Qt5：与 lambda 的一次性连接
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【中文标题】Qt5：与 lambda 的一次性连接【英文标题】：Qt5: one-shot connection to lambda 【发布时间】：2019-03-05 10:24:05 【问题描述】：如何使用 Qt5.12 创建一次性连接（即第一次激活时自动断开的连接）？我正在寻找一个优雅的解决方案，没有过多的冗长，可以清楚地传达含义。
我正在使用
QObject::connect(instance,Class::signal,this,[this]()
QObject::disconnect(instance,Class::signal,this,0);
/* ... */
);
只有在没有连接其他信号时才有效。
这篇帖子https://forum.qt.io/post/328402建议
QMetaObject::Connection * const connection = new QMetaObject::Connection;
*connection = connect(_textFadeOutAnimation, &QPropertyAnimation::finished, [this, text, connection]()
QObject::disconnect(*connection);
delete connection;
);
即使在存在其他连接的情况下也可以工作，但同样不是很优雅。
SO 有几个关于这个主题的问题，但似乎没有一个解决方案对我有用。例如，https://***.com/a/42989833/761090 使用虚拟对象：
QObject *dummy=new QObject(this);
QObject::connect(instance,Class::signal,[dummy]()
dummy->deleteLater();
);
产生运行时警告：
QObject: Cannot create children for a parent that is in a different thread.
(Parent is ClassInstance(0x561c14ce3a60), parent's thread is QThread(0x561c14e1b050), current thread is QThread(0x561c14c2b530)
模板化解决方案（https://***.com/a/26554206/761090 的第二部分）不能用 c++17 编译。
有更好的建议吗？
编辑：我将此作为请求 Qt 错误跟踪器提交：https://bugreports.qt.io/browse/QTBUG-74547。
【问题讨论】：
【参考方案1】：我从 OPs 问题中采用了（对我来说）最有希望的方法
QMetaObject::Connection * const connection = new QMetaObject::Connection;
*connection = connect(_textFadeOutAnimation, &QPropertyAnimation::finished, [this, text, connection]()
QObject::disconnect(*connection);
delete connection;
);
并考虑如何将其包装成一个函数。实际上，它必须是一个模板函数才能使其可用于任何 Qt 信号：
template <typename Sender, typename Emitter, typename Slot, typename... Args>
QMetaObject::Connection connectOneShot(
Sender *pSender, void (Emitter::*pSignal)(Args ...args), Slot slot)
QMetaObject::Connection *pConnection = new QMetaObject::Connection;
*pConnection
= QObject::connect(pSender, pSignal,
[pConnection, slot](Args... args)
QObject::disconnect(*pConnection);
delete pConnection;
slot(args...);
);
return *pConnection;
我在MCVE 中试过这个：
#include <functional>
#include <QtWidgets>
template <typename Sender, typename Emitter, typename Slot, typename... Args>
QMetaObject::Connection connectOneShot(Sender *pSender, void (Emitter::*pSignal)(Args ...args), Slot slot)
QMetaObject::Connection *pConnection = new QMetaObject::Connection;
*pConnection
= QObject::connect(pSender, pSignal,
[pConnection, slot](Args... args)
QObject::disconnect(*pConnection);
delete pConnection;
slot(args...);
);
return *pConnection;
template <typename Sender, typename Emitter,
typename Receiver, typename Slot, typename... Args>
QMetaObject::Connection connectOneShot(
Sender *pSender, void (Emitter::*pSignal)(Args ...args),
Receiver *pRecv, Slot slot)
QMetaObject::Connection *pConnection = new QMetaObject::Connection;
*pConnection
= QObject::connect(pSender, pSignal,
[pConnection, pRecv, slot](Args... args)
QObject::disconnect(*pConnection);
delete pConnection;
(pRecv->*slot)(args...);
);
return *pConnection;
void onBtnClicked(bool)
static int i = 0;
qDebug() << "onBtnClicked() called:" << ++i;
struct PushButton: public QPushButton
int i = 0;
using QPushButton::QPushButton;
virtual ~PushButton() = default;
void onClicked(bool)
++i;
qDebug() << this << "PushButton::onClicked() called:" << i;
setText(QString("Clicked %1.").arg(i));
;
int main(int argc, char **argv)
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
// setup user interface
QWidget qWinMain;
QGridLayout qGrid;
qGrid.addWidget(new QLabel("Multi Shot"), 0, 1);
qGrid.addWidget(new QLabel("One Shot"), 0, 2);
auto addRow
= [](
QGridLayout &qGrid, const QString &qText,
QWidget &qWidgetMShot, QWidget &qWidgetOneShot)
const int i = qGrid.rowCount();
qGrid.addWidget(new QLabel(qText), i, 0);
qGrid.addWidget(&qWidgetMShot, i, 1);
qGrid.addWidget(&qWidgetOneShot, i, 2);
;
QPushButton qBtnMShotFunc("Click me!");
QPushButton qBtnOneShotFunc("Click me!");
addRow(qGrid, "Function:", qBtnMShotFunc, qBtnOneShotFunc);
PushButton qBtnMShotMemFunc("Click me!");
PushButton qBtnOneShotMemFunc("Click me!");
addRow(qGrid, "Member Function:", qBtnMShotMemFunc, qBtnOneShotMemFunc);
QPushButton qBtnMShotLambda("Click me!");
QPushButton qBtnOneShotLambda("Click me!");
addRow(qGrid, "Lambda:", qBtnMShotLambda, qBtnOneShotLambda);
QLineEdit qEditMShot("Edit me!");
QLineEdit qEditOneShot("Edit me!");
addRow(qGrid, "Lambda:", qEditMShot, qEditOneShot);
qWinMain.setLayout(&qGrid);
qWinMain.show();
// install signal handlers
QObject::connect(&qBtnMShotFunc, &QPushButton::clicked,
&onBtnClicked);
connectOneShot(&qBtnOneShotFunc, &QPushButton::clicked,
&onBtnClicked);
QObject::connect(&qBtnMShotMemFunc, &QPushButton::clicked,
&qBtnMShotMemFunc, &PushButton::onClicked);
connectOneShot(&qBtnOneShotMemFunc, &QPushButton::clicked,
&qBtnOneShotMemFunc, &PushButton::onClicked);
QObject::connect(&qBtnMShotLambda, &QPushButton::clicked,
[&](bool)
qDebug() << "[&](bool) qBtnMShotLambda called.";
static int i = 0;
qBtnMShotLambda.setText(QString("Clicked %1.").arg(++i));
);
connectOneShot(&qBtnOneShotLambda, &QPushButton::clicked,
[&](bool)
qDebug() << "[&](bool) for qBtnOneShotLambda called.";
static int i = 0;
qBtnOneShotLambda.setText(QString("Clicked %1.").arg(++i));
);
QObject::connect(&qEditMShot, &QLineEdit::editingFinished,
[&]()
qDebug() << "[&]() for qEditMShot called. Input:" << qEditMShot.text();
qEditMShot.setText("Well done.");
);
connectOneShot(&qEditOneShot, &QLineEdit::editingFinished,
[&]()
qDebug() << "[&]() for qEditOneShot called. Input:" << qEditOneShot.text();
qEditOneShot.setText("No more input accepted.");
qEditOneShot.setEnabled(false);
);
// run
return app.exec();
对应的Qt项目：
SOURCES = testQSignalOneShot.cc
QT += widgets
在 Windows 10 上的 VS2017 中测试：
在cygwin64 (X11) 中使用 g++ 测试：
$ qmake-qt5 testQSignalOneShot.pro
$ make && ./testQSignalOneShot
g++ -c -fno-keep-inline-dllexport -D_GNU_SOURCE -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtWidgets -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtCore -I. -I/usr/lib/qt5/mkspecs/cygwin-g++ -o testQSignalOneShot.o testQSignalOneShot.cc
g++ -o testQSignalOneShot.exe testQSignalOneShot.o -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread
Qt Version: 5.9.4
onBtnClicked() called: 1
onBtnClicked() called: 2
onBtnClicked() called: 3
QPushButton(0xffffcb60) PushButton::onClicked() called: 1
QPushButton(0xffffcb60) PushButton::onClicked() called: 2
QPushButton(0xffffcba0) PushButton::onClicked() called: 1
[&](bool) qBtnMShotLambda called.
[&](bool) qBtnMShotLambda called.
[&](bool) for qBtnOneShotLambda called.
[&]() for qEditMShot called. Input: "abc123"
[&]() for qEditMShot called. Input: "def456"
[&]() for qEditMShot called. Input: "Well done."
[&]() for qEditOneShot called. Input: "abc456"
注意：
我必须承认这个解决方案有一点缺陷。如果一次性连接没有“触发”而是断开连接（例如，因为发送者对象被删除），那么QMetaObject::Connection 不会被删除并成为内存泄漏。我思考了一会儿如何在没有一个好主意的情况下解决这个问题。最后，我决定按原样发送。所以，请对它持保留态度——它还没有“准备好生产”。至少，它显示了这个想法。
最后，我解决了内存泄漏的问题。
连接的关键在于它必须传递给内部的“蹦床”lambda。
但是，按值传递连接将确保正确的存储管理，但此时它尚未初始化。
通过引用传递它可以解决这个问题，但这会捕获对局部变量的引用（致命）。
因此，new QMetaObject::Connection 的解决方案似乎是唯一可行的方法，因为指针可以按值传递，但之后可以更新实例。由于使用new 进行分配，应用程序可以控制QMetaObject::Connection 实例的生命周期。
但是如果没有发出信号怎么办。我的解决方案：否则，发件人对象可能负责删除它。这可以在 Qt 中通过将QObject“附加”到另一个来实现（将后者设置为前者的父级）。
基于此，一个改进的解决方案，我将QMetaObject::Connection 存储在派生自QObject 的包装器中：
#include <functional>
#include <QtWidgets>
struct ConnectionWrapper: QObject
ConnectionWrapper(QObject *pQParent): QObject(pQParent)
ConnectionWrapper(const ConnectionWrapper&) = delete;
ConnectionWrapper& operator=(const ConnectionWrapper&) = delete;
virtual ~ConnectionWrapper()
qDebug() << "ConnectionWrapper::~ConnectionWrapper()";
QMetaObject::Connection connection;
;
template <typename Sender, typename Emitter, typename Slot, typename... Args>
QMetaObject::Connection connectOneShot(Sender *pSender, void (Emitter::*pSignal)(Args ...args), Slot slot)
ConnectionWrapper *pConn = new ConnectionWrapper(pSender);
pConn->connection
= QObject::connect(pSender, pSignal,
[pConn, slot](Args... args)
QObject::disconnect(pConn->connection);
delete pConn;
slot(args...);
);
return pConn->connection;
int main(int argc, char **argv)
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
// setup user interface
QWidget qWinMain;
QGridLayout qGrid;
auto addRow
= [](QGridLayout &qGrid, const QString &qText, QWidget &qWidgetMShot, QWidget &qWidgetOneShot)
const int i = qGrid.rowCount();
qGrid.addWidget(new QLabel(qText), i, 0);
qGrid.addWidget(&qWidgetMShot, i, 1);
qGrid.addWidget(&qWidgetOneShot, i, 2);
;
QPushButton qBtn("One Shot");
QPushButton qBtnDisconnect("Disconnect");
addRow(qGrid, "Disconnect Test:", qBtnDisconnect, qBtn);
qWinMain.setLayout(&qGrid);
qWinMain.show();
// install signal handlers
QMetaObject::Connection connectionBtn
= connectOneShot(&qBtn, &QPushButton::clicked,
[&](bool)
qDebug() << "[&](bool) for qBtn called.";
static int i = 0;
qBtn.setText(QString("Clicked %1.").arg(++i));
);
QObject::connect(&qBtnDisconnect, &QPushButton::clicked,
[&](bool)
QObject::disconnect(connectionBtn);
qDebug() << "qBtn disconnected.";
);
// run
return app.exec();
对应的Qt项目：
SOURCES = testQSignalOneShot2.cc
QT += widgets
我在 VS2017（Windows 10“本机”）和带有 X11 的 cygwin64 上再次进行了测试。在后者的会话下方：
$ qmake-qt5 testQSignalOneShot2.pro
$ make && ./testQSignalOneShot2
/usr/bin/qmake-qt5 -o Makefile testQSignalOneShot2.pro
g++ -c -fno-keep-inline-dllexport -D_GNU_SOURCE -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtWidgets -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtCore -I. -I/usr/lib/qt5/mkspecs/cygwin-g++ -o testQSignalOneShot2.o testQSignalOneShot2.cc
g++ -o testQSignalOneShot2.exe testQSignalOneShot2.o -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread
Qt Version: 5.9.4
ConnectionWrapper::~ConnectionWrapper()
[&](bool) for qBtn called.
qBtn disconnected.
在这种情况下，我首先按下 One Shot，然后按下 Disconnect。再次测试相反的顺序：
$ make && ./testQSignalOneShot2
make: Nothing to be done for 'first'.
Qt Version: 5.9.4
qBtn disconnected.
ConnectionWrapper::~ConnectionWrapper()
在这种情况下，ConnectionWrapper::~ConnectionWrapper() 的输出在我退出应用程序之前没有出现。 （有道理——当main() 的范围离开时，发件人qBtn 被删除。）
【讨论】：
@eudoxos 刚刚添加了答案的更新 - 我找到了解决潜在内存泄漏的方法。【参考方案2】：有一个更简单的方法：删除接收者来破坏连接。
auto receiver = new QObject(this);
connect(_textFadeOutAnimation, &QPropertyAnimation::finished, receiver, [this, text, receiver]()
receiver.deleteLater();
);
【讨论】：
【参考方案3】：前段时间我为单次连接写了一个包装器。似乎工作正常，也许对某人有用。
template <typename SenderFunc, typename ReceiverFunc>
void singleShotConnect(const typename QtPrivate::FunctionPointer<SenderFunc>::Object* sender, SenderFunc signal, const QObject* context, ReceiverFunc receiverFunc)
auto connection = std::make_shared<QMetaObject::Connection>();
connection = QObject::connect(sender, signal, context, [connection, receiverFunc](auto ...args)
QObject::disconnect(*connection);
std::invoke(receiverFunc, args...);
);
;
示例用法：
auto* senderObj = new SenderObj();
auto* receiverObj = new ReceiverObj();
singleShotConnect(senderObj, &SenderObj:someSignal, receiverObj, &ReceiverObj::someSlot);
【讨论】：
我相信您在函数声明中缺少signal。【参考方案4】：
the accepted answer 的小问题（我认为所有其他答案）是它无法处理发送者和接收者在不同线程中的情况。如果接收器很忙（例如处理另一个信号），那么信号可能会在断开之前多次触发。断开信号并不会删除任何已排队的连接。
解决方案是使用两个连接对其进行稍微变体。第一个连接到预期的接收器（并在接收器的线程上运行）。第二个在发送者的线程上运行并断开连接。
template <typename SenderT, typename Func1T, typename ...Args>
QMetaObject::Connection connectOneShot(SenderT* sender, const Func1T& signal, Args&&...args)
auto connections = std::make_shared<std::pair<QMetaObject::Connection, QMetaObject::Connection>>();
// The order of connections is important here - the target slot must be connected first (so that it's called before the disconnection has happened)
connections->first = QObject::connect(sender, signal, std::forward<Args>(args)...);
// This gets run as a direct connection on the sender's thread, thus ensuring that's it's called immediately (before the signal has the change to repeat)
connections->second = QObject::connect(sender, signal, sender, [connections]()
QObject::disconnect(connections->first);
QObject::disconnect(connections->second);
);
return connections->first;
与原始版本的另一个细微变化是我使用std::shared_ptr 来确保清除堆分配的连接 - 就像在@Mieszko's answer 中一样。
【讨论】：
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