Slots in D-Bus adaptors are declared just like normal, public slots, but their parameters must follow certain rules (see The Qt D-Bus Type System for more information). Slots whose parameters do not follow those rules or that are not public will not be accessible via D-Bus.
Slots can have one parameter of type const QDBusMessage &, which must appear at the end of the input parameter list, before any output parameters. This parameter, if present, will be initialized with a copy of the current message being processed, which allows the callee to obtain information about the caller, such as its connection name.
In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques − QtCore.QObject.connect(widget, QtCore.SIGNAL(‘signalname’), slotfunction) A more convenient way to call a slotfunction, when a signal is emitted by a widget is as follows − widget.signal.connect(slotfunction). Traditional syntax: SIGNAL and SLOT QtCore.SIGNAL and QtCore.SLOT macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax. You can use Qt's meta-object data to ask about its type (as a string), or ask which calls it supports. That means you can then dynamically find, and get call signatures and through Qt's signals/slots, invoke via text an object.
Slots can be of three kinds:
- Asynchronous
- Input-only
- Input-and-output
The first thing Qt does when doing a connection is to find out the index of the signal and the slot. Qt will look up in the string tables of the meta object to find the corresponding indexes. Then a QObjectPrivate::Connection object is created and added in the internal linked lists. What information needs to be stored for each connection? Qt Call Slot With Parameter only. No deposit winnings capped at €/$20. 50x wagering requirement applies to FS no deposit. Min €/$20 deposit required to claim each deposit bonus as part of the welcome bonus. 100 free spins on first deposit issued 25 spins per Qt Call Slot With Parameter day for four days. Max payout €/$50.
Asynchronous Slots
Asynchronous slots are those that do not normally return any reply to the caller. For that reason, they cannot take any output parameters. In most cases, by the time the first line of the slot is run, the caller function has already resumed working.
However, slots must not rely on that behavior. Scheduling and message-dispatching issues could change the order in which the slot is run. Code intending to synchronize with the caller should provide its own method of synchronization.
Asynchronous slots are marked by the keyword Q_NOREPLY in the method signature, before the void return type and the slot name. The quit() slot in the D-Bus Complex Ping Pong Example is an example of this.
Input-Only Slots
Input-only slots are normal slots that take parameters passed by value or by constant reference. However, unlike asynchronous slots, the caller is usually waiting for completion of the callee before resuming operation. Therefore, non-asynchronous slots should not block or should state it its documentation that they may do so.
Input-only slots have no special marking in their signature, except that they take only parameters passed by value or by constant reference. Optionally, slots can take a QDBusMessage parameter as a last parameter, which can be used to perform additional analysis of the method call message.
Input and Output Slots
Like input-only slots, input-and-output slots are those that the caller is waiting for a reply. Unlike input-only ones, though, this reply will contain data. Slots that output data may contain non-constant references and may return a value as well. However, the output parameters must all appear at the end of the argument list and may not have input arguments interleaved. Optionally, a QDBusMessage argument may appear between the input and the output arguments.
Automatic Replies
Method replies are generated automatically with the contents of the output parameters (if there were any) by the Qt D-Bus implementation. Slots need not worry about constructing proper QDBusMessage objects and sending them over the connection.
However, the possibility of doing so remains there. Should the slot find out it needs to send a special reply or even an error, it can do so by using QDBusMessage::createReply() or QDBusMessage::createErrorReply() on the QDBusMessage parameter and send it with QDBusConnection::send(). The Qt D-Bus implementation will not generate any reply if the slot did so.
Warning: When a caller places a method call and waits for a reply, it will only wait for a limited amount of time. Slots intending to take a long time to complete should make that fact clear in documentation so that callers properly set higher timeouts.
Delayed Replies
In some circumstances, the called slot may not be able to process the request immediately. This is frequently the case when the request involves an I/O or networking operation which may block.
If this is the case, the slot should return control to the application's main loop to avoid freezing the user interface, and resume the process later. To accomplish this, it should make use of the extra QDBusMessage parameter at the end of the input parameter list and request a delayed reply.
We do this by writing a slot that stores the request data in a persistent structure, indicating to the caller using QDBusMessage::setDelayedReply(true) that the response will be sent later.
In this case, the return value is unimportant; we return an arbitrary value to satisfy the compiler.
When the request is processed and a reply is available, it should be sent using the QDBusMessage object that was obtained. In our example, the reply code could be something as follows:
As can be seen in the example, when a delayed reply is in place, the return value(s) from the slot will be ignored by Qt D-Bus. They are used only to determine the slot's signature when communicating the adaptor's description to remote applications, or in case the code in the slot decides not to use a delayed reply.
The delayed reply itself is requested from Qt D-Bus by calling QDBusMessage::reply() on the original message. It then becomes the resposibility of the called code to eventually send a reply to the caller.
Warning: When a caller places a method call and waits for a reply, it will only wait for a limited amount of time. Slots intending to take a long time to complete should make that fact clear in documentation so that callers properly set higher timeouts.
See also Using Qt D-Bus Adaptors, Declaring Signals in D-Bus Adaptors, The Qt D-Bus Type System, QDBusConnection, and QDBusMessage.
© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.
The QMetaMethod class provides meta-data about a member function. More...
Public Types
| enum | Access { Private, Protected, Public } |
| enum | MethodType { Method, Signal, Slot, Constructor } |
Public Functions
| QMetaMethod::Access | access() const |
| bool | invoke(QObject *object, Qt::ConnectionType connectionType, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument(nullptr), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | invoke(QObject *object, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument(0), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | invoke(QObject *object, Qt::ConnectionType connectionType, QGenericArgument val0 = QGenericArgument(0), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | invoke(QObject *object, QGenericArgument val0 = QGenericArgument(0), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | invokeOnGadget(void *gadget, QGenericReturnArgument returnValue, QGenericArgument val0 = QGenericArgument(nullptr), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | invokeOnGadget(void *gadget, QGenericArgument val0 = QGenericArgument(0), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument()) const |
| bool | isValid() const |
| int | methodIndex() const |
| QByteArray | methodSignature() const |
| QMetaMethod::MethodType | methodType() const |
| QByteArray | name() const |
| int | parameterCount() const |
| QList<QByteArray> | parameterNames() const |
| int | parameterType(int index) const |
| QList<QByteArray> | parameterTypes() const |
| int | returnType() const |
| int | revision() const |
| const char * | tag() const |
| const char * | typeName() const |
Static Public Members
| QMetaMethod | fromSignal(PointerToMemberFunction signal) |
Related Non-Members
| bool | operator!=(const QMetaMethod &m1, const QMetaMethod &m2) |
| bool | operator(const QMetaMethod &m1, const QMetaMethod &m2) |
Macros
Detailed Description
Call Slot In Qt Code
A QMetaMethod has a methodType(), a methodSignature(), a list of parameterTypes() and parameterNames(), a return typeName(), a tag(), and an access() specifier. You can use invoke() to invoke the method on an arbitrary QObject.
See also QMetaObject, QMetaEnum, QMetaProperty, and Qt's Property System.
Member Type Documentation
enum QMetaMethod::Access
This enum describes the access level of a method, following the conventions used in C++.
| Constant | Value |
|---|---|
QMetaMethod::Private | 0 |
QMetaMethod::Protected | 1 |
QMetaMethod::Public | 2 |
enum QMetaMethod::MethodType
| Constant | Value | Description |
|---|---|---|
QMetaMethod::Method | 0 | The function is a plain member function. |
QMetaMethod::Signal | 1 | The function is a signal. |
QMetaMethod::Slot | 2 | The function is a slot. |
QMetaMethod::Constructor | 3 | The function is a constructor. |
Member Function Documentation
QMetaMethod::Access QMetaMethod::access() const
Returns the access specification of this method (private, protected, or public).
Note: Signals are always public, but you should regard that as an implementation detail. It is almost always a bad idea to emit a signal from outside its class.
See also methodType().
[static] template <typename PointerToMemberFunction> QMetaMethod QMetaMethod::fromSignal(PointerToMemberFunctionsignal)
Qt Call Slot In Another Thread
Returns the meta-method that corresponds to the given signal, or an invalid QMetaMethod if signal is not a signal of the class.
Example:
This function was introduced in Qt 5.0.
bool QMetaMethod::invoke(QObject *object, Qt::ConnectionTypeconnectionType, QGenericReturnArgumentreturnValue, QGenericArgumentval0 = QGenericArgument(nullptr), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
Invokes this method on the object object. Returns true if the member could be invoked. Returns false if there is no such member or the parameters did not match.
The invocation can be either synchronous or asynchronous, depending on the connectionType:
- If connectionType is Qt::DirectConnection, the member will be invoked immediately.
- If connectionType is Qt::QueuedConnection, a QEvent will be posted and the member is invoked as soon as the application enters the main event loop.
- If connectionType is Qt::AutoConnection, the member is invoked synchronously if object lives in the same thread as the caller; otherwise it will invoke the member asynchronously.
The return value of this method call is placed in returnValue. If the invocation is asynchronous, the return value cannot be evaluated. You can pass up to ten arguments (val0, val1, val2, val3, val4, val5, val6, val7, val8, and val9) to this method call.
QGenericArgument and QGenericReturnArgument are internal helper classes. Because signals and slots can be dynamically invoked, you must enclose the arguments using the Q_ARG() and Q_RETURN_ARG() macros. Q_ARG() takes a type name and a const reference of that type; Q_RETURN_ARG() takes a type name and a non-const reference.
To asynchronously invoke the animateClick() slot on a QPushButton:
With asynchronous method invocations, the parameters must be of types that are known to Qt's meta-object system, because Qt needs to copy the arguments to store them in an event behind the scenes. If you try to use a queued connection and get the error message
call qRegisterMetaType() to register the data type before you call QMetaMethod::invoke().
To synchronously invoke the compute(QString, int, double) slot on some arbitrary object obj retrieve its return value:
QMetaObject::normalizedSignature() is used here to ensure that the format of the signature is what invoke() expects. E.g. extra whitespace is removed.
If the 'compute' slot does not take exactly one QString, one int and one double in the specified order, the call will fail.
Warning: this method will not test the validity of the arguments: object must be an instance of the class of the QMetaObject of which this QMetaMethod has been constructed with. The arguments must have the same type as the ones expected by the method, else, the behaviour is undefined.
See also Q_ARG(), Q_RETURN_ARG(), qRegisterMetaType(), and QMetaObject::invokeMethod().
bool QMetaMethod::invoke(QObject *object, QGenericReturnArgumentreturnValue, QGenericArgumentval0 = QGenericArgument(0), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
This function overloads invoke().
This overload always invokes this method using the connection type Qt::AutoConnection.
bool QMetaMethod::invoke(QObject *object, Qt::ConnectionTypeconnectionType, QGenericArgumentval0 = QGenericArgument(0), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
This function overloads invoke().
This overload can be used if the return value of the member is of no interest.
bool QMetaMethod::invoke(QObject *object, QGenericArgumentval0 = QGenericArgument(0), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
This function overloads invoke().
This overload invokes this method using the connection type Qt::AutoConnection and ignores return values.
bool QMetaMethod::invokeOnGadget(void *gadget, QGenericReturnArgumentreturnValue, QGenericArgumentval0 = QGenericArgument(nullptr), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
Invokes this method on a Q_GADGET. Returns true if the member could be invoked. Returns false if there is no such member or the parameters did not match.
The pointer gadget must point to an instance of the gadget class.
The invocation is always synchronous.
The return value of this method call is placed in returnValue. You can pass up to ten arguments (val0, val1, val2, val3, val4, val5, val6, val7, val8, and val9) to this method call.
Warning: this method will not test the validity of the arguments: gadget must be an instance of the class of the QMetaObject of which this QMetaMethod has been constructed with. The arguments must have the same type as the ones expected by the method, else, the behavior is undefined.
This function was introduced in Qt 5.5.
See also Q_ARG(), Q_RETURN_ARG(), qRegisterMetaType(), and QMetaObject::invokeMethod().
bool QMetaMethod::invokeOnGadget(void *gadget, QGenericArgumentval0 = QGenericArgument(0), QGenericArgumentval1 = QGenericArgument(), QGenericArgumentval2 = QGenericArgument(), QGenericArgumentval3 = QGenericArgument(), QGenericArgumentval4 = QGenericArgument(), QGenericArgumentval5 = QGenericArgument(), QGenericArgumentval6 = QGenericArgument(), QGenericArgumentval7 = QGenericArgument(), QGenericArgumentval8 = QGenericArgument(), QGenericArgumentval9 = QGenericArgument()) const
This is an overloaded function.
This overload invokes this method for a gadget and ignores return values.
This function was introduced in Qt 5.5.
bool QMetaMethod::isValid() const
Returns true if this method is valid (can be introspected and invoked), otherwise returns false.
This function was introduced in Qt 5.0.
int QMetaMethod::methodIndex() const
Returns this method's index.
This function was introduced in Qt 4.6.
QByteArray QMetaMethod::methodSignature() const
Returns the signature of this method (e.g., setValue(double)).
This function was introduced in Qt 5.0.
See also parameterTypes() and parameterNames().
QMetaMethod::MethodType QMetaMethod::methodType() const
Returns the type of this method (signal, slot, or method).
See also access().
QByteArray QMetaMethod::name() const
Returns the name of this method.
This function was introduced in Qt 5.0.
See also methodSignature() and parameterCount().
int QMetaMethod::parameterCount() const
Returns the number of parameters of this method.
This function was introduced in Qt 5.0.
See also parameterType() and parameterNames().
QList<QByteArray> QMetaMethod::parameterNames() const
Returns a list of parameter names.
See also parameterTypes() and methodSignature().
int QMetaMethod::parameterType(intindex) const
Returns the type of the parameter at the given index.
The return value is one of the types that are registered with QMetaType, or QMetaType::UnknownType if the type is not registered.
This function was introduced in Qt 5.0.
See also parameterCount(), returnType(), and QMetaType.
QList<QByteArray> QMetaMethod::parameterTypes() const
Returns a list of parameter types.
See also parameterNames() and methodSignature().
int QMetaMethod::returnType() const
Returns the return type of this method.
The return value is one of the types that are registered with QMetaType, or QMetaType::UnknownType if the type is not registered.
This function was introduced in Qt 5.0.
See also parameterType(), QMetaType, and typeName().
int QMetaMethod::revision() const
Returns the method revision if one was specified by Q_REVISION, otherwise returns 0.
This function was introduced in Qt 5.1.
const char *QMetaMethod::tag() const
Returns the tag associated with this method.
Tags are special macros recognized by moc that make it possible to add extra information about a method.
Tag information can be added in the following way in the function declaration:
and the information can be accessed by using:
For the moment, moc will extract and record all tags, but it will not handle any of them specially. You can use the tags to annotate your methods differently, and treat them according to the specific needs of your application.
Note: Since Qt 5.0, moc expands preprocessor macros, so it is necessary to surround the definition with #ifndefQ_MOC_RUN, as shown in the example above. This was not required in Qt 4. The code as shown above works with Qt 4 too.
const char *QMetaMethod::typeName() const
Returns the return type name of this method.
See also returnType() and QMetaType::type().
Related Non-Members
booloperator!=(const QMetaMethod &m1, const QMetaMethod &m2)
This is an overloaded function.
Returns true if method m1 is not equal to method m2, otherwise returns false.
This function was introduced in Qt 5.0.
booloperator(const QMetaMethod &m1, const QMetaMethod &m2)
This is an overloaded function.
Call Slot In Qt Download
Returns true if method m1 is equal to method m2, otherwise returns false.
Call Slot Directly Qt
This function was introduced in Qt 5.0.
Macro Documentation
Q_METAMETHOD_INVOKE_MAX_ARGS
Equals maximum number of arguments available for execution of the method via QMetaMethod::invoke()
© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.