给我用qt写一段功能封装成类的代码

好的，以下是一个用Qt编写的功能类的示例代码：

#include <QObject>
#include <QFile>

class FileHandler : public QObject
{
    Q_OBJECT

public:
    explicit FileHandler(QObject *parent = nullptr);

signals:
    void fileOpened(QString);
    void fileClosed();
    void errorOccurred(QString);

public slots:
    void openFile(QString filePath);
    void closeFile();
    void writeText(QString text);

private:
    QFile* m_file;
};

FileHandler::FileHandler(QObject *parent)
    : QObject(parent)
    , m_file(nullptr)
{
}

void FileHandler::openFile(QString filePath)
{
    if (m_file != nullptr) {
        emit errorOccurred("File is already opened.");
        return;
    }

    m_file = new QFile(filePath, this);
    if (!m_file->open(QIODevice::WriteOnly | QIODevice::Text)) {
        emit errorOccurred(QString("Failed to open file: %1").arg(m_file->errorString()));
        delete m_file;
        m_file = nullptr;
        return;
    }

    emit fileOpened(filePath);
}

void FileHandler::closeFile()
{
    if (m_file == nullptr) {
        emit errorOccurred("File is not opened.");
        return;
    }

    m_file->close();
    delete m_file;
    m_file = nullptr;

    emit fileClosed();
}

void FileHandler::writeText(QString text)
{
    if (m_file == nullptr) {
        emit errorOccurred("File is not opened.");
        return;
    }

    m_file->write(text.toUtf8());
}

这个类名为`FileHandler`，它可以打开、关闭和向文件写入文本。它有三个信号：`fileOpened`，`fileClosed`和`errorOccurred`，分别在文件被打开、关闭和错误发生时发出。它也有三个槽：`openFile`，`closeFile`和`writeText`，分别用于打开、关闭和写入文本。