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diff --git a/doc/src/howtos/custom-types.qdoc b/doc/src/howtos/custom-types.qdoc new file mode 100644 index 0000000..997c8bc --- /dev/null +++ b/doc/src/howtos/custom-types.qdoc @@ -0,0 +1,179 @@ +/**************************************************************************** +** +** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:LGPL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the either Technology Preview License Agreement or the +** Beta Release License Agreement. +** +** GNU Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain +** additional rights. These rights are described in the Nokia Qt LGPL +** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this +** package. +** +** GNU General Public License Usage +** Alternatively, this file may be used under the terms of the GNU +** General Public License version 3.0 as published by the Free Software +** Foundation and appearing in the file LICENSE.GPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU General Public License version 3.0 requirements will be +** met: http://www.gnu.org/copyleft/gpl.html. +** +** If you are unsure which license is appropriate for your use, please +** contact the sales department at http://qt.nokia.com/contact. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \page custom-types.html + \title Creating Custom Qt Types + \brief How to create and register new types with Qt. + + \ingroup best-practices + + \tableofcontents + + \section1 Overview + + When creating user interfaces with Qt, particularly those with specialized controls and + features, developers sometimes need to create new data types that can be used alongside + or in place of Qt's existing set of value types. + + Standard types such as QSize, QColor and QString can all be stored in QVariant objects, + used as the types of properties in QObject-based classes, and emitted in signal-slot + communication. + + In this document, we take a custom type and describe how to integrate it into Qt's object + model so that it can be stored in the same way as standard Qt types. We then show how to + register the custom type to allow it to be used in signals and slots connections. + + \section1 Creating a Custom Type + + Before we begin, we need to ensure that the custom type we are creating meets all the + requirements imposed by QMetaType. In other words, it must provide: + + \list + \o a public default constructor, + \o a public copy constructor, and + \o a public destructor. + \endlist + + The following \c Message class definition includes these members: + + \snippet examples/tools/customtype/message.h custom type definition + + The class also provides a constructor for normal use and two public member functions + that are used to obtain the private data. + + \section1 Declaring the Type with QMetaType + + The \c Message class only needs a suitable implementation in order to be usable. + However, Qt's type system will not be able to understand how to store, retrieve + and serialize instances of this class without some assistance. For example, we + will be unable to store \c Message values in QVariant. + + The class in Qt responsible for custom types is QMetaType. To make the type known + to this class, we invoke the Q_DECLARE_METATYPE() macro on the class in the header + file where it is defined: + + \snippet examples/tools/customtype/message.h custom type meta-type declaration + + This now makes it possible for \c Message values to be stored in QVariant objects + and retrieved later. See the \l{Custom Type Example} for code that demonstrates + this. + + The Q_DECLARE_METATYPE() macro also makes it possible for these values to be used as + arguments to signals, but \e{only in direct signal-slot connections}. + To make the custom type generally usable with the signals and slots mechanism, we + need to perform some extra work. + + \section1 Creating and Destroying Custom Objects + + Although the declaration in the previous section makes the type available for use + in direct signal-slot connections, it cannot be used for queued signal-slot + connections, such as those that are made between objects in different threads. + This is because the meta-object system does not know how to handle creation and + destruction of objects of the custom type at run-time. + + To enable creation of objects at run-time, call the qRegisterMetaType() template + function to register it with the meta-object system. This also makes the type + available for queued signal-slot communication as long as you call it before you + make the first connection that uses the type. + + The \l{Queued Custom Type Example} declares a \c Block class which is registered + in the \c{main.cpp} file: + + \snippet examples/threads/queuedcustomtype/main.cpp main start + \dots + \snippet examples/threads/queuedcustomtype/main.cpp register meta-type for queued communications + \dots + \snippet examples/threads/queuedcustomtype/main.cpp main finish + + This type is later used in a signal-slot connection in the \c{window.cpp} file: + + \snippet examples/threads/queuedcustomtype/window.cpp Window constructor start + \dots + \snippet examples/threads/queuedcustomtype/window.cpp connecting signal with custom type + \dots + \snippet examples/threads/queuedcustomtype/window.cpp Window constructor finish + + If a type is used in a queued connection without being registered, a warning will be + printed at the console; for example: + + \code + QObject::connect: Cannot queue arguments of type 'Block' + (Make sure 'Block' is registered using qRegisterMetaType().) + \endcode + + \section1 Making the Type Printable + + It is often quite useful to make a custom type printable for debugging purposes, + as in the following code: + + \snippet examples/tools/customtype/main.cpp printing a custom type + + This is achieved by creating a streaming operator for the type, which is often + defined in the header file for that type: + + \snippet examples/tools/customtype/message.h custom type streaming operator + + The implementation for the \c Message type in the \l{Custom Type Example} + goes to some effort to make the printable representation as readable as + possible: + + \snippet examples/tools/customtype/message.cpp custom type streaming operator + + The output sent to the debug stream can, of course, be made as simple or as + complicated as you like. Note that the value returned by this function is + the QDebug object itself, though this is often obtained by calling the + maybeSpace() member function of QDebug that pads out the stream with space + characters to make it more readable. + + \section1 Further Reading + + The Q_DECLARE_METATYPE() macro and qRegisterMetaType() function documentation + contain more detailed information about their uses and limitations. + + The \l{Custom Type Example}{Custom Type}, + \l{Custom Type Sending Example}{Custom Type Sending} + and \l{Queued Custom Type Example}{Queued Custom Type} examples show how to + implement a custom type with the features outlined in this document. + + The \l{Debugging Techniques} document provides an overview of the debugging + mechanisms discussed above. +*/ |