data" to inform the user.</li>
</ul>
<p>We then set <tt>DropArea</tt>'s <a href="qwidget.html#backgroundRole">backgroundRole</a> to <a href="qpalette.html#ColorRole-enum">QPalette::Dark</a> and we accept <tt>event</tt>'s proposed action.</p>
<pre> setBackgroundRole(QPalette::Dark);
event->acceptProposedAction();
}</pre>
<p>The <a href="qwidget.html#dragLeaveEvent">dragLeaveEvent()</a> event handler is called when a drag is in progress and the mouse leaves the widget.</p>
<pre> void DropArea::dragLeaveEvent(QDragLeaveEvent *event)
{
clear();
event->accept();
}</pre>
<p>For <tt>DropArea</tt>'s implementation, we clear invoke <tt>clear()</tt> and then accept the proposed event.</p>
<p>The <tt>clear()</tt> function sets the text in <tt>DropArea</tt> to "<drop content>" and sets the <a href="qwidget.html#backgroundRole">backgroundRole</a> to <a href="qpalette.html#ColorRole-enum">QPalette::Dark</a>. Lastly, it emits the <tt>changed()</tt> signal.</p>
<pre> void DropArea::clear()
{
setText(tr("<drop content>"));
setBackgroundRole(QPalette::Dark);
emit changed();
}</pre>
<a name="dropsitewindow-class-definition"></a>
<h2>DropSiteWindow Class Definition</h2>
<p>The <tt>DropSiteWindow</tt> class contains a constructor and a public slot, <tt>updateFormatsTable()</tt>.</p>
<pre> class DropSiteWindow : public QWidget
{
Q_OBJECT
public:
DropSiteWindow();
public slots:
void updateFormatsTable(const QMimeData *mimeData);
private:
DropArea *dropArea;
QLabel *abstractLabel;
QTableWidget *formatsTable;
QPushButton *clearButton;
QPushButton *quitButton;
QDialogButtonBox *buttonBox;
};</pre>
<p>The class also contains a private instance of <tt>DropArea</tt>, <tt>dropArea</tt>, <a href="qlabel.html">QLabel</a>, <tt>abstractLabel</tt>, <a href="qtablewidget.html">QTableWidget</a>, <tt>formatsTable</tt>, <a href="qdialogbuttonbox.html">QDialogButtonBox</a>, <tt>buttonBox</tt>, and two <a href="qpushbutton.html">QPushButton</a> objects, <tt>clearButton</tt> and <tt>quitButton</tt>.</p>
<a name="dropsitewindow-class-implementation"></a>
<h2>DropSiteWindow Class Implementation</h2>
<p>In the constructor of <tt>DropSiteWindow</tt>, we instantiate <tt>abstractLabel</tt> and set its <a href="qlabel.html#wordWrap-prop">wordWrap</a> property to <tt>true</tt>. We also call the <a href="qwidget.html#adjustSize">adjustSize()</a> function to adjust <tt>abstractLabel</tt>'s size according to its contents.</p>
<pre> DropSiteWindow::DropSiteWindow()
{
abstractLabel = new QLabel(tr("This example accepts drags from other "
"applications and displays the MIME types "
"provided by the drag object."));
abstractLabel->setWordWrap(true);
abstractLabel->adjustSize();</pre>
<p>Then we instantiate <tt>dropArea</tt> and connect its <tt>changed()</tt> signal to <tt>DropSiteWindow</tt>'s <tt>updateFormatsTable()</tt> slot.</p>
<pre> dropArea = new DropArea;
connect(dropArea, SIGNAL(changed(const QMimeData*)),
this, SLOT(updateFormatsTable(const QMimeData*)));</pre>
<p>We now set up the <a href="qtablewidget.html">QTableWidget</a> object, <tt>formatsTable</tt>. Its horizontal header is set using a <a href="qstringlist.html">QStringList</a> object, <tt>labels</tt>. The number of columms are set to two and the table is not editable. Also, the <tt>formatTable</tt>'s horizontal header is formatted to ensure that its second column stretches to occupy additional space available.</p>
<pre> QStringList labels;
labels << tr("Format") << tr("Content");
formatsTable = new QTableWidget;
formatsTable->setColumnCount(2);
formatsTable->setEditTriggers(QAbstractItemView::NoEditTriggers);
formatsTable->setHorizontalHeaderLabels(labels);
formatsTable->horizontalHeader()->setStretchLastSection(true);</pre>
<p>Two <a href="qpushbutton.html">QPushButton</a> objects, <tt>clearButton</tt> and <tt>quitButton</tt>, are instantiated and added to <tt>buttonBox</tt> - a <a href="qdialogbuttonbox.html">QDialogButtonBox</a> object. We use <a href="qdialogbuttonbox.html">QDialogButtonBox</a> here to ensure that the push buttons are presented in a layout that conforms to the platform's style.</p>
<pre> clearButton = new QPushButton(tr("Clear"));
quitButton = new QPushButton(tr("Quit"));
buttonBox = new QDialogButtonBox;
buttonBox->addButton(clearButton, QDialogButtonBox::ActionRole);
buttonBox->addButton(quitButton, QDialogButtonBox::RejectRole);
connect(quitButton, SIGNAL(pressed()), this, SLOT(close()));
connect(clearButton, SIGNAL(pressed()), dropArea, SLOT(clear()));</pre>
<p>The <a href="qabstractbutton.html#clicked">clicked()</a> signals for <tt>quitButton</tt> and <tt>clearButton</tt> are connected to <a href="qwidget.html#close">close()</a> and <tt>clear()</tt>, respectively.</p>
<p>For the layout, we use a <a href="qvboxlayout.html">QVBoxLayout</a>, <tt>mainLayout</tt>, to arrange our widgets vertically. We also set the window title to "Drop Site" and the minimum size to 350x500 pixels.</p>
<pre> QVBoxLayout *mainLayout = new QVBoxLayout;
mainLayout->addWidget(abstractLabel);
mainLayout->addWidget(dropArea);
mainLayout->addWidget(formatsTable);
mainLayout->addWidget(buttonBox);
setLayout(mainLayout);
setWindowTitle(tr("Drop Site"));
setMinimumSize(350, 500);
}</pre>
<p>We move on to the <tt>updateFormatsTable()</tt> function. This function updates the <tt>formatsTable</tt>, displaying the MIME formats of the object dropped onto the <tt>DropArea</tt> object. First, we set <a href="qtablewidget.html">QTableWidget</a>'s <a href="qtablewidget.html#rowCount-prop">rowCount</a> property to 0. Then, we validate to ensure that the <a href="qmimedata.html">QMimeData</a> object passed in is a valid object.</p>
<pre> void DropSiteWindow::updateFormatsTable(const QMimeData *mimeData)
{
formatsTable->setRowCount(0);
if (!mimeData)
return;</pre>
<p>Once we are sure that <tt>mimeData</tt> is valid, we iterate through its supported formats using the <a href="containers.html#the-foreach-keyword">foreach keyword</a>. This keyword has the following format:</p>
<pre> foreach(variable, container)</pre>
<p>In our example, <tt>format</tt> is the <i>variable</i> and the <i>container</i> is a <a href="qstringlist.html">QStringList</a>, obtained from <tt>mimeData->formats()</tt>.</p>
<p><b>Note:</b> The <a href="qmimedata.html#formats">formats()</a> function returns a <a href="qstringlist.html">QStringList</a> object, containing all the formats supported by the <tt>mimeData</tt>.</p>
<pre> foreach (QString format, mimeData->formats()) {
QTableWidgetItem *formatItem = new QTableWidgetItem(format);
formatItem->setFlags(Qt::ItemIsEnabled);
formatItem->setTextAlignment(Qt::AlignTop | Qt::AlignLeft);</pre>
<p>Within each iteration, we create a <a href="qtablewidgetitem.html">QTableWidgetItem</a>, <tt>formatItem</tt> and we set its <a href="qtablewidgetitem.html#setFlags">flags</a> to <a href="qt.html#ItemFlag-enum">Qt::ItemIsEnabled</a>, and its <a href="qtablewidgetitem.html#setTextAlignment">text alignment</a> to <a href="qt.html#AlignmentFlag-enum">Qt::AlignTop</a> and <a href="qt.html#AlignmentFlag-enum">Qt::AlignLeft</a>.</p>
<p>A <a href="qstring.html">QString</a> object, <tt>text</tt>, is customized to display data according to the contents of <tt>format</tt>. We invoke {<a href="qstring.html">QString</a>}'s <a href="qstring.html#simplified">simplified()</a> function on <tt>text</tt>, to obtain a string that has no additional space before, after or in between words.</p>
<pre> QString text;
if (format == "text/plain") {
text = mimeData->text().simplified();
} else if (format == "text/html") {
text = mimeData->html().simplified();
} else if (format == "text/uri-list") {
QList<QUrl> urlList = mimeData->urls();
for (int i = 0; i < urlList.size() && i < 32; ++i)
text.append(urlList[i].toString() + " ");
} else {
QByteArray data = mimeData->data(format);
for (int i = 0; i < data.size() && i < 32; ++i) {
QString hex = QString("%1").arg(uchar(data[i]), 2, 16,
QChar('0'))
.toUpper();
text.append(hex + " ");
}
}</pre>
<p>If <tt>format</tt> contains a list of URLs, we iterate through them, using spaces to separate them. On the other hand, if <tt>format</tt> contains an image, we display the data by converting the text to hexadecimal.</p>
<pre> int row = formatsTable->rowCount();
formatsTable->insertRow(row);
formatsTable->setItem(row, 0, new QTableWidgetItem(format));
formatsTable->setItem(row, 1, new QTableWidgetItem(text));
}
formatsTable->resizeColumnToContents(0);
}</pre>
<p>Once <tt>text</tt> has been customized to contain the appropriate data, we insert both <tt>format</tt> and <tt>text</tt> into <tt>formatsTable</tt> with <a href="qtablewidget.html#setItem">setItem()</a>. Lastly, we invoke <a href="qtableview.html#resizeColumnToContents">resizeColumnToContents()</a> on <tt>formatsTable</tt>'s first column.</p>
<a name="the-main-function"></a>
<h2>The main() Function</h2>
<p>Within the <tt>main()</tt> function, we instantiate <tt>DropSiteWindow</tt> and invoke its <a href="qwidget.html#show">show()</a> function.</p>
<pre> int main(int argc, char *argv[])
{
QApplication app(argc, argv);
DropSiteWindow window;
window.show();
return app.exec();
}</pre>
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<td width="40%" align="left">Copyright © 2010 Nokia Corporation and/or its subsidiary(-ies)</td>
<td width="20%" align="center"><a href="trademarks.html">Trademarks</a></td>
<td width="40%" align="right"><div align="right">Qt 4.6.3</div></td>
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