QSignalSpy Class

Detailed Description

The QSignalSpy class enables introspection of signal emission.

QSignalSpy can connect to any signal of any object and records its emission. QSignalSpy itself is a list of QVariant lists. Each emission of the signal will append one item to the list, containing the arguments of the signal.

The following example records all signal emissions for the clicked() signal of a QCheckBox:

 QCheckBox *box = ...;
 QSignalSpy spy(box, SIGNAL(clicked(bool)));

 // do something that triggers the signal
 box->animateClick();

 QCOMPARE(spy.count(), 1); // make sure the signal was emitted exactly one time
 QList<QVariant> arguments = spy.takeFirst(); // take the first signal

 QVERIFY(arguments.at(0).toBool() == true); // verify the first argument

spy.takeFirst() returns the arguments for the first emitted signal, as a list of QVariant objects. The clicked() signal has a single bool argument, which is stored as the first entry in the list of arguments.

The example below catches a signal from a custom object:

 QSignalSpy spy(myCustomObject, SIGNAL(mySignal(int, QString, double)));

 myCustomObject->doSomething(); // trigger emission of the signal

 QList<QVariant> arguments = spy.takeFirst();
 QVERIFY(arguments.at(0).type() == QVariant::Int);
 QVERIFY(arguments.at(1).type() == QVariant::QString);
 QVERIFY(arguments.at(2).type() == QVariant::double);

Note: Non-standard data types need to be registered, using the qRegisterMetaType() function, before you can create a QSignalSpy. For example:

 qRegisterMetaType<QModelIndex>("QModelIndex");
 QSignalSpy spy(&model, SIGNAL(whatever(QModelIndex)));

To retrieve the QModelIndex, you can use qvariant_cast:

 // get the first argument from the first received signal:
 QModelIndex result = qvariant_cast<QModelIndex>(spy.at(0).at(0));