关注这几个接口

 	 void insertSpacing(int index, int size);
    void insertStretch(int index, int stretch = 0);
    void insertSpacerItem(int index, QSpacerItem *spacerItem);
    void insertWidget(int index, QWidget *widget, int stretch = 0, Qt::Alignment alignment = Qt::Alignment());
    void insertLayout(int index, QLayout *layout, int stretch = 0);
    void insertItem(int index, QLayoutItem *);

class Q_WIDGETS_EXPORT QBoxLayout : public QLayout
{
    Q_OBJECT
    Q_DECLARE_PRIVATE(QBoxLayout)
public:
    enum Direction { LeftToRight, RightToLeft, TopToBottom, BottomToTop,
                     Down = TopToBottom, Up = BottomToTop };

    explicit QBoxLayout(Direction, QWidget *parent = nullptr);

    ~QBoxLayout();

    Direction direction() const;
    void setDirection(Direction);

    void addSpacing(int size);
    void addStretch(int stretch = 0);
    void addSpacerItem(QSpacerItem *spacerItem);
    void addWidget(QWidget *, int stretch = 0, Qt::Alignment alignment = Qt::Alignment());
    void addLayout(QLayout *layout, int stretch = 0);
    void addStrut(int);
    void addItem(QLayoutItem *) override;

    void insertSpacing(int index, int size);
    void insertStretch(int index, int stretch = 0);
    void insertSpacerItem(int index, QSpacerItem *spacerItem);
    void insertWidget(int index, QWidget *widget, int stretch = 0, Qt::Alignment alignment = Qt::Alignment());
    void insertLayout(int index, QLayout *layout, int stretch = 0);
    void insertItem(int index, QLayoutItem *);

    int spacing() const;
    void setSpacing(int spacing);

    bool setStretchFactor(QWidget *w, int stretch);
    bool setStretchFactor(QLayout *l, int stretch);
    void setStretch(int index, int stretch);
    int stretch(int index) const;

    QSize sizeHint() const override;
    QSize minimumSize() const override;
    QSize maximumSize() const override;

    bool hasHeightForWidth() const override;
    int heightForWidth(int) const override;
    int minimumHeightForWidth(int) const override;

    Qt::Orientations expandingDirections() const override;
    void invalidate() override;
    QLayoutItem *itemAt(int) const override;
    QLayoutItem *takeAt(int) override;
    int count() const override;
    void setGeometry(const QRect&) override;

private:
    Q_DISABLE_COPY(QBoxLayout)
};