QT C++如何调用lambda one

Question

我想调用某个lambda函数A一次，下一次我想调用lambda函数B。

例如：

connect(someButton, &QPushButton::clicked, this, [=]()
{
QMessageBox::information(this, "Function A", "This is FIRST Message");
});

然后我想断开这个功能，连接到第二个：

connect(someButton, &QPushButton::clicked, this, [=]()
{
QMessageBox::information(this, "Function B", "This is SECOND Message");
});

预期结果：

Button clicked first time - "This is FIRST Message"
Button clicked second time - "This is SECOND Message"
…
Button clicked 10th time - "This is TENTH Message"

Answer 1

您可以使用具有可变状态的lambda。例如：

[counter = 0]() mutable {
if (counter++ == 0)
; // first time
else
; // afterwards
};

我认为这样的捕获需要C++14。

Answer 2

另一个不使用mutable的例子是放入一个静态变量进行检查。类似的东西

[](){
static bool isFirstTime = true;
if (isFirstTime) {
// first time code
isFirstTime = false;
}
else {
// subsequent times code
}
}

Answer 3

解决方案取决于开发环境的现代化程度：

void CPP14() {  // C++14 & up
connect(button, &QPushButton::clicked, this, [first = true]() mutable {
if (first) {
qDebug() << "first call";
first = false;
} else {
qDebug() << "second call";
}
});
}
void CPP11() {  // C++11
int first = true;
connect(button, &QPushButton::clicked, this, [first]() mutable {
if (first) {
qDebug() << "first call";
first = false;
} else {
qDebug() << "second call";
}
});
}

// C++98 or Qt 4
class OnClick : public QObject {
Q_OBJECT
public:
bool first;
OnClick(QObject *parent) : QObject(parent), first(true) {}
Q_SLOT void slot() {
if (first) {
qDebug() << "first call";
first = false;
} else {
qDebug() << "second call";
}
}
};
class Class : public QWidget {
QPushButton *button;
void CPP98() {
connect(button, SIGNAL(clicked(bool)), new OnClick(this), SLOT(slot()));
}
};