Qt：让任意线程执行一个匿名函数

本类主要功能是在当前线程（比如说主线程），指派任意一个线程（比如说某个工作线程）去执行一个匿名函数。

注意，这个和QtConcurrent配合QThreadPool不一样，QtConcurrent配合QThreadPool只能指派回调到QThreadPool中的线程。
而这个类可以指派一个回调到任意线程。

两个主要接口

JasonQt_InvokeFromThread::invoke：非阻塞，只是将回调放到队列中，等待执行

JasonQt_InvokeFromThread::waitForInvoke：阻塞，函数执行完成后才回返回

代码部分：

.h文件内容：

#include <QThread>
#include <QMap>
#include <QDebug>

class JasonQt_InvokeFromThreadHelper: public QObject
{
Q_OBJECT

private:
QMutex m_mutex;
QList< std::function<void()> > m_waitCallbacks;

public:
void addCallback(const std::function<void()> &callback);

public slots:
void onRun();
};

class JasonQt_InvokeFromThread
{
private:
static QMutex g_mutex;
static QMap< QThread *, JasonQt_InvokeFromThreadHelper * > g_helpers;

public:
static void invoke(QThread *thread, const std::function<void()> &callback);

static void waitForInvoke(QThread *thread, const std::function<void()> &callback);
};

.cpp文件内容

// JasonQt_InvokeFromThreadHelper
void JasonQt_InvokeFromThreadHelper::addCallback(const std::function<void ()> &callback)
{
m_mutex.lock();
m_waitCallbacks.push_back(callback);
m_mutex.unlock();
}

void JasonQt_InvokeFromThreadHelper::onRun()
{
if(!m_waitCallbacks.isEmpty())
{
m_mutex.lock();

auto callback = m_waitCallbacks.first();
m_waitCallbacks.pop_front();

m_mutex.unlock();

callback();
}
}

// JasonQt_InvokeFromThread
QMutex JasonQt_InvokeFromThread::g_mutex;
QMap< QThread *, JasonQt_InvokeFromThreadHelper * > JasonQt_InvokeFromThread::g_helpers;

void JasonQt_InvokeFromThread::invoke(QThread *thread, const std::function<void ()> &callback)
{
if(!(thread->isRunning()))
{
qDebug() << "JasonQt_InvokeFromThread::invoke: Target thread" << thread << "is not running!";
return;
}

g_mutex.lock();

auto it = g_helpers.find(thread);

if(it == g_helpers.end())
{
auto helper = new JasonQt_InvokeFromThreadHelper;
helper->moveToThread(thread);

QObject::connect(thread, &QThread::finished, [=]()
{
g_mutex.lock();

auto it = g_helpers.find(thread);
if(it != g_helpers.end())
{
g_helpers.erase(it);
}

g_mutex.unlock();
});

g_helpers.insert( thread, helper );
it = g_helpers.find(thread);
}

it.value()->addCallback(callback);

QMetaObject::invokeMethod(it.value(), "onRun", Qt::QueuedConnection);

g_mutex.unlock();
}

void JasonQt_InvokeFromThread::waitForInvoke(QThread *thread, const std::function<void ()> &callback)
{
if(!(thread->isRunning()))
{
qDebug() << "JasonQt_InvokeFromThread::waitForInvoke: Target thread" << thread << "is not running!";
return;
}

g_mutex.lock();

auto it = g_helpers.find(thread);

if(it == g_helpers.end())
{
auto helper = new JasonQt_InvokeFromThreadHelper;
helper->moveToThread(thread);

QObject::connect(thread, &QThread::finished, [=]()
{
g_mutex.lock();

auto it = g_helpers.find(thread);
if(it != g_helpers.end())
{
g_helpers.erase(it);
}

g_mutex.unlock();
});

g_helpers.insert( thread, helper );
it = g_helpers.find(thread);
}

it.value()->addCallback([&]()
{
g_mutex.unlock();
callback();
});

QMetaObject::invokeMethod(it.value(), "onRun", Qt::QueuedConnection);

g_mutex.lock();
g_mutex.unlock();
}

测试代码：

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

qDebug() << "Main thread:" << QThread::currentThread();

constexpr auto threadCount = 5;
QVector< QObject * > objects;
QThreadPool threadPool;

objects.resize(threadCount);
threadPool.setMaxThreadCount(threadCount);

for(auto i = 0; i < threadCount; i++)
{
QtConcurrent::run([&objects, i]()
{
objects[i] = new QObject;

qDebug() << "Thread started:" << QThread::currentThread();

QEventLoop().exec();
});
}

// 等待测试线程启动
QThread::sleep(3);

// 调用
for(auto i = 0; i < (threadCount * 2); i++)
{
// 第一个参数是目标线程，第二个是回调
JasonQt_InvokeFromThread::invoke(objects[i % threadCount]->thread(), [=]()
{
qDebug() << "Current thread:" << QThread::currentThread() << ", Flag:" << i;
});
}

return a.exec();
}

执行输出

Main thread: QThread(0x7f821951bf30)
Thread started: QThread(0x7f8219705ac0, name = "Thread (pooled)")
Thread started: QThread(0x7f8219705f90, name = "Thread (pooled)")
Thread started: QThread(0x7f82197055f0, name = "Thread (pooled)")
Thread started: QThread(0x7f8219705120, name = "Thread (pooled)")
Thread started: QThread(0x7f8219704950, name = "Thread (pooled)")
Current thread: QThread(0x7f8219704950, name = "Thread (pooled)") , Flag: 0
Current thread: QThread(0x7f8219705ac0, name = "Thread (pooled)") , Flag: 3
Current thread: QThread(0x7f8219705120, name = "Thread (pooled)") , Flag: 1
Current thread: QThread(0x7f82197055f0, name = "Thread (pooled)") , Flag: 2
Current thread: QThread(0x7f8219705f90, name = "Thread (pooled)") , Flag: 4
Current thread: QThread(0x7f8219704950, name = "Thread (pooled)") , Flag: 5
Current thread: QThread(0x7f8219705ac0, name = "Thread (pooled)") , Flag: 8
Current thread: QThread(0x7f8219705120, name = "Thread (pooled)") , Flag: 6
Current thread: QThread(0x7f8219705f90, name = "Thread (pooled)") , Flag: 9
Current thread: QThread(0x7f82197055f0, name = "Thread (pooled)") , Flag: 7

可以看见，回调被执行在了测试线程中。

注：目标线程需要有运行Qt的事件循环，这是必须的。