Qt 5.11 | Qt Widgets | Drop Site Example

Qt 5.11.3 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` also contains a private instance of QLabel and 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;

      private:
          QLabel *label;
      };

## 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, plain text, or a list of URLs.

          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)
                  text += urlList.at(i).path() + QLatin1Char('\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 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.

          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);

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

          QPushButton *clearButton;
          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.

          QStringList labels;
          labels << tr("Format") << tr("Content");

          formatsTable = new QTableWidget;
          formatsTable->setColumnCount(2);
          formatsTable->setEditTriggers(QAbstractItemView::NoEditTriggers);
          formatsTable->setHorizontalHeaderLabels(labels);
          formatsTable->horizontalHeader()->setStretchLastSection(true);

Two QPushButton objects, `clearButton` 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"));
          quitButton = new QPushButton(tr("Quit"));

          buttonBox = new QDialogButtonBox;
          buttonBox->addButton(clearButton, QDialogButtonBox::ActionRole);
          buttonBox->addButton(quitButton, QDialogButtonBox::RejectRole);

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

The clicked() signals for `quitButton` and `clearButton` are connected to close() and `clear()`, 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"));
          setMinimumSize(350, 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);
          if (!mimeData)
              return;

Once we are sure that `mimeData` is valid, we iterate through its supported formats using the foreach keyword. This keyword has the following format:

      foreach(variable, container)

In our example, `format` is the variable and the container is a QStringList, obtained from `mimeData->formats()`.

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

          for (const QString &format : mimeData->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 == QLatin1String("text/plain")) {
                  text = mimeData->text().simplified();
              } else if (format == QLatin1String("text/html")) {
                  text = mimeData->html().simplified();
              } else if (format == QLatin1String("text/uri-list")) {
                  QList<QUrl> urlList = mimeData->urls();
                  for (int i = 0; i < urlList.size() && i < 32; ++i)
                      text.append(urlList.at(i).toString() + QLatin1Char(' '));
              } else {
                  QByteArray data = mimeData->data(format);
                  for (int i = 0; i < data.size() && i < 32; ++i)
                      text.append(QStringLiteral("%1 ").arg(uchar(data[i]), 2, 16, QLatin1Char('0')).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);
      }

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();
      }

Files:

- draganddrop/dropsite/droparea.cpp
- draganddrop/dropsite/droparea.h
- draganddrop/dropsite/dropsite.pro
- draganddrop/dropsite/dropsitewindow.cpp
- draganddrop/dropsite/dropsitewindow.h
- draganddrop/dropsite/main.cpp