Substitute for sleep function in Qt/C++

Question

So I am writing a program that displays each letter of a word for 1 second with a 1 second interval between the letters. (It's for a spelling exercise for grade 1). I am currently using the sleep function to "pause" the program for 1 second before it "updates" again. After that it displays the word for a second and then removes it. I repaint before the sleep function, else it does not seem to update in time.

Here is the basic function:

QString word = "apple";
QThread thread;

for(int i = 0; i < word.size(); i++)
{
    ui->label1->setText(word[i]);
    ui->label1->repaint();
    thread.sleep(1);
    ui->label1->setText("");
    thread.sleep(1);
}

ui->label1->setText(word);
ui->label1->repaint();
thread.sleep(1);
ui->label1->setText("");

This works fine, except the program stops responding (even though I can see the correct output is still displaying) until the whole function is done executing then it works fine again. Is there another way I can accomplish this goal without using sleep? I am quite new to Qt.

Update I made. I made a new class that will handle the timer, but it does not seem to actually connect the signal and slot. Here is the .h file:

#ifndef TIMERDISPLAY_H
#define TIMERDISPLAY_H

#include <QTimer>
#include <QObject>

class TimerDisplay:public QObject
{
    Q_OBJECT

public:
    TimerDisplay();

public slots:
    void expired();

private:
    QTimer timer;
};

#endif // TIMERDISPLAY_H

and the .cpp file:

#include "timerdisplay.h"
#include <QDebug>

TimerDisplay::TimerDisplay()
{
    connect(&timer, SIGNAL(timeout()), this, SLOT(expired()));
    timer.setSingleShot(false);
    timer.setInterval(1000);
    timer.start();
}

void TimerDisplay::expired()
{
    qDebug()<<"timer expired";
}

Answer 1

Use QTimer or QElapsedTimer if you need more precision.

main.cpp

#include <QTimer>
#include <QCoreApplication>
#include <QString>
#include <QTextStream>
#include <QDebug>

int main(int argc, char **argv)
{
    QCoreApplication application(argc, argv);
    QTimer timer;
    QTextStream textStream(stdout);
    QString word = "apple";
    int i = 0;
    QObject::connect(&timer, &QTimer::timeout, [&textStream, word, &i] () {
        if (i < word.size()) {
            textStream << word.at(i) << flush;
            ++i;
        }
    });
    timer.start(1000);
    return application.exec();
}

main.pro

TEMPLATE = app
TARGET = main
QT = core
CONFIG += c++11
SOURCES += main.cpp

Build and Run

qmake && make && ./main

Output

apple

Answer 2

This is happening, because you're blocking the thread. In most cases you're interested in using event loop.

You can use timer and increment a counter in a slot function. Every QObject has support for timer. You can start it using int QObject::startTimer(int interval) and it will call virtual void timerEvent(QTimerEvent *event) every interval miliseconds (it's a virtual method - reimplement it).

You can also use QTimer and connect QTimer::timeout() signal to accomplish the same thing.

Inside timer handler increment a counter and print the character.

It's a good idea to put your hands on finite state machine concept. FSMs are a great tool for solving similar problems problems. In fact, using a timer callback you create a state machine, but very simple one.

Answer 3

I still don't know what the problem is, but I found a make-shift solution. In my program I include one class (eg class1) into my mainwindow.cpp and from that class I include the timer class. I solved the problem by removing the timer class and adding all those functions to class1. Doesn't make sense to me, but it works.