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  设计模式之工厂模式(一)

        在阅读Qt网络部分源码时候，发现在某处运用了工厂模式，而且编程技巧也用的好，于是就想分享出来，供大家参考，理解的不对的地方请多多指点。

        以下是我整理出来的类图：

关键说明：

1.Q_GLOBAL_STATIC(QSocketEngineHandlerList, socketHandlers)

Qt实现单例模式：Q_GLOBAL_STATIC和Q_GLOBAL_STATIC_WITH_ARGS_qt 单例宏-CSDN博客

Q_GLOBAL_STATIC宏定义了一个全局变量，这个全局变量是定义在qabstractsocketengine.cpp中，它的定义如下：

class QSocketEngineHandlerList : public QList<QSocketEngineHandler*>
{
public:QMutex mutex;
};Q_GLOBAL_STATIC(QSocketEngineHandlerList, socketHandlers)

在cpp中定义，其它地方是访问不到这个全局变量，隐藏了实现，封装性比较好。

2.QAbstractSocketEngine和QHttpSocketEngine

在工厂模式中这个类就相当于是需要生产的对象，它是一个接口类，一般都是通过继承它来实现具体的功能。本例中QHttpSocketEngine就是实际的具有某个功能的类。

3.QSocketEngineHandler

这个类类似工厂模式中的工厂，它的定义如下：

class Q_AUTOTEST_EXPORT QSocketEngineHandler
{
protected:QSocketEngineHandler();virtual ~QSocketEngineHandler();virtual QAbstractSocketEngine *createSocketEngine(QAbstractSocket::SocketType socketType,const QNetworkProxy &, QObject *parent) = 0;virtual QAbstractSocketEngine *createSocketEngine(qintptr socketDescriptor, QObject *parent) = 0;private:friend class QAbstractSocketEngine;
};

QSocketEngineHandler::QSocketEngineHandler()
{if (!socketHandlers())return;QMutexLocker locker(&socketHandlers()->mutex);socketHandlers()->prepend(this);
}QSocketEngineHandler::~QSocketEngineHandler()
{if (!socketHandlers())return;QMutexLocker locker(&socketHandlers()->mutex);socketHandlers()->removeAll(this);
}

从上面的代码可以看出，在QSocketEngineHandler的构造函数和析构函数分别去注册和移除工厂，让继承QSocketEngineHandler的类也不用暴露socketHandlers，这个地方也是写的比较巧妙的地方。

4.QHttpSocketEngineHandler

具体的工厂类，负责生产QHttpSocketEngine，代码如下：

class Q_AUTOTEST_EXPORT QHttpSocketEngineHandler : public QSocketEngineHandler
{
public:virtual QAbstractSocketEngine *createSocketEngine(QAbstractSocket::SocketType socketType,const QNetworkProxy &, QObject *parent) override;virtual QAbstractSocketEngine *createSocketEngine(qintptr socketDescripter, QObject *parent) override;
};

QAbstractSocketEngine *QHttpSocketEngineHandler::createSocketEngine(QAbstractSocket::SocketType socketType,const QNetworkProxy &proxy,QObject *parent)
{if (socketType != QAbstractSocket::TcpSocket)return 0;// proxy type must have been resolved by nowif (proxy.type() != QNetworkProxy::HttpProxy)return 0;// we only accept active socketsif (!qobject_cast<QAbstractSocket *>(parent))return 0;QHttpSocketEngine *engine = new QHttpSocketEngine(parent);engine->setProxy(proxy);return engine;
}QAbstractSocketEngine *QHttpSocketEngineHandler::createSocketEngine(qintptr, QObject *)
{return 0;
}

5.createSocketEngine()

在QAbstractSocketEngine类有两个静态函数，就是生产对象的入口：

class Q_AUTOTEST_EXPORT QAbstractSocketEngine : public QObject
{Q_OBJECT
public:static QAbstractSocketEngine *createSocketEngine(QAbstractSocket::SocketType socketType, const QNetworkProxy &, QObject *parent);static QAbstractSocketEngine *createSocketEngine(qintptr socketDescriptor, QObject *parent);QAbstractSocketEngine(QObject *parent = 0);...
};

QAbstractSocketEngine *QAbstractSocketEngine::createSocketEngine(QAbstractSocket::SocketType socketType, const QNetworkProxy &proxy, QObject *parent)
{
#ifndef QT_NO_NETWORKPROXY// proxy type must have been resolved by nowif (proxy.type() == QNetworkProxy::DefaultProxy)return 0;
#endifQMutexLocker locker(&socketHandlers()->mutex);for (int i = 0; i < socketHandlers()->size(); i++) {if (QAbstractSocketEngine *ret = socketHandlers()->at(i)->createSocketEngine(socketType, proxy, parent))return ret;}#ifndef QT_NO_NETWORKPROXY// only NoProxy can have reached hereif (proxy.type() != QNetworkProxy::NoProxy)return 0;
#endifreturn new QNativeSocketEngine(parent);
}QAbstractSocketEngine *QAbstractSocketEngine::createSocketEngine(qintptr socketDescripter, QObject *parent)
{QMutexLocker locker(&socketHandlers()->mutex);for (int i = 0; i < socketHandlers()->size(); i++) {if (QAbstractSocketEngine *ret = socketHandlers()->at(i)->createSocketEngine(socketDescripter, parent))return ret;}return new QNativeSocketEngine(parent);
}

在函数createSocketEngine中依次循环调用socketHandlers()来创建QAbstractSocketEngine，这里的类型过滤是在具体的创建函数中，比如QHttpSocketEngineHandler::createSocketEngine。

6.总结

        虽然工厂模式并不复杂，但是要在实际项目中灵活运用，也不是一件容易的事，希望我的分享会对你更好的理解工厂模式。