Qt Reference Documentation

# QSignalSpy Class Reference

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

     #include <QSignalSpy>

Inherits: QObject and QList<QList<QVariant> >.

- List of all members, including inherited members

## Public Functions

| QSignalSpy ( QObject * object, const char * signal )
bool | isValid () const
QByteArray | signal () const

- 29 public functions inherited from QObject
- 67 public functions inherited from QList

### Additional Inherited Members

- 1 property inherited from QObject
- 1 public slot inherited from QObject
- 1 signal inherited from QObject
- 7 static public members inherited from QObject
- 3 static public members inherited from QList
- 8 protected functions inherited from QObject

## 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));

## Member Function Documentation

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

Constructs a new QSignalSpy that listens for emissions of the signal from the QObject object. Neither signal nor object can be null.

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.