如何将Lambda函数排队到Qt的事件循环中? [英] How to queue lambda function into Qt's event loop?
问题描述
基本上,我需要在Java中执行相同的操作:
Basically I need the same thing that is done like this in Java:
SwingUtilities.invokeLater(()->{/* function */});
或者在javascript中这样:
Or like this in javascript:
setTimeout(()=>{/* function */}, 0);
但是使用Qt和lambda.所以是一些伪代码:
But with Qt and lambda. So some pseudocode:
Qt::queuePushMagic([]() { /* function */ });
作为一个附加的麻烦,我需要它在多线程上下文中工作.我实际上想做的是在正确的线程中自动运行某些方法.代码如下所示:
As an additional complication, I need this to work in multithreaded context. What I'm actually trying to do is to automatically run certain methods in correct thread. What the code would then look:
SomeClass::threadSafeAsyncMethod() {
if(this->thread() != QThread::currentThread()) {
Qt::queuePushMagic([this]()=>{ this->threadSafeAsyncMethod() });
return;
}
}
如何执行此操作?
推荐答案
Your problem is of How to leverage Qt to make a QObject method thread-safe? Let's adapt the solutions offered there to your use case. First, let's factor out the safety check:
bool isSafe(QObject * obj) {
Q_ASSERT(obj->thread() || qApp && qApp->thread() == QThread::currentThread());
auto thread = obj->thread() ? obj->thread() : qApp->thread();
return thread == QThread::currentThread();
}
您建议的方法采用函子,并让编译器处理将函子中的参数(如果有的话)打包:
The approach you suggested takes a functor, and lets the compiler deal with packing up the arguments (if any) within the functor:
template <typename Fun> void postCall(QObject * obj, Fun && fun) {
qDebug() << __FUNCTION__;
struct Event : public QEvent {
using F = typename std::decay<Fun>::type;
F fun;
Event(F && fun) : QEvent(QEvent::None), fun(std::move(fun)) {}
Event(const F & fun) : QEvent(QEvent::None), fun(fun) {}
~Event() { fun(); }
};
QCoreApplication::postEvent(
obj->thread() ? obj : qApp, new Event(std::forward<Fun>(fun)));
}
第二种方法是在事件内显式存储所有参数的副本,并且不使用函子:
A second approach stores the copies of all the parameters explicitly within the event and doesn't use a functor:
template <typename Class, typename... Args>
struct CallEvent : public QEvent {
// See https://stackoverflow.com/a/7858971/1329652
// See also https://stackoverflow.com/a/15338881/1329652
template <int ...> struct seq {};
template <int N, int... S> struct gens { using type = typename gens<N-1, N-1, S...>::type; };
template <int ...S> struct gens<0, S...> { using type = seq<S...>; };
template <int ...S> void callFunc(seq<S...>) { (obj->*method)(std::get<S>(args)...); }
Class * obj;
void (Class::*method)(Args...);
std::tuple<typename std::decay<Args>::type...> args;
CallEvent(Class * obj, void (Class::*method)(Args...), Args&&... args) :
QEvent(QEvent::None), obj(obj), method(method), args(std::move<Args>(args)...) {}
~CallEvent() { callFunc(typename gens<sizeof...(Args)>::type()); }
};
template <typename Class, typename... Args> void postCall(Class * obj, void (Class::*method)(Args...), Args&& ...args) {
qDebug() << __FUNCTION__;
QCoreApplication::postEvent(
obj->thread() ? static_cast<QObject*>(obj) : qApp, new CallEvent<Class, Args...>{obj, method, std::forward<Args>(args)...});
}
它的用法如下:
struct Class : QObject {
int num{};
QString str;
void method1(int val) {
if (!isSafe(this))
return postCall(this, [=]{ method1(val); });
qDebug() << __FUNCTION__;
num = val;
}
void method2(const QString &val) {
if (!isSafe(this))
return postCall(this, &Class::method2, val);
qDebug() << __FUNCTION__;
str = val;
}
};
测试工具:
// https://github.com/KubaO/stackoverflown/tree/master/questions/safe-method-40382820
#include <QtCore>
// above code
class Thread : public QThread {
public:
Thread(QObject * parent = nullptr) : QThread(parent) {}
~Thread() { quit(); wait(); }
};
void moveToOwnThread(QObject * obj) {
Q_ASSERT(obj->thread() == QThread::currentThread());
auto thread = new Thread{obj};
thread->start();
obj->moveToThread(thread);
}
int main(int argc, char ** argv) {
QCoreApplication app{argc, argv};
Class c;
moveToOwnThread(&c);
const auto num = 44;
const auto str = QString::fromLatin1("Foo");
c.method1(num);
c.method2(str);
postCall(&c, [&]{ c.thread()->quit(); });
c.thread()->wait();
Q_ASSERT(c.num == num && c.str == str);
}
输出:
postCall
postCall
postCall
method1
method2
上面的代码可以与Qt 4或Qt 5一起编译并使用.
The above compiles and works with either Qt 4 or Qt 5.
另请参见此问题,探讨在Qt中其他线程上下文中调用函子的各种方法.
See also this question, exploring various ways of invoking functors in other thread contexts in Qt.
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