在每次循环迭代时停止执行代码数毫秒

Question

I'm calling a function inside a loop:

for (Item *item : *items) {
    item->func();
}

The called function emits a signal and waits for 100 millisecond:

void Item::func()
{
    // Emit a signal
    emit castSignal();

    QTimer::singleShot(100 /* msec */, [](){
        qDebug() << "Wait for 100 milliseconds";
    });
}

I have a slot which logs to console when receives a signal:

QObject::connect(this, &Item::castSignal, this, &Item::handleSignal);

void Item::handleSignal()
{
    qDebug() << "Signal received";
}

I expect the logs to be like this:

Signal received
Wait for 100 milliseconds
Signal received
Wait for 100 milliseconds
Signal received
Wait for 100 milliseconds

But the actual log is like this:

Signal received
Signal received
Signal received
Wait for 100 milliseconds
Wait for 100 milliseconds
Wait for 100 milliseconds

I cannot figure out why! Can anybody help.

Answer 1

singleShot adds a timer, then keeps executing. When the time interval has passed, then your lambda will be called. So you're starting three timers, and all three signals get received before the timeout of the first timer has passed.

If you really need func to wait for 100ms use a sleep function.

Answer 2

Let's analyze your code step by step:

for (Item *item : *items) {
    item->func();
}

Is equivalent to:

emit castSignal();
QTimer::singleShot(100 /* msec */, [](){
    qDebug() << "Wait for 100 milliseconds";
});
emit castSignal();
QTimer::singleShot(100 /* msec */, [](){
    qDebug() << "Wait for 100 milliseconds";
});
emit castSignal();
QTimer::singleShot(100 /* msec */, [](){
    qDebug() << "Wait for 100 milliseconds";
});
// ...

The following must be taken into account:

• In your case, the receiver and the sender begin same thread, so the slot is invoked immediately since Qt::AutoConnection is used by default.

• The timers are launched when the synchronous part ends and at the indicated time

Considering the above and seeing that handleSignal consumes very little time your code is equivalent to:

qDebug() << "Signal received";
qDebug() << "Signal received";
qDebug() << "Signal received";

T: 100 ms

qDebug() << "Wait for 100 milliseconds";
qDebug() << "Wait for 100 milliseconds";
qDebug() << "Wait for 100 milliseconds";

getting the behavior you get.

The same is what @1201ProgramAlarm indicates but using sleep is a bad option since it will block the event loop preventing other signals, events, etc run correctly, instead a possible option is to use QEventLoop:

void Item::func()
{
    // Emit a signal
    emit castSignal();
    QEventLoop loop;

    QTimer::singleShot(100 /* msec */, [&loop](){
        qDebug() << "Wait for 100 milliseconds";
        loop.quit();
    });
    loop.exec();
}