- Qt 6.9
- Qt Widgets
- Drop Site Example
- Qt 6.9.3 Reference Documentation

# Drop Site Example

The example shows how to distinguish the various MIME formats available in a drag and drop operation.

The Drop Site example accepts drops from other applications, and displays the MIME formats provided by the drag object.

There are two classes, `DropArea` and `DropSiteWindow`, and a `main()` function in this example. A `DropArea` object is instantiated in `DropSiteWindow`; a `DropSiteWindow` object is then invoked in the `main()` function.

#### DropArea Class Definition

The `DropArea` class is a subclass of QLabel with a public slot, `clear()`, and a `changed()` signal.

     class DropArea : public QLabel
     {
         Q_OBJECT

     public:
         explicit DropArea(QWidget *parent = nullptr);

     public slots:
         void clear();

     signals:
         void changed(const QMimeData *mimeData = nullptr);

In addition, `DropArea` contains reimplementations of four QWidget event handlers:

1. dragEnterEvent()
2. dragMoveEvent()
3. dragLeaveEvent()
4. dropEvent()

These event handlers are further explained in the implementation of the `DropArea` class.

     protected:
         void dragEnterEvent(QDragEnterEvent *event) override;
         void dragMoveEvent(QDragMoveEvent *event) override;
         void dragLeaveEvent(QDragLeaveEvent *event) override;
         void dropEvent(QDropEvent *event) override;
     };

#### DropArea Class Implementation

In the `DropArea` constructor, we set the minimum size to 200x200 pixels, the frame style to both QFrame::Sunken and QFrame::StyledPanel, and we align its contents to the center.

     DropArea::DropArea(QWidget *parent)
         : QLabel(parent)
     {
         setMinimumSize(200, 200);
         setFrameStyle(QFrame::Sunken | QFrame::StyledPanel);
         setAlignment(Qt::AlignCenter);
         setAcceptDrops(true);
         setAutoFillBackground(true);
         clear();
     }

Also, we enable drop events in `DropArea` by setting the acceptDrops property to `true`. Then, we enable the autoFillBackground property and invoke the `clear()` function.

The dragEnterEvent() event handler is called when a drag is in progress and the mouse enters the `DropArea` object. For the `DropSite` example, when the mouse enters `DropArea`, we set its text to "<drop content>" and highlight its background.

     void DropArea::dragEnterEvent(QDragEnterEvent *event)
     {
         setText(tr("<drop content>"));
         setBackgroundRole(QPalette::Highlight);

         event->acceptProposedAction();
         emit changed(event->mimeData());
     }

Then, we invoke acceptProposedAction() on event, setting the drop action to the one proposed. Lastly, we emit the `changed()` signal, with the data that was dropped and its MIME type information as a parameter.

For dragMoveEvent(), we just accept the proposed QDragMoveEvent object, event, with acceptProposedAction().

     void DropArea::dragMoveEvent(QDragMoveEvent *event)
     {
         event->acceptProposedAction();
     }

The `DropArea` class's implementation of dropEvent() extracts the event's mime data and displays it accordingly.

     void DropArea::dropEvent(QDropEvent *event)
     {
         const QMimeData *mimeData = event->mimeData();

The `mimeData` object can contain one of the following objects: an image, HTML text, Markdown text, plain text, or a list of URLs.

         if (mimeData->hasImage()) {
             setPixmap(qvariant_cast<QPixmap>(mimeData->imageData()));
         } else if (mimeData->hasColor()) {
             const auto color = mimeData->colorData().value<QColor>();
             setText(tr("Color: %1").arg(color.name()));
             setPalette(QPalette(color));
             setBackgroundRole(QPalette::Button);
         } else if (mimeData->hasFormat(u"text/markdown"_s)) {
             setText(QString::fromUtf8(mimeData->data(u"text/markdown"_s)));
             setTextFormat(Qt::MarkdownText);
         } 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 (qsizetype i = 0, count = qMin(urlList.size(), qsizetype(32)); i < count; ++i)
                 text += urlList.at(i).path() + u'\n';
             setText(text);
         } else {
             setText(tr("Cannot display data"));
         }

- If `mimeData` contains an image, we display it in `DropArea` with setPixmap().
- If `mimeData` contains HTML, we display it with setText() and set `DropArea`'s text format as Qt::RichText.
- If `mimeData` contains Markdown, we display it with setText() and set `DropArea`'s text format as Qt::MarkdownText.
- If `mimeData` contains plain text, we display it with setText() and set `DropArea`'s text format as Qt::PlainText. In the event that `mimeData` contains URLs, we iterate through the list of URLs to display them on individual lines.
- If `mimeData` contains other types of objects, we set `DropArea`'s text, with setText() to "Cannot display data" to inform the user.

We then set `DropArea`'s backgroundRole to QPalette::Dark and we accept `event`'s proposed action.

         if (!mimeData->hasColor()) {
             setPalette({});
             setBackgroundRole(QPalette::Dark);
         }
         event->acceptProposedAction();
     }

The dragLeaveEvent() event handler is called when a drag is in progress and the mouse leaves the widget.

     void DropArea::dragLeaveEvent(QDragLeaveEvent *event)
     {
         clear();
         event->accept();
     }

For `DropArea`'s implementation, we clear invoke `clear()` and then accept the proposed event.

The `clear()` function sets the text in `DropArea` to "<drop content>" and sets the backgroundRole to QPalette::Dark. Lastly, it emits the `changed()` signal.

     void DropArea::clear()
     {
         setText(tr("<drop content>"));
         setBackgroundRole(QPalette::Dark);

         emit changed();
     }

#### DropSiteWindow Class Definition

The `DropSiteWindow` class contains a constructor and a public slot, `updateFormatsTable()`.

     class DropSiteWindow : public QWidget
     {
         Q_OBJECT

     public:
         DropSiteWindow();

     public slots:
         void updateFormatsTable(const QMimeData *mimeData);
         void copy();

     private:
         DropArea *dropArea;
         QLabel *abstractLabel;
         QTableWidget *formatsTable;

         QPushButton *clearButton;
         QPushButton *copyButton;
         QPushButton *quitButton;
         QDialogButtonBox *buttonBox;
     };

The class also contains a private instance of `DropArea`, `dropArea`, QLabel, `abstractLabel`, QTableWidget, `formatsTable`, QDialogButtonBox, `buttonBox`, and two QPushButton objects, `clearButton` and `quitButton`.

#### DropSiteWindow Class Implementation

In the constructor of `DropSiteWindow`, we instantiate `abstractLabel` and set its wordWrap property to `true`. We also call the adjustSize() function to adjust `abstractLabel`'s size according to its contents.

     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();

Then we instantiate `dropArea` and connect its `changed()` signal to `DropSiteWindow`'s `updateFormatsTable()` slot.

         dropArea = new DropArea;
         connect(dropArea, &DropArea::changed,
                 this, &DropSiteWindow::updateFormatsTable);

We now set up the QTableWidget object, `formatsTable`. Its horizontal header is set using a QStringList object, `labels`. The number of columms are set to two and the table is not editable. Also, the `formatTable`'s horizontal header is formatted to ensure that its second column stretches to occupy additional space available.

         formatsTable = new QTableWidget;
         formatsTable->setColumnCount(2);
         formatsTable->setEditTriggers(QAbstractItemView::NoEditTriggers);
         formatsTable->setHorizontalHeaderLabels({tr("Format"),  tr("Content")});
         formatsTable->horizontalHeader()->setStretchLastSection(true);

Three QPushButton objects, `clearButton`, `copyButton`, and `quitButton`, are instantiated and added to `buttonBox` - a QDialogButtonBox object. We use QDialogButtonBox here to ensure that the push buttons are presented in a layout that conforms to the platform's style.

         clearButton = new QPushButton(tr("Clear"));
         copyButton = new QPushButton(tr("Copy"));
         quitButton = new QPushButton(tr("Quit"));

         buttonBox = new QDialogButtonBox;
         buttonBox->addButton(clearButton, QDialogButtonBox::ActionRole);
         buttonBox->addButton(copyButton, QDialogButtonBox::ActionRole);
     #if !QT_CONFIG(clipboard)
         copyButton->setVisible(false);
     #endif

         buttonBox->addButton(quitButton, QDialogButtonBox::RejectRole);

         connect(quitButton, &QAbstractButton::clicked, this, &QWidget::close);
         connect(clearButton, &QAbstractButton::clicked, dropArea, &DropArea::clear);
         connect(copyButton, &QAbstractButton::clicked, this, &DropSiteWindow::copy);

The clicked() signals for `copyButton`, `clearButton`, and `quitButton` are connected to `copy()`, `clear()` and close(), respectively.

For the layout, we use a QVBoxLayout, `mainLayout`, to arrange our widgets vertically. We also set the window title to "Drop Site" and the minimum size to 350x500 pixels.

         QVBoxLayout *mainLayout = new QVBoxLayout(this);
         mainLayout->addWidget(abstractLabel);
         mainLayout->addWidget(dropArea);
         mainLayout->addWidget(formatsTable);
         mainLayout->addWidget(buttonBox);

         setWindowTitle(tr("Drop Site"));
         resize(700, 500);
     }

We move on to the `updateFormatsTable()` function. This function updates the `formatsTable`, displaying the MIME formats of the object dropped onto the `DropArea` object. First, we set QTableWidget's rowCount property to 0. Then, we validate to ensure that the QMimeData object passed in is a valid object.

     void DropSiteWindow::updateFormatsTable(const QMimeData *mimeData)
     {
         formatsTable->setRowCount(0);
         copyButton->setEnabled(false);
         if (!mimeData)
             return;

Once we are sure that `mimeData` is valid, we iterate through its supported formats.

Note: The formats() function returns a QStringList object, containing all the formats supported by the `mimeData`.

         const QStringList formats = mimeData->formats();
         for (const QString &format : formats) {
             QTableWidgetItem *formatItem = new QTableWidgetItem(format);
             formatItem->setFlags(Qt::ItemIsEnabled);
             formatItem->setTextAlignment(Qt::AlignTop | Qt::AlignLeft);

Within each iteration, we create a QTableWidgetItem, `formatItem` and we set its flags to Qt::ItemIsEnabled, and its text alignment to Qt::AlignTop and Qt::AlignLeft.

A QString object, `text`, is customized to display data according to the contents of `format`. We invoke QString's simplified() function on `text`, to obtain a string that has no additional space before, after or in between words.

             QString text;
             if (format == u"text/plain") {
                 text = mimeData->text().simplified();
             } else if (format == u"text/markdown") {
                 text = QString::fromUtf8(mimeData->data(u"text/markdown"_s));
             } else if (format == u"text/html") {
                 text = mimeData->html().simplified();
             } else if (format == u"text/uri-list") {
                 QList<QUrl> urlList = mimeData->urls();
                 for (qsizetype i = 0, count = qMin(urlList.size(), qsizetype(32)); i < count; ++i)
                     text.append(urlList.at(i).toString() + u' ');
             } else {
                 QByteArray data = mimeData->data(format);
                 if (data.size() > 32)
                     data.truncate(32);
                 text = QString::fromLatin1(data.toHex(' ')).toUpper();
             }

If `format` contains a list of URLs, we iterate through them, using spaces to separate them. On the other hand, if `format` contains an image, we display the data by converting the text to hexadecimal.

             int row = formatsTable->rowCount();
             formatsTable->insertRow(row);
             formatsTable->setItem(row, 0, new QTableWidgetItem(format));
             formatsTable->setItem(row, 1, new QTableWidgetItem(text));
         }

         formatsTable->resizeColumnToContents(0);
     #if QT_CONFIG(clipboard)
         copyButton->setEnabled(formatsTable->rowCount() > 0);
     #endif
     }

Once `text` has been customized to contain the appropriate data, we insert both `format` and `text` into `formatsTable` with setItem(). Lastly, we invoke resizeColumnToContents() on `formatsTable`'s first column.

#### The main() Function

Within the `main()` function, we instantiate `DropSiteWindow` and invoke its show() function.

     int main(int argc, char *argv[])
     {
         QApplication app(argc, argv);
         DropSiteWindow window;
         window.show();
         return app.exec();
     }

Example project @ code.qt.io