Question

如何使用Qt5.12创建一个一次性连接（即第一次激活时会自动断开连接）？我正在寻找一种优雅的解决方案，而无需过多的冗长，以明确传达其含义。

我当前正在使用

QObject::connect(instance,Class::signal,this,[this](){
    QObject::disconnect(instance,Class::signal,this,0);
    /* ... */
});

仅在没有其他信号连接时有效。

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，但又不是很优雅。

在SO上有几个与此主题有关的问题，但是似乎没有一种解决方案对我有用。例如，https://stackoverflow.com/a/42989833/761090使用伪对象：

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

还有更好的建议吗？

编辑：我将其作为请求的Qt错误跟踪程序：https://bugreports.qt.io/browse/QTBUG-74547提交。

Answer 1

我从OP问题中采用了（对我而言）最有前途的方法

QMetaObject::Connection * const connection = new QMetaObject::Connection;
*connection = connect(_textFadeOutAnimation, &QPropertyAnimation::finished, [this, text, connection](){
    QObject::disconnect(*connection);
    delete connection;
});

，然后考虑如何将其包装到函数中。实际上，它必须是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中尝试过：

#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();
}

相应的Qt项目：

SOURCES = testQSignalOneShot.cc

QT += widgets

在Windows 10的VS2017中测试：

在cygwin64（X11）中使用g ++进行了测试：

$ 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：

我必须承认此解决方案有一点缺陷。如果单触发连接不是“触发”而是断开连接（例如，由于删除了发送方对象），则QMetaObject::Connection不会被删除并会导致内存泄漏。我考虑了一会儿如何解决这个问题，却没有一个好主意。最后，我决定按原样发送此邮件。因此，请带一点盐食用-尚未“准备生产”。至少，它表明了这个想法。

最后，我解决了内存泄漏的问题。

关键在于必须将其传递给内部的“蹦床” lambda。

但是，通过值传递连接可以确保适当的存储管理，但目前尚未初始化。

通过引用传递它可以解决此问题，但这将捕获对局部变量（致命）的引用。

因此，使用new QMetaObject::Connection的解决方案似乎是唯一可行的方法，因为可以按值传递指针，但此后可以更新实例。由于使用new进行分配，因此应用程序可以控制QMetaObject::Connection实例的生命周期。

但是，如果不发出信号怎么办。我的解决方案：否则，发件人对象可能负责删除它。在Qt中，这可以通过将QObject“附加”到另一个（将后者设置为前者的父级）来实现。

基于此，这是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();
}

相应的Qt项目：

SOURCES = testQSignalOneShot2.cc

QT += widgets

我再次在VS2017（Windows 10“ native”）和带有X11的cygwin64上进行了测试。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，我首先按下一个镜头，然后按下第二个 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，我退出应用程序之前没有出现ConnectionWrapper::~ConnectionWrapper()的输出。（可以理解–离开qBtn的范围时，发件人main()被删除。）

Answer 2

还有一种更简单的方法：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

前一段时间，我为单次连接编写了一个包装。似乎工作正常，也许对某些人有用。

Z::Z(const Z &X){
    z1 = new int(*X.first() );
    z2 = new int(*X.second() );
}

Z& operator=(const Z& X)
{
  *z1 = *(X.z1);
  *z2 = *(X.z2);

  return *this;
}

用法示例：

    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...);
    });
};

Qt5：与lambda的一次性连接

3 个答案: