QSignalSpy::wait(...) fails, when QTest::qWait(...) succeeds

Question

It's a confusing situation i'm in. While testing state transitions for a QStateMachine, the following code fails to spy on the signal that causes the transition.

// Test transition to SS_STARTING
QSignalSpy spy(test_obj_, SIGNAL(StateChanged(int)));

// emmit StateChanged signal
test_obj_->SetState(SS_STARTING); 
// Current state property should be SS_STARTING
QVERIFY(spy.wait()); //<--- fails test
QVERIFY(test_obj_->GetCurrentState() == SS_STARTING);
QCOMPARE(spy.count(), 1);

The following code passes the test!

// Test transition to SS_STARTING
QSignalSpy spy(test_obj_, SIGNAL(StateChanged(int)));

// emmit StateChanged signal
test_obj_->SetState(SS_STARTING); 
// Current state property should be SS_STARTING
QTest::qWait(20); //<--- passes test
QVERIFY(test_obj_->GetCurrentState() == SS_STARTING);
QCOMPARE(spy.count(), 1);

I can also verify externally that the signal is being emitted using dbus-monitor.

I can go ahead and use QTest::qWait, it's no big deal, but i'm just confused as to why spy.wait isn't working.

Cheers, Simon

Answer 1

Your test is incorrect, as soon as you set setState() the signal is emitted, so spy.wait() will no longer receive it. So the idea is to emit the signal a moment after spy.wait() starts using a QTimer:

// Test transition to SS_STARTING
QSignalSpy spy(test_obj_, SIGNAL(StateChanged(int)));
// emmit StateChanged signal
// test_obj_->SetState(SS_STARTING); 
QTimer::singleShot(0, [test_obj_](){ test_obj_->setState(SS_STARTING);}); // <----
QVERIFY(spy.wait());
QVERIFY(test_obj_->GetCurrentState() == SS_STARTING);
QCOMPARE(spy.count(), 1);

In its second example, the QTest::qWait() is not necessary because the status changed synchronously