//mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

class QTextEdit;

class MainWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void openFile();
    void saveFile();
    
private:
    QAction *openAction;
    QAction *saveAction;

    QTextEdit *textEdit;
};

#endif // MAINWINDOW_H

//mainwindow.cpp
#include <QtGui>
#include <QtWidgets>
#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    openAction = new QAction(QIcon(":/images/file-open"), tr("&Open..."), this);
    openAction->setShortcuts(QKeySequence::Open);
    openAction->setStatusTip(tr("Open an existing file"));
    connect(openAction, &QAction::triggered, this, &MainWindow::openFile);

    saveAction = new QAction(QIcon(":/images/file-save"), tr("&Save..."), this);
    saveAction->setShortcuts(QKeySequence::Save);
    saveAction->setStatusTip(tr("Save a new file"));
    connect(saveAction, &QAction::triggered, this, &MainWindow::saveFile);

    QMenu *file = menuBar()->addMenu(tr("&File"));
    file->addAction(openAction);
    file->addAction(saveAction);

    QToolBar *toolBar = addToolBar(tr("&File"));
    toolBar->addAction(openAction);
    toolBar->addAction(saveAction);

    statusBar() ;

    textEdit = new QTextEdit(this);
    setCentralWidget(textEdit);
}

MainWindow::~MainWindow()
{
}

void MainWindow::openFile()
{
    QString path = QFileDialog::getOpenFileName(this, tr("Open File"), ".", tr("Text Files(*.txt)"));
    if(!path.isEmpty()) {
        QFile file(path);
        if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) {
            QMessageBox::warning(this, tr("Read File"), tr("Cannot open file:\n%1").arg(path));
            return;
        }
        QTextStream in(&file);
        textEdit->setText(in.readAll());
        file.close();
    } else {
        QMessageBox::warning(this, tr("Path"), tr("You did not select any file."));
    }
}

void MainWindow::saveFile()
{
    QString path = QFileDialog::getSaveFileName(this, tr("Save File"), ".", tr("Text Files(*.txt)"));
    if(!path.isEmpty()) {
        QFile file(path);
        if (!file.open(QIODevice::WriteOnly | QIODevice::Text)) {
            QMessageBox::warning(this, tr("Write File"), tr("Cannot open file:\n%1").arg(path));
            return;
        }
        QTextStream out(&file);
        out << textEdit->toPlainText();
        file.close();
    } else {
        QMessageBox::warning(this, tr("Path"), tr("You did not select any file."));
    }
}

QString getOpenFileName(QWidget * parent = 0,                        //父窗口
                        const QString & caption = QString(),         //对话框标题
                        const QString & dir = QString(),             //打开的默认目录
                        const QString & filter = QString(),          //过滤器
                        QString * selectedFilter = 0,                //默认选择的过滤器
                        Options options = 0)                         //对话框的一些参数设定，比如只显示文件夹等等，它的取值是enum