练习：

登录界面增加注册功能(在本地增加用户文件进行比对用户)

LoginWindow.h

#ifndef LOGINWINDOW_H
#define LOGINWINDOW_H

#include <QMainWindow>
#include <QIcon>
#include <QLabel>
#include <QLineEdit>
#include <QPushButton>
#include <QDebug>
#include <QMessageBox>
#include <QFile>

class LoginWindow : public QMainWindow
{
    Q_OBJECT

signals:
    void pushToHomeWindow();

public slots:
    void loginButtonEvent();
    void registerEvent();

public:
    LoginWindow(QWidget *parent = 0);
    ~LoginWindow();

private:
    QLineEdit *usernameInput;
    QLineEdit *passwdInput;
};

#endif // LOGINWINDOW_H

LoginWindow.cpp

#include "loginwindow.h"

const QString USER_TABLE_PATH = "./userTable.txt";

void showAlert(QString msg) {
    QMessageBox::information(nullptr, "提醒", msg);
}

LoginWindow::LoginWindow(QWidget *parent)
    : QMainWindow(parent)
{
    this->resize(600, 800);

    this->setWindowIcon(QIcon(":/images/QQ.png"));
    this->setWindowTitle("登录界面");
    this->setStyleSheet("background-color:white");

    int offsetX = (this->width()-480)/2, offsetY = 10;

    //LOGO
    QLabel *logoLabel = new QLabel(this);
    logoLabel->setPixmap(QPixmap(":/images/logo.png"));
    logoLabel->setGeometry(offsetX, offsetY, 480, 266);
    offsetY += (266 + 20);

    //登录框
    offsetX = (this->width()-260)/2;
    QLabel *loginIcon = new QLabel(this);
    loginIcon->setPixmap(QPixmap(":/images/userName.jpg"));
    loginIcon->setScaledContents(true);
    loginIcon->setGeometry(offsetX, offsetY, 50, 50);
    offsetX += (50+10);

    QLineEdit *usernameInput = new QLineEdit(this);
    this->usernameInput = usernameInput;
    usernameInput->setPlaceholderText("请输入用户名");
    usernameInput->setGeometry(offsetX, offsetY, 200, 50);
    usernameInput->setStyleSheet("border:none");

    QWidget *loginBottomLine = new QWidget(this);
    loginBottomLine->setStyleSheet("background-color:black");
    loginBottomLine->setGeometry(usernameInput->x(), usernameInput->y()+usernameInput->height(), usernameInput->width(), 1);
    offsetY += (50+10);

    offsetX = (this->width()-260)/2;
    QLabel *passwdIcon = new QLabel(this);
    passwdIcon->setPixmap(QPixmap(":/images/passwd.jpg"));
    passwdIcon->setScaledContents(true);
    passwdIcon->setGeometry(offsetX, offsetY, 50, 50);
    offsetX += (50+10);

    QLineEdit *passwdInput = new QLineEdit(this);
    this->passwdInput = passwdInput;
    passwdInput->setPlaceholderText("请输入密码");
    passwdInput->setGeometry(offsetX, offsetY, 200, 50);
    passwdInput->setEchoMode(QLineEdit::Password);
    passwdInput->setStyleSheet("border:none");

    QWidget *passwdBottomLine = new QWidget(this);
    passwdBottomLine->setStyleSheet("background-color:black");
    passwdBottomLine->setGeometry(passwdInput->x(), passwdInput->y()+passwdInput->height(), passwdInput->width(), 1);
    offsetY += 60;

    //登录按钮
    offsetX = (this->width()-150)/2;
    QPushButton *loginBtn = new QPushButton(this);
    loginBtn->setIcon(QIcon(":/images/login.png"));
    loginBtn->setText("登录");
    loginBtn->setIconSize(QSize(20, 20));
    loginBtn->setGeometry(offsetX, offsetY, 80, 44);
    connect(loginBtn, &QPushButton::clicked, this, &LoginWindow::loginButtonEvent);
    offsetX += (80 + 20);

    //取消按钮
    QPushButton *cancelBtn = new QPushButton(this);
    cancelBtn->setIcon(QIcon(":/images/cancel.png"));
    cancelBtn->setText("取消");
    cancelBtn->setIconSize(QSize(20, 20));
    cancelBtn->setGeometry(offsetX, offsetY, 80, 44);
    connect(cancelBtn, &QPushButton::clicked, [=]() {
        //取消按钮点击事件
        int res = QMessageBox::question(this, "提醒", "是否要退出登录", QMessageBox::Yes | QMessageBox::No);
        if (res == QMessageBox::Yes) {
            this->close();
        }
    });
    offsetX += (80 + 20);

    //注册
    QPushButton *registerBtn = new QPushButton(this);
    registerBtn->setText("注册");
    registerBtn->setGeometry(offsetX, offsetY, 80, 44);
    connect(registerBtn, &QPushButton::clicked, this, &LoginWindow::registerEvent);

    offsetY += 70;

    this->resize(this->width(), offsetY);
}

bool checkUser(QString username, QString passwd) {
    QFile file(USER_TABLE_PATH);
    if (!file.exists()) {
        showAlert("登录失败");
        return false;
    }
    if (!file.open(QFile::ReadOnly)) {
        showAlert("打开用户表失败");
        return false;
    }
    while(!file.atEnd()) {
        QByteArray byteArray = file.readLine();
        QString orgStr = QString(byteArray);
        QStringList list = orgStr.split(" ");
        QString tempUsername = list[0];
        QString tempPasswd = list[1];
        tempPasswd.remove(tempPasswd.size()-1, 1);
        if (tempUsername == username && tempPasswd == passwd) {
            return true;
        }
    }
    return false;
}

void LoginWindow::loginButtonEvent() {
    //登录事件
    if (checkUser(this->usernameInput->text(), this->passwdInput->text())) {
        QMessageBox box(QMessageBox::Information, "提醒", "登录成功", QMessageBox::Ok, this);
        //box.button(QMessageBox::Ok)->setText("aaa");
        int res = box.exec();
        if (res == QMessageBox::Ok) {
            qDebug() << "登录成功";
            emit pushToHomeWindow();
            this->close();
        }
    } else {
        int res = QMessageBox::warning(this, "提醒", "账号密码不匹配，是否重新登录?", QMessageBox::Ok | QMessageBox::Cancel);
        if (res == QMessageBox::Ok) {
            this->passwdInput->setText("");
        } else if (res == QMessageBox::Cancel) {
            this->close();
        }
    }
}

//注册事件
void LoginWindow::registerEvent() {
    if (this->usernameInput->text().length() <= 0 || this->passwdInput->text().length() <= 0) {
        showAlert("请输入用户名和密码");
        return;
    }
    QFile file(USER_TABLE_PATH);

    QString username = this->usernameInput->text();
    QString passwd = this->passwdInput->text();

    QString res = username + " " + passwd + "\n";
    if (!file.open(QFileDevice::Append)) {
        showAlert("打开文件失败");
        return;
    }
    file.write(res.toLocal8Bit().data());

    showAlert("注册成功");
}

LoginWindow::~LoginWindow()
{

}

结果展示：