掉落站点范例

范例展示如何区分可用于拖放操作中的各种 MIME (多用途 Internet 邮件扩展) 格式。

Screenshot of the Drop Site example

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 ，和 main() function in this example. A DropArea object is instantiated in DropSiteWindow ; a DropSiteWindow object is then invoked in the main() 函数。

DropArea 类定义

The DropArea 类是子类化的 QLabel 具有公共槽 clear() ，和 changed() 信号。

class DropArea : public QLabel
{
    Q_OBJECT
public:
    explicit DropArea(QWidget *parent = nullptr);
public slots:
    void clear();
signals:
    void changed(const QMimeData *mimeData = nullptr);

此外， DropArea contains reimplementations of four QWidget 事件处理程序：

These event handlers are further explained in the implementation of the DropArea 类。

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

DropArea 类实现

在 DropArea 构造函数，设置最小大小为 200x200 像素，框架样式 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 特性到 true . Then, we enable the autoFillBackground property and invoke the clear() 函数。

The dragEnterEvent () event handler is called when a drag is in progress and the mouse enters the DropArea 对象。对于 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 () 在 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 对象， event ，采用 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->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"));
    }

若 mimeData 包含图像，显示它在 DropArea with setPixmap ().
若 mimeData 包含 HTML，显示它采用 setText () and set DropArea 的文本格式为 Qt::RichText .
若 mimeData contains Markdown, we display it with setText () and set DropArea 的文本格式为 Qt::MarkdownText .
若 mimeData 包含纯文本，显示它采用 setText () and set DropArea 的文本格式为 Qt::PlainText . In the event that mimeData contains URLs, we iterate through the list of URLs to display them on individual lines.
若 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() 信号。

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

DropSiteWindow 类定义

The DropSiteWindow 类包含构造函数和公共槽 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 对象， clearButton and quitButton .

DropSiteWindow 类实现

In the constructor of DropSiteWindow , we instantiate abstractLabel and set its wordWrap 特性到 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 和连接它的 changed() signal to DropSiteWindow 's updateFormatsTable() 槽。

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

现在设置 QTableWidget 对象， formatsTable . Its horizontal header is set using a QStringList 对象， 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 对象， clearButton , copyButton ，和 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 ，和 quitButton 被连接到 copy() , clear() and close () 分别。

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.

注意： The formats () 函数返回 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 文本对齐 to Qt::AlignTop and Qt::AlignLeft .

A QString 对象， 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();
        }

若 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
}

一旦 text has been customized to contain the appropriate data, we insert both format and text into formatsTable with setItem (). Lastly, we invoke resizeColumnToContents () 在 formatsTable 's first column.

main() 函数

在 main() 函数，实例化 DropSiteWindow 并援引其 show () 函数。

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

范例工程 @ code.qt.io