1. 把主类指针this传给其他类，tcpClientSocket = new TcpClient(this);

//ex2.cpp
#include "ex2.h"
#include "ui_ex2.h"

Ex2::Ex2(QWidget *parent)
    : QDialog(parent)
    , ui(new Ui::Ex2)
{
    ui->setupUi(this);

    tcpClientSocket = new TcpClient(this);

}

Ex2::~Ex2()
{
    delete ui;
}

void Ex2::on_pushButtonTcpConnect_clicked()
{
    tcpClientSocket->Test();
}

2. 把ui类改为公共类

//ex2.h
#ifndef EX2_H
#define EX2_H

#include <QDialog>
#include "tcpclient.h"


QT_BEGIN_NAMESPACE
namespace Ui { class Ex2; }
QT_END_NAMESPACE

class Ex2 : public QDialog
{
    Q_OBJECT

public:
    Ex2(QWidget *parent = nullptr);
    ~Ex2();
    Ui::Ex2 *ui;		//改为公共类
    TcpClient *tcpClientSocket;

private slots:
    void on_pushButtonTcpConnect_clicked();

private:
    //Ui::Ex2 *ui;		//私有类
    QSerialPort *serial;
};
#endif // EX2_H

3. 保存主类传来的指针保存，通过该指针调用UI中的控件

//tcpclient.cpp
#include "tcpclient.h"
#include "ex2.h"

TcpClient::TcpClient(Ex2 *parent)
{
    socket = new QTcpSocket();
    pUi = parent;
}


TcpClient::~TcpClient()
{
    delete socket;
}

void TcpClient::Connect()
{

}

void TcpClient::Test()
{
    pUi->ui->textEditMy1->setText("Test");
}

//tcpclient.h
#ifndef TCPCLIENT_H
#define TCPCLIENT_H

#include <QTcpSocket>
//#include "ex2.h"
#include "ui_ex2.h"

class Ex2;      // 声明类
class TcpClient
{
public:
    TcpClient(Ex2 *parent);
    ~TcpClient();
    void Connect();
    void Test();
    Ex2 *pUi;
private:
    QTcpSocket *socket;
};

#endif // TCPCLIENT_H

4. 在pro文件中增加QT += network

//ex2.pro
QT       += core gui

QT += serialport

QT += network

# disable C4819 warning
QMAKE_CXXFLAGS_WARN_ON += -wd4819


greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    ex2.cpp \
    tcpclient.cpp

HEADERS += \
    ex2.h \
    tcpclient.h

FORMS += \
    ex2.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

5. 效果