c++ - 如何在 Qt、GCD 风格的给定线程中执行仿函数或 lambda？

Question

在带有 GCD 的 ObjC 中，有一种方法可以在任何旋转事件循环的线程中执行 lambda。例如:

dispatch_sync(dispatch_get_main_queue(), ^{ /* do sth */ });

或者:

dispatch_async(dispatch_get_main_queue(), ^{ /* do sth */ });

它在主线程的队列中执行一些(相当于 C++ 中的 []{ /* do sth */ })，要么阻塞要么异步。

我如何在 Qt 中做同样的事情？

从我读过的内容来看，我想解决方案是以某种方式向主线程的某个对象发送信号。但什么对象？刚刚 QApplication::instance() ? (那是当时唯一存在于主线程中的对象。)什么信号？

从目前的答案和我目前的研究来看，似乎我真的需要一些虚拟对象来坐在主线程中，并带有一些插槽，它只是等待进入一些代码来执行。

所以，我决定子类 QApplication补充一点。我当前的代码不起作用(但也许您可以提供帮助):

#include <QApplication>
#include <QThread>
#include <QMetaMethod>
#include <functional>
#include <assert.h>

class App : public QApplication
{
    Q_OBJECT

public:
    App();

signals:

public slots:
    void genericExec(std::function<void(void)> func) {
        func();
    }

private:
    // cache this
    QMetaMethod genericExec_method;
public:
    void invokeGenericExec(std::function<void(void)> func, Qt::ConnectionType connType) {
        if(!genericExec_method) {
            QByteArray normalizedSignature = QMetaObject::normalizedSignature("genericExec(std::function<void(void)>)");
            int methodIndex = this->metaObject()->indexOfSlot(normalizedSignature);
            assert(methodIndex >= 0);
            genericExec_method = this->metaObject()->method(methodIndex);
        }
        genericExec_method.invoke(this, connType, Q_ARG(std::function<void(void)>, func));
    }

};

static inline
void execInMainThread_sync(std::function<void(void)> func) {
    if(qApp->thread() == QThread::currentThread())
        func();
    else {
        ((App*) qApp)->invokeGenericExec(func, Qt::BlockingQueuedConnection);
    }
}

static inline
void execInMainThread_async(std::function<void(void)> func) {
    ((App*) qApp)->invokeGenericExec(func, Qt::QueuedConnection);
}

Answer 1

这当然是可能的。任何解决方案都以传递一个事件为中心，该事件将仿函数包装到驻留在所需线程中的消费者对象。我们将此操作称为元调用发布。可以以多种方式执行细节。

Qt 5.10 及更高版本 TL;DR

// invoke on the main thread
QMetaObject::invokeMethod(qApp, []{ ... });

// invoke on an object's thread
QMetaObject::invokeMethod(obj, []{ ... });

// invoke on a particular thread
QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thread),
                         []{ ... });

TL; DR 用于仿函数

// https://github.com/KubaO/stackoverflown/tree/master/questions/metacall-21646467

// Qt 5.10 & up - it's all done

template <typename F>
static void postToObject(F &&fun, QObject *obj = qApp) {
  QMetaObject::invokeMethod(obj, std::forward<F>(fun));
}

template <typename F>
static void postToThread(F && fun, QThread *thread = qApp->thread()) {
   auto *obj = QAbstractEventDispatcher::instance(thread);
   Q_ASSERT(obj);
   QMetaObject::invokeMethod(obj, std::forward<F>(fun));
}

// Qt 5/4 - preferred, has least allocations

namespace detail {
template <typename F>
struct FEvent : public QEvent {
   using Fun = typename std::decay<F>::type;
   Fun fun;
   FEvent(Fun && fun) : QEvent(QEvent::None), fun(std::move(fun)) {}
   FEvent(const Fun & fun) : QEvent(QEvent::None), fun(fun) {}
   ~FEvent() { fun(); }
}; }

template <typename F>
static void postToObject(F && fun, QObject * obj = qApp) {
   if (qobject_cast<QThread*>(obj))
      qWarning() << "posting a call to a thread object - consider using postToThread";
   QCoreApplication::postEvent(obj, new detail::FEvent<F>(std::forward<F>(fun)));
}

template <typename F>
static void postToThread(F && fun, QThread * thread = qApp->thread()) {
   QObject * obj = QAbstractEventDispatcher::instance(thread);
   Q_ASSERT(obj);
   QCoreApplication::postEvent(obj, new detail::FEvent<F>(std::forward<F>(fun)));
}

// Qt 5 - alternative version

template <typename F>
static void postToObject2(F && fun, QObject * obj = qApp) {
   if (qobject_cast<QThread*>(obj))
      qWarning() << "posting a call to a thread object - consider using postToThread";
   QObject src;
   QObject::connect(&src, &QObject::destroyed, obj, std::forward<F>(fun),
                    Qt::QueuedConnection);
}

template <typename F>
static void postToThread2(F && fun, QThread * thread = qApp->thread()) {
   QObject * obj = QAbstractEventDispatcher::instance(thread);
   Q_ASSERT(obj);
   QObject src;
   QObject::connect(&src, &QObject::destroyed, obj, std::forward<F>(fun),
                    Qt::QueuedConnection);
}

void test1() {
   QThread t;
   QObject o;
   o.moveToThread(&t);

   // Execute in given object's thread
   postToObject([&]{ o.setObjectName("hello"); }, &o);
   // or
   postToObject(std::bind(&QObject::setObjectName, &o, "hello"), &o);

   // Execute in given thread
   postToThread([]{ qDebug() << "hello from worker thread"; });

   // Execute in the main thread
   postToThread([]{ qDebug() << "hello from main thread"; });
}

TL;DR 用于方法/插槽

// Qt 5/4
template <typename T, typename R>
static void postToObject(T * obj, R(T::* method)()) {
   struct Event : public QEvent {
      T * obj;
      R(T::* method)();
      Event(T * obj, R(T::*method)()):
         QEvent(QEvent::None), obj(obj), method(method) {}
      ~Event() { (obj->*method)(); }
   };
   if (qobject_cast<QThread*>(obj))
      qWarning() << "posting a call to a thread object - this may be a bug";
   QCoreApplication::postEvent(obj, new Event(obj, method));
}

void test2() {
   QThread t;
   struct MyObject : QObject { void method() {} } obj;
   obj.moveToThread(&t);

   // Execute in obj's thread
   postToObject(&obj, &MyObject::method);
}

TL;DR:单发定时器怎么样？

以上所有方法都适用于没有事件循环的线程。由于 QTBUG-66458 ，方便的拨款 QTimer::singleShot在源线程中也需要一个事件循环。然后 postToObject变得非常简单，你可能只需要使用 QTimer::singleShot直接，虽然这是一个尴尬的名字，对那些不熟悉这个习语的人隐藏了意图。即使您不需要类型检查，通过名为更好地指示意图的函数进行的间接也是有意义的:

template <typename F>
static void postToObject(F && fun, QObject * obj = qApp) {
   if (qobject_cast<QThread*>(obj))
      qWarning() << "posting a call to a thread object - consider using postToThread";
   QTimer::singleShot(0, obj, std::forward<F>(fun));
}

通用代码

让我们根据以下通用代码来定义我们的问题。最简单的解决方案是将事件发布到应用程序对象(如果目标线程是主线程)或任何其他给定线程的事件调度程序。由于事件调度器仅在 QThread::run 之后才存在。已输入，我们通过从 needsRunningThread 返回 true 来指示线程运行的要求。 .

#ifndef HAS_FUNCTORCALLCONSUMER
namespace FunctorCallConsumer {
   bool needsRunningThread() { return true; }
   QObject * forThread(QThread * thread) {
      Q_ASSERT(thread);
      QObject * target = thread == qApp->thread()
            ? static_cast<QObject*>(qApp) : QAbstractEventDispatcher::instance(thread);
      Q_ASSERT_X(target, "postMetaCall", "the receiver thread must have an event loop");
      return target;
   }
}
#endif

元调用发布函数，以其最简单的形式，要求仿函数调用消费者为给定线程提供对象，并实例化仿函数调用事件。事件的实现还在前面，是各种实现之间的本质区别。

第二个重载接受仿函数的右值引用，可能会保存对仿函数的复制操作。如果延续包含复制成本高的数据，这将很有帮助。

#ifndef HAS_POSTMETACALL
void postMetaCall(QThread * thread, const std::function<void()> & fun) {
   auto receiver = FunctorCallConsumer::forThread(thread);
   QCoreApplication::postEvent(receiver, new FunctorCallEvent(fun, receiver));
}

void postMetaCall(QThread * thread, std::function<void()> && fun) {
   auto receiver = FunctorCallConsumer::forThread(thread);
   QCoreApplication::postEvent(receiver,
                               new FunctorCallEvent(std::move(fun), receiver));
}
#endif

出于演示目的，工作线程首先向主线程发布元调用，然后遵循 QThread::run()启动一个事件循环来监听来自其他线程的可能的元调用。如果消费者的实现需要，互斥用于允许线程用户以简单的方式等待线程启动。对于上面给出的默认事件使用者，这种等待是必要的。

class Worker : public QThread {
   QMutex m_started;
   void run() {
      m_started.unlock();
      postMetaCall(qApp->thread(), []{
         qDebug() << "worker functor executes in thread" << QThread::currentThread();
      });
      QThread::run();
   }
public:
   Worker(QObject * parent = 0) : QThread(parent) { m_started.lock(); }
   ~Worker() { quit(); wait(); }
   void waitForStart() { m_started.lock(); m_started.unlock(); }
};

最后，我们启动上面的工作线程，向主(应用程序)线程发布元调用，应用程序线程向工作线程发布元调用。

int main(int argc, char *argv[])
{
   QCoreApplication a(argc, argv);
   a.thread()->setObjectName("main");
   Worker worker;
   worker.setObjectName("worker");
   qDebug() << "worker thread:" << &worker;
   qDebug() << "main thread:" << QThread::currentThread();
   if (FunctorCallConsumer::needsRunningThread()) {
      worker.start();
      worker.waitForStart();
   }
   postMetaCall(&worker, []{ qDebug() << "main functor executes in thread" << QThread::currentThread(); });
   if (!FunctorCallConsumer::needsRunningThread()) worker.start();
   QMetaObject::invokeMethod(&a, "quit", Qt::QueuedConnection);
   return a.exec();
}

在所有实现中，输出将大致如下所示。仿函数跨线程:在主线程中创建的仿函数在工作线程中执行，反之亦然。

worker thread: QThread(0x7fff5692fc20, name = "worker") 
main thread: QThread(0x7f86abc02f00, name = "main") 
main functor executes in thread QThread(0x7fff5692fc20, name = "worker") 
worker functor executes in thread QThread(0x7f86abc02f00, name = "main") 

Qt 5 使用临时对象作为信号源的解决方案

Qt 5 最简单的方法是使用临时 QObject作为信号源，并将仿函数连接到它的 destroyed(QObject*)信号。当 postMetaCall返回， signalSource被破坏，发出它的 destroyed信号，并将元调用发布到代理对象。

这可能是 C++11 风格中最简洁直接的实现。 signalSource对象以 C++11 RAII 方式用于其销毁的副作用。短语“副作用”在 C++11 的语义中具有含义，不应被解释为“不可靠”或“不受欢迎”——它绝非如此。 QObject与我们的合约是发射 destroyed在其析构函数执行期间的某个时间。我们非常欢迎使用这一事实。

#include <QtCore>
#include <functional>

namespace FunctorCallConsumer { QObject * forThread(QThread*); }

#define HAS_POSTMETACALL
void postMetaCall(QThread * thread, const std::function<void()> & fun) {
   QObject signalSource;
   QObject::connect(&signalSource, &QObject::destroyed,
                    FunctorCallConsumer::forThread(thread), [=](QObject*){ fun(); });
}
#ifdef __cpp_init_captures
void postMetaCall(QThread * thread, std::function<void()> && fun) {
   QObject signalSource;
   QObject::connect(&signalSource, &QObject::destroyed,
                    FunctorCallConsumer::forThread(thread), [fun(std::move(fun))](QObject*){ fun(); });
}
#endif
// Common Code follows here

如果我们只打算发布到主线程，代码几乎变得微不足道:

void postToMainThread(const std::function<void()> & fun) {
  QObject signalSource;
  QObject::connect(&signalSource, &QObject::destroyed, qApp, [=](QObject*){
    fun();
  });
}

#ifdef __cpp_init_captures
void postToMainThread(std::function<void()> && fun) {
  QObject signalSource;
  QObject::connect(&signalSource, &QObject::destroyed, qApp, [fun(std::move(fun))](QObject*){
    fun();
  });
}
#endif

使用 QEvent 析构函数的 Qt 4/5 解决方案

同样的方法可以应用于 QEvent直接地。事件的虚拟析构函数可以调用仿函数。事件在被消费者对象的线程的事件调度器传递后立即被删除，因此它们总是在正确的线程中执行。这在 Qt 4/5 中不会改变。

#include <QtCore>
#include <functional>

class FunctorCallEvent : public QEvent {
   std::function<void()> m_fun;
   QThread * m_thread;
public:
   FunctorCallEvent(const std::function<void()> & fun, QObject * receiver) :
      QEvent(QEvent::None), m_fun(fun), m_thread(receiver->thread()) {}
   FunctorCallEvent(std::function<void()> && fun, QObject * receiver) :
      QEvent(QEvent::None), m_fun(std::move(fun)), m_thread(receiver->thread()) { qDebug() << "move semantics"; }
   ~FunctorCallEvent() {
      if (QThread::currentThread() == m_thread)
         m_fun();
      else
         qWarning() << "Dropping a functor call destined for thread" << m_thread;
   }
};
// Common Code follows here

仅发布到主线程，事情变得更加简单:

class FunctorCallEvent : public QEvent {
   std::function<void()> m_fun;
public:
   FunctorCallEvent(const std::function<void()> & fun) :
      QEvent(QEvent::None), m_fun(fun) {}
   FunctorCallEvent(std::function<void()> && fun, QObject * receiver) :
      QEvent(QEvent::None), m_fun(std::move(fun)) {}
   ~FunctorCallEvent() {
      m_fun();
   }
};

void postToMainThread(const std::function<void()> & fun) {
   QCoreApplication::postEvent(qApp, new FunctorCallEvent(fun);
}

void postToMainThread(std::function<void()> && fun) {
   QCoreApplication::postEvent(qApp, new FunctorCallEvent(std::move(fun)));
}

使用私有(private) QMetaCallEvent 的 Qt 5 解决方案

仿函数可以包裹在 QMetaCallEvent 的 Qt 5 插槽对象负载中。 .仿函数将由 QObject::event 调用，因此可以发布到目标线程中的任何对象。此解决方案使用 Qt 5 的私有(private)实现细节。

#include <QtCore>
#include <private/qobject_p.h>
#include <functional>

class FunctorCallEvent : public QMetaCallEvent {
public:
   template <typename Functor>
   FunctorCallEvent(Functor && fun, QObject * receiver) :
      QMetaCallEvent(new QtPrivate::QFunctorSlotObject<Functor, 0, typename QtPrivate::List_Left<void, 0>::Value, void>
                     (std::forward<Functor>(fun)), receiver, 0, 0, 0, (void**)malloc(sizeof(void*))) {}
   // Metacalls with slot objects require an argument array for the return type, even if it's void.
};
// Common Code follows here

使用自定义事件和使用者的 Qt 4/5 解决方案

我们重新实现了 event()对象的方法，并让它调用仿函数。这在仿函数被发布到的每个线程中调用一个显式的事件消费者对象。对象在其线程完成时被清除，或者对于主线程，当应用程序实例被破坏时。它适用于 Qt 4 和 Qt 5。右值引用的使用避免了临时仿函数的复制。

#include <QtCore>
#include <functional>

class FunctorCallEvent : public QEvent {
   std::function<void()> m_fun;
public:
   FunctorCallEvent(const std::function<void()> & fun, QObject *) :
      QEvent(QEvent::None), m_fun(fun) {}
   FunctorCallEvent(std::function<void()> && fun, QObject *) :
      QEvent(QEvent::None), m_fun(std::move(fun)) { qDebug() << "move semantics"; }
   void call() { m_fun(); }
};

#define HAS_FUNCTORCALLCONSUMER
class FunctorCallConsumer : public QObject {
   typedef QMap<QThread*, FunctorCallConsumer*> Map;
   static QObject * m_appThreadObject;
   static QMutex m_threadObjectMutex;
   static Map m_threadObjects;
   bool event(QEvent * ev) {
      if (!dynamic_cast<FunctorCallEvent*>(ev)) return QObject::event(ev);
      static_cast<FunctorCallEvent*>(ev)->call();
      return true;
   }
   FunctorCallConsumer() {}
   ~FunctorCallConsumer() {
      qDebug() << "consumer done for thread" << thread();
      Q_ASSERT(thread());
      QMutexLocker lock(&m_threadObjectMutex);
      m_threadObjects.remove(thread());
   }
   static void deleteAppThreadObject() {
      delete m_appThreadObject;
      m_appThreadObject = nullptr;
   }
public:
   static bool needsRunningThread() { return false; }
   static FunctorCallConsumer * forThread(QThread * thread) {
      QMutexLocker lock(&m_threadObjectMutex);
      Map map = m_threadObjects;
      lock.unlock();
      Map::const_iterator it = map.find(thread);
      if (it != map.end()) return *it;
      FunctorCallConsumer * consumer = new FunctorCallConsumer;
      consumer->moveToThread(thread);
      if (thread != qApp->thread())
         QObject::connect(thread, SIGNAL(finished()), consumer, SLOT(deleteLater()));
      lock.relock();
      it = m_threadObjects.find(thread);
      if (it == m_threadObjects.end()) {
         if (thread == qApp->thread()) {
            Q_ASSERT(! m_appThreadObject);
            m_appThreadObject = consumer;
            qAddPostRoutine(&deleteAppThreadObject);
         }
         m_threadObjects.insert(thread, consumer);
         return consumer;
      } else {
         delete consumer;
         return *it;
      }
   }
};

QObject * FunctorCallConsumer::m_appThreadObject = nullptr;
QMutex FunctorCallConsumer::m_threadObjectMutex;
FunctorCallConsumer::Map FunctorCallConsumer::m_threadObjects;
// Common Code follows here