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QSignalSpy Class

The QSignalSpy class enables introspection of signal emission. More...

Header: #include <QSignalSpy>
qmake: QT += testlib
Inherits: QObject and QList

Public Functions

QSignalSpy(const QObject *obj, const QMetaMethod &signal)
QSignalSpy(const QObject *object, PointerToMemberFunction signal)
QSignalSpy(const QObject *object, const char *signal)
bool isValid() const
QByteArray signal() const
bool wait(int timeout = 5000)

Detailed Description

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::String);
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<SomeStruct>();
QSignalSpy spy(&model, SIGNAL(whatever(SomeStruct)));

To retrieve the instance, you can use qvariant_cast:

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

Verifying Signal Emissions

The QSignalSpy class provides an elegant mechanism for capturing the list of signals emitted by an object. However, you should verify its validity after construction. The constructor does a number of sanity checks, such as verifying that the signal to be spied upon actually exists. To make the diagnosis of test failures easier, the results of these checks should be checked by calling QVERIFY(spy.isValid()) before proceeding further with a test.

See also QVERIFY().

Member Function Documentation

QSignalSpy::QSignalSpy(const QObject *obj, const QMetaMethod &signal)

Constructs a new QSignalSpy that listens for emissions of the signal from the QObject obj. If QSignalSpy is not able to listen for a valid signal (for example, because obj is nullptr or signal does not denote a valid signal of obj), an explanatory warning message will be output using qWarning() and subsequent calls to isValid() will return false.

This constructor is convenient to use when Qt's meta-object system is heavily used in a test.

Basic usage example:

QObject object;
auto mo = object.metaObject();
auto signalIndex = mo->indexOfSignal("objectNameChanged(QString)");
auto signal = mo->method(signalIndex);

QSignalSpy spy(&object, signal);
object.setObjectName("A new object name");
QCOMPARE(spy.count(), 1);

Imagine we need to check whether all properties of the QWindow class that represent minimum and maximum dimensions are properly writable. The following example demonstrates one of the approaches:

void tst_QWindow::writeMinMaxDimensionalProps_data()
    QTest::addColumn<int>("propertyIndex");

    // Collect all relevant properties
    static const auto mo = QWindow::staticMetaObject;
    for (int i = mo.propertyOffset(); i < mo.propertyCount(); ++i) {
        auto property = mo.property(i);

        // ...that have type int
        if (property.type() == QVariant::Int) {
            static const QRegularExpression re("^minimum|maximum");
            const auto name = property.name();

            // ...and start with "minimum" or "maximum"
            if (re.match(name).hasMatch()) {
                QTest::addRow("%s", name) << i;
            }
        }
    }
}

void tst_QWindow::writeMinMaxDimensionalProps()
{
    QFETCH(int, propertyIndex);

    auto property = QWindow::staticMetaObject.property(propertyIndex);
    QVERIFY(property.isWritable());
    QVERIFY(property.hasNotifySignal());

    QWindow window;
    QSignalSpy spy(&window, property.notifySignal());

    QVERIFY(property.write(&window, 42));
    QCOMPARE(spy.count(), 1);
}

This function was introduced in Qt 5.14.

QSignalSpy::QSignalSpy(const QObject *object, PointerToMemberFunction signal)

Constructs a new QSignalSpy that listens for emissions of the signal from the QObject object. If QSignalSpy is not able to listen for a valid signal (for example, because object is nullptr or signal does not denote a valid signal of object), an explanatory warning message will be output using qWarning() and subsequent calls to isValid() will return false.

Example:

QSignalSpy spy(myPushButton, &QPushButton::clicked);

This function was introduced in Qt 5.4.

QSignalSpy::QSignalSpy(const QObject *object, const char *signal)

Constructs a new QSignalSpy that listens for emissions of the signal from the QObject object. If QSignalSpy is not able to listen for a valid signal (for example, because object is nullptr or signal does not denote a valid signal of object), an explanatory warning message will be output using qWarning() and subsequent calls to isValid() will return false.

Example:

QSignalSpy spy(myPushButton, SIGNAL(clicked(bool)));

bool QSignalSpy::isValid() const

Returns true if the signal spy listens to a valid signal, otherwise false.

QByteArray QSignalSpy::signal() const

Returns the normalized signal the spy is currently listening to.

bool QSignalSpy::wait(int timeout = 5000)

Starts an event loop that runs until the given signal is received. Optionally the event loop can return earlier on a timeout (in milliseconds).

Returns true if the signal was emitted at least once in timeout milliseconds, otherwise returns false.

Example:

QVERIFY(spy.wait(1000));

This function was introduced in Qt 5.0.

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