下列代码定义了一个窗口，窗口采用竖直布局：一个按钮及一个label。按下按钮时候，窗口扩张，显示label控件。再次按下按钮时，窗口收缩，隐藏label控件。

详细代码如下：

#include <QApplication>
#include <QWidget>
#include <QHBoxLayout>
#include <QTimer>
#include <QLabel>
#include <QVBoxLayout>
#include <QPushButton>
#include <QDebug>

class MyWidget : public QWidget
{
	//Q_OBJECT
public:
	MyWidget(QWidget* parent = nullptr) : QWidget(parent), m_bD(true)
	{
		setFixedWidth(100);
	//	setStyleSheet("background-color:red;");
		m_vLayout = new QVBoxLayout();
		m_vLayout->setContentsMargins(0, 0, 0, 0);
		setLayout(m_vLayout);

		m_hLayout = new QHBoxLayout();
		m_pushBtn = new QPushButton("d!", this);
		m_pushBtn->setFixedSize(20, 20);
		m_hLayout->addStretch(1);
		m_hLayout->addWidget(m_pushBtn);

		m_vLayout->addLayout(m_hLayout);

		m_label = new QLabel(this);
		m_label->setText("just a label text!");
		m_vLayout->addWidget(m_label);
		m_vLayout->addStretch(1);

		//setFixedHeight(m_pushBtn->height() + m_label->height());
		setFixedHeight(m_pushBtn->height());
		m_label->hide();

		QObject::connect(m_pushBtn, &QPushButton::clicked, [=]() {
			if (m_bD)
			{
				//m_vLayout->addWidget(m_label);
				m_label->show();
				m_pushBtn->setText("d!");

				qDebug() << "push btn h:" << m_pushBtn->height() << ", label h:" << m_label->height();
				setFixedHeight(m_pushBtn->height() + m_label->height());
				m_bD = false;
			}
			else
			{
				m_bD = true;
				//m_vLayout->removeWidget(m_label);
				m_label->hide();
				m_pushBtn->setText("p!");
				qDebug() << "push btn h:" << m_pushBtn->height() << ", label h:" << m_label->height();
				setFixedHeight(m_pushBtn->height());
			}
			});
	}



private:
	QHBoxLayout* m_hLayout;
	QPushButton* m_pushBtn;
	bool m_bD;

	QVBoxLayout* m_vLayout;
	QLabel *m_label;
};

int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	MyWidget w;

	w.show();

	return app.exec();
}

页面效果如下：