Qt小技巧（1）

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一、Qt实现睡眠函数

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自己定义qSleep(int ms)

#ifdef Q_OS_WIN
#include <windows.h> // for Sleep
#endif

#ifdef Q_OS_UNIX
#include <time.h>
#endif

void qSleep(int ms)
{
#ifdef Q_OS_WIN
    Sleep( uint(ms) );
#else
    struct timespec ts = {
        ms / 1000, (ms % 1000) * 1000 * 1000
    };
    nanosleep(&ts, NULL);
#endif
}

二、qDebug()输出unicode码的问题

用qDebug()的时候，无意间发现了一个问题。输出QString,如果QString里面有中文，则输出其unicode码。
看下面代码：

QString s("你好");
qDebug() << s;
//输出："\u4F60\u597D"。输出unicode码，带双引号。

qDebug().noquote() << s;
//输出为：你好。输出了中文并且不带引号。

qDebug() << s.toStdString().c_str();
//输出为：你好。输出了中文并且不带引号。

为什么一个去掉引号就不一样了呢？我打开Qt帮助文档仔细看了一下QDebug类，愣是没看出什么来。于是在网上搜了一下QDebug，看到了官方在线帮助文档的QDebug类的说明，才明白了。

QDebug & QDebug::operator<<(const QString & s)

**Writes the string, s, to the stream and returns a reference to the stream. Normally, QDebug prints the string inside quotes and transforms non-printable characters to their Unicode values (\u1234). **

To print non-printable characters without transformation, enable the noquote() functionality. Note that some QDebug backends might not be 8-bit clean.

我就纳闷了，为什么我本地的帮助文档和Qt官方网站的在线文档竟然不一样？幸运的是，只要有网，不用*就可以看......

三、Qt实现应用程序的单例化

通过编写一个QSingleApplication类，来实现Qt程序的单例化。

两种方法：

方案一：使用Qt中的QSharedMemory,QLocalServer和QLocalSocket实现(不过需要在你的.pro里加上QT += network)

//"single_application.h"
#ifndef SINGLE_APPLICATION_H

#ifndef SINGLE_APPLICATION_H
#define SINGLE_APPLICATION_H

#include <QApplication>
#include <QSharedMemory>
#include <QLocalServer>

class SingleApplication : public QApplication
{
    Q_OBJECT

public:
    SingleApplication(int &argc, char *argv[], const QString uniqueKey);
    bool isRunning();
    bool sendMessage(const QString &message);

public slots:
    void receiveMessage();

signals:
    void messageAvailable(QString message);

private:
    bool _isRunning;
    QString _uniqueKey;
    QSharedMemory sharedMemory;
    QLocalServer *localServer;
    static const int timeout = 1000;
};

#endif // SINGLE_APPLICATION_H

// "single_application.cpp"
#include <QLocalSocket>
#include "single_application.h"

SingleApplication::SingleApplication(int &argc, char *argv[], const QString uniqueKey) : QApplication(argc, argv), _uniqueKey(uniqueKey)
{
    sharedMemory.setKey(_uniqueKey);
    if (sharedMemory.attach())
        _isRunning = true;
    else
    {
        _isRunning = false;
        // create shared memory.
        if (!sharedMemory.create(1))
        {
            qDebug("Unable to create single instance.");
            return;
        }

        // create local server and listen to incomming messages from other instances.
        localServer = new QLocalServer(this);
        connect(localServer, SIGNAL(newConnection()), this, SLOT(receiveMessage()));
        localServer->listen(_uniqueKey);
    }
}

// public slots.
void SingleApplication::receiveMessage()
{
    QLocalSocket *localSocket = localServer->nextPendingConnection();
    if (!localSocket->waitForReadyRead(timeout))
    {
        qDebug(localSocket->errorString().toLatin1());
        return;
    }

    QByteArray byteArray = localSocket->readAll();
    QString message = QString::fromUtf8(byteArray.constData());
    emit messageAvailable(message);
    localSocket->disconnectFromServer();
}

// public functions.
bool SingleApplication::isRunning()
{
    return _isRunning;
}

bool SingleApplication::sendMessage(const QString &message)
{
    if (!_isRunning)
        return false;
    QLocalSocket localSocket(this);
    localSocket.connectToServer(_uniqueKey, QIODevice::WriteOnly);
    if (!localSocket.waitForConnected(timeout))
    {
        qDebug(localSocket.errorString().toLatin1());
        return false;
    }

    localSocket.write(message.toUtf8());
    if (!localSocket.waitForBytesWritten(timeout))
    {
        qDebug(localSocket.errorString().toLatin1());
        return false;
    }

    localSocket.disconnectFromServer();
    return true;
}

方案二：使用Qt中的QSharedMemory,和QTimert实现

// "single_application.h"
#ifndef SINGLE_APPLICATION_H
#define SINGLE_APPLICATION_H

#include <QApplication>
#include <QSharedMemory>

class SingleApplication : public QApplication
{
    Q_OBJECT
public:
    SingleApplication(int &argc, char *argv[], const QString uniqueKey);
    bool isRunning();
    bool sendMessage(const QString &message);
public slots:
    void checkForMessage();
signals:
    void messageAvailable(QString message);
private:
    bool _isRunning;
    QSharedMemory sharedMemory;
};

#endif // SINGLE_APPLICATION_H

// "single_application.cpp"
#include <QTimer>
#include <QByteArray>
#include "single_application.h"

SingleApplication::SingleApplication(int &argc, char *argv[], const QString uniqueKey) : QApplication(argc, argv)
{
    sharedMemory.setKey(uniqueKey);
    if (sharedMemory.attach())
        _isRunning = true;
    else
    {
        _isRunning = false;
        // attach data to shared memory.
        QByteArray byteArray("0"); // default value to note that no message is available.
        if (!sharedMemory.create(byteArray.size()))
        {
            qDebug("Unable to create single instance.");
            return;
        }
        sharedMemory.lock();
        char *to = (char*)sharedMemory.data();
        const char *from = byteArray.data();
        memcpy(to, from, qMin(sharedMemory.size(), byteArray.size()));
        sharedMemory.unlock();

                // start checking for messages of other instances.
        QTimer *timer = new QTimer(this);
        connect(timer, SIGNAL(timeout()), this, SLOT(checkForMessage()));
        timer->start(1000);
    }
}

// public slots.
void SingleApplication::checkForMessage()
{
    sharedMemory.lock();
    QByteArray byteArray = QByteArray((char*)sharedMemory.constData(), sharedMemory.size());
    sharedMemory.unlock();
    if (byteArray.left(1) == "0")
        return;
    byteArray.remove(0, 1);
    QString message = QString::fromUtf8(byteArray.constData());
    emit messageAvailable(message);

    // remove message from shared memory.
    byteArray = "0";
    sharedMemory.lock();
    char *to = (char*)sharedMemory.data();
    const char *from = byteArray.data();
    memcpy(to, from, qMin(sharedMemory.size(), byteArray.size()));
    sharedMemory.unlock();
}

// public functions.
bool SingleApplication::isRunning()
{
    return _isRunning;
}

bool SingleApplication::sendMessage(const QString &message)
{
    if (!_isRunning)
        return false;

    QByteArray byteArray("1");
    byteArray.append(message.toUtf8());
    byteArray.append('/0'); // < should be as char here, not a string!
    sharedMemory.lock();
    char *to = (char*)sharedMemory.data();
    const char *from = byteArray.data();
    memcpy(to, from, qMin(sharedMemory.size(), byteArray.size()));
    sharedMemory.unlock();
    return true;
}

使用:

// "main.cpp"
#include "single_application.h"
int main(int argc, char *argv[])
{
    SingleApplication app(argc, argv, "some unique key string");
    if(app.isRunning())
    {
        app.sendMessage( QObject::tr("message from other instance.") );
        QMessageBox::critical( 0, QObject::tr("myScreenShot"),QObject::tr("myScreenShot is already running !!") );
        return 0;
    }

    MainWindow *mainWindow = new MainWindow();
    mainWindow->show();

    return app.exec();
}

总结

这些技巧在做实际项目的时候绝对会用到的，也许不在今天，就在明天。