Qt5：一次性连接到 lambda

Question

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，不过分冗长，它清楚地传达了意思。

我目前正在使用

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能工作，但又不是很优雅。

SO 上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://stackoverflow.com/a/42989833/761090使用虚拟 object：

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://stackoverflow.com/a/26554206/761090的第二部分）不使用 c++17 进行编译。

有更好的建议吗？

编辑：我将此作为请求提交 Qt 错误跟踪器： https://bugreports.qt.io/browse/QTBUG-74547 。

Answer 1

我从 OPs 问题中采用了（对我而言）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;
});

并思考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++ 进行测试：

$ 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"

笔记：

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被“触发”而是断开连接（例如因为发送方对象被删除），那么QMetaObject::Connection不会被删除并成为内存泄漏。 我想了一会儿如何解决这个没有一个好主意。 最后，我决定按原样发送。 所以，请对此持保留态度——它还不是“生产就绪”。 至少，它显示了这个想法。

---

最后，我解决了内存泄漏的问题。

连接的关键是它必须传递到内部的“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（致命）的引用。

因此，使用new QMetaObject::Connection的解决方案似乎是唯一有效的方法，因为指针可以按值传递，但实例可以在之后更新。 由于使用new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呢？ 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“附加”一个QObject到另一个（将后者设置为前者的父级）来实现。

基于此，改进的解决方案，我将QMetaObject::Connection存储在从QObject派生的包装器中：

#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和第二个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()作用域被保留时，发送者qBtn被删除了。）

Answer 2

有一个更简单的方法：删除接收者以破坏连接。

auto receiver = new QObject(this);
connect(_textFadeOutAnimation, &QPropertyAnimation::finished, receiver, [this, text, receiver](){
    receiver.deleteLater();
});

Answer 3

前段时间我为单次连接编写了一个包装器。 似乎工作正常，也许它对某人有用。

    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);

Answer 4

接受的答案（我认为所有其他答案）的一个小问题是它不处理发送者和接收者在不同线程中的情况。 如果接收器正忙（例如处理另一个信号），则信号可能会在断开连接之前多次触发。 断开信号不会删除任何已经排队的连接。

解决方案是用两个连接做一个稍微变体。 第一个连接到预期的接收器（并在接收器的线程上运行）。 第二个在发件人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;
}

与原始版本相比的另一个细微变化是我使用std::shared_ptr来确保清理堆分配的连接——就像@Mieszko 的回答一样。