//代码占位符
==========================================
mainwindow.cpp
=========================================
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
#include <QTextEdit>
#include <QTextDocument>
#include <QTextCursor>
#include <QMainWindow>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
  ui->setupUi(this);
  QTextDocument *doc = new QTextDocument;
  QTextCursor cur(doc);
  QTextEdit *edit = new QTextEdit(this);
  connect(doc,&QTextDocument::contentsChange,this,&MainWindow::DocContentsChange);
  edit->setDocument(doc);
  setCentralWidget(edit);
}
MainWindow::~MainWindow()
{
  delete ui;
}
void MainWindow::DocContentsChange(int aFrom, int aCharsRemoves, int acharsAdded)
{
   qDebug() << "aFrom" << aFrom << "aCharsRemoves" << aCharsRemoves << "acharsAdded" << acharsAdded;
}
============================================
mainwindow.h
=============================================
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
namespace Ui {
  class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
  public:
explicit MainWindow(QWidget *parent = 0);
  ~MainWindow();
public slots:
   void DocContentsChange(int aFrom,int aCharsRemoves,int acharsAdded);private:
   Ui::MainWindow *ui;
};#endif // MAINWINDOW_H

==========================================
main.cpp
==========================================
#include "mainwindow.h"
#include <QApplication>int main(int argc, char *argv[])
{
   QApplication a(argc, argv);
   MainWindow w;
   w.show();
   return a.exec();
}