Qt Reference Documentation

QSignalSpy Class Reference

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

 #include <QSignalSpy>

Inherits QObject and QList<QList<QVariant> >.

Public Functions

QSignalSpy ( QObject * object, const char * signal )
bool isValid () const
QByteArray signal () const
  • 29 public functions inherited from QObject
  • 66 public functions inherited from QList

Additional Inherited Members

  • 1 property inherited from QObject
  • 1 public slot inherited from QObject
  • 1 signal inherited from QObject
  • 5 static public members inherited from QObject
  • 3 static public members inherited from QList
  • 7 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

 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:

 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.


 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.


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