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authorLars Knoll <lars.knoll@nokia.com>2009-03-23 09:18:55 (GMT)
committerSimon Hausmann <simon.hausmann@nokia.com>2009-03-23 09:18:55 (GMT)
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+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Qt Software Information (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 qt-sales@nokia.com.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+ \title Moving from QSA to Qt Script
+ \page porting-qsa.html
+ \ingroup porting
+
+ The purpose of this document is to map the differences between Qt
+ Script for Applications (QSA) and Qt Script, the ECMAScript compatible
+ engine supplied with Qt 4.3. This document is not supposed to be a
+ complete function by function porting guide, but will cover the most
+ obvious aspects.
+
+ First of all it is important to realize that Qt Script is only an
+ interpreter, it does not provide an editor, completion or script project
+ management, like QSA does. Qt Script however does provides almost full
+ compliance with the ECMAScript standard and performs significantly
+ better than the script engine provided by QSA.
+
+ \tableofcontents
+
+ \section1 The Scripting Language
+
+ The scripting language used in QSA, from here on referred to as QSA,
+ was derived from ECMAScript 3.0 and 4.0 and is a hybrid of these
+ standards. Most of the run-time logic, such as classes and scoping
+ rules, is based on the ECMAScript 4.0 proposal, while the library
+ implementation is based on the ECMAScript 3.0 standard.
+ Qt Script on the other hand is solely based on the ECMAScript 3.0
+ standard. Though the languages look identical at first glance,
+ there are a few differences that we'll cover in the sections below.
+
+
+ \section2 Classes vs. Objects and Properties
+
+ QSA implements classes and inheritance much in a familiar way to users
+ of other object oriented languages, like C++ and Java. However, the
+ ECMAScript 3.0 standard defines that everything is an object, and objects
+ can have named properties. For instance to create an point object with
+ the properties x and y one would write the following Qt Script code:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 0
+
+ The object \c point in this case is constructed as a plain object and
+ we assign two properties, \c x and \c y, to it with the values 12 and
+ 35. The \c point object is assigned to the "Global Object" as the
+ named property \c{point}. The global object can be considered the
+ global namespace of the script engine. Similarly, global functions are
+ named properties of the global object; for example:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 1
+
+ An equivalent construction that illustrates that the function is a
+ property of the global object is the following assignment:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 2
+
+ Since functions are objects, they can be assigned to objects as
+ properties, becoming member functions:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 3
+
+ In the code above, we see the first subtle difference between
+ QSA and Qt Script. In QSA one would write the point class like this:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 4
+
+ where in the \c manhattanLength() function we access \c x and \c y
+ directly because, when the function is called, the \c this object is
+ implicitly part of the current scope, as in C++. In Qt Script,
+ however, this is not the case, and we need to explicitly access
+ the \c x and \c y values via \c{this}.
+
+ All the code above runs with QSA except the assignment of a function
+ to \c{point.manhattanLength}, which we repeat here for clarity:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 5
+
+ This is because, in QSA, the value of \c this is decided based on
+ the location of the declaration of the function it is used in. In the
+ code above, the function is assigned to an object, but it is declared
+ in the global scope, hence there will be no valid \c this value.
+ In Qt Script, the value of \c this is decided at run-time,
+ hence you could have assigned the \c manhattanLength() function to any
+ object that had \c x and \c y values.
+
+
+ \section2 Constructors
+
+ In the code above, we use a rather awkward method for constructing
+ the objects, by first instantiating them, then manually
+ assigning properties to them. In QSA, the proper way to solve this
+ is to implement a constructor in the class:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 6
+
+ The equivalent in Qt Script is to create a constructor function:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 7
+
+ As we can see, the constructor is just a normal function. What is
+ special with is how we call it, namely prefixed with the \c new
+ keyword. This will create a new object and call the \c Car()
+ function with the newly created object as the \c this pointer.
+ So, in a sense, it is equivalent to:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 8
+
+ This is similar to the manhattenLength() example above. Again, the
+ main difference between QSA and Qt Script is that one has to
+ explicitly use the keyword \c this to access the members and that
+ instead of declaring the variable, \c regNumber, we just extend the
+ \c this object with the property.
+
+
+ \section2 Member Functions and Prototypes
+
+ As we saw above, one way of creating member functions of a Qt Script
+ object is to assign the member function to the object as a property
+ and use the \c this object inside the functions. So, if we add a
+ \c toString function to the \c Car class
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 9
+
+ one could write this in Qt Script as:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 10
+
+ In QSA, the member functions were part of the class declaration,
+ and were therefore shared between all instances of a given class.
+ In Qt Script, each instance has a instance member for each function.
+ This means that more memory is used when multiple instances are used.
+ Qt Script uses prototypes to remedy this.
+
+ The basic prototype-based inheritance mechanism works as follows.
+ Each Qt Script object has an internal link to another object, its
+ prototype. When a property is looked up in an object, and the object
+ itself does not have the property, the interpreter searches for the
+ property in the prototype object instead; if the prototype has the
+ property then that property is returned. If the prototype object does
+ not have the property, the interpreter searches for the property in
+ the prototype of the prototype object, and so on.
+
+ This chain of objects constitutes a prototype chain. The chain of
+ prototype objects is followed until the property is found or the end
+ of the chain is reached.
+
+ To make the \c toString() function part of the prototype, we write
+ code like this:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 11
+
+ Here, we made the \c toString() function part of the prototype so
+ that, when we call \c{car.toString()} it will be resolved via the
+ internal prototype object of the car object. Note, however, that the
+ \c this object is still the original object that the function was
+ called on, namely \c{car}.
+
+
+ \section2 Inheritance
+
+ Now that we've seen how to use prototypes to create a "class" members
+ in Qt Script, let's see how we can use prototypes to create
+ polymorphism. In QSA you would write
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 12
+
+ With Qt Script, we acheive the same effect by creating a prototype
+ chain. The default prototype of an object is a plain \c Object
+ without any special members, but it is possible to replace this
+ object with another prototype object.
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 13
+
+ In the code above, we have a constructor, \c{GasolineCar}, which
+ calls the "base class" implementation of the constructor to
+ initialize the \c this object with the property \c{regNumber},
+ based on the values passed in the constructor. The interesting line
+ in this case is the line after the constructor where we change the
+ default prototype for \c GasolineCar to be an instance of type
+ \c{Car}. This means that all members available in a \c Car object
+ are now available in all \c GasolineCar objects. In the last line,
+ we replace the \c toString() function in the prototype with our own,
+ thus overriding the \c toString() for all instances of
+ \c{GasolineCar}.
+
+
+ \section2 Static Members
+
+ QSA allowed users to declare static members in classes, and these
+ could be accessed both through instances of the class and through
+ the class itself. For example, the following variable is accessed
+ through the \c Car class:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 14
+
+ The equivalent in Qt Script is to assign variables that should appear
+ as static members as properties of the constructor function. For
+ example:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 15
+
+ Note that in QSA, static member variables were also accessible in
+ instances of the given class. In Qt Script, with the approach
+ illustrated above, the variable is a member of the constructor
+ object only, and thus only accessible through \c{Car.globalCount}.
+
+
+ \section1 The Built-in Functions and Library
+
+ The built-in functions in QSA are based on those defined in the
+ ECMAScript 3.0 standard, the same standard used for Qt Script, but
+ QSA adds some extensions to this, specifically for the \c String
+ and \c RegExp types. QSA also lacked some functions from the
+ standard, most notably the \c Date type. Below we list all the
+ differences. All changes made to Qt Script are to increase
+ compliance with ECMAScript 3.0.
+
+ \table
+ \header \o QSA Function \o Notes about Equivalent Qt Script Functions
+ \row \o eval()
+ \o The eval function in QSA opened a new scope for code being
+ executed in the eval function, so locally declared variables were not
+ accessible outside. In Qt Script, the eval() function shares the
+ current scope, making locally declared variables accessible outside
+ the eval() call.
+
+ \row \o debug()
+ \o This function is not available in Qt Script. Use print() instead.
+
+ \row \o connect()
+ \o QSA had closures, meaning that a member function
+ reference implicitly contained its \c this object. Qt Script does not
+ support this. See the Qt Script documentation for details on using the
+ connect function.
+
+ \row \o String.arg()
+ \o This function is not available in Qt Script. Use replace() or concat() instead.
+
+ \row \o String.argDec()
+ \o This function is not available in Qt Script. Use replace() or concat() instead.
+
+ \row \o String.argInt()
+ \o This function is not available in Qt Script. Use replace() or concat() instead.
+
+ \row \o String.argStr()
+ \o This function is not available in Qt Script. Use replace() or concat() instead.
+
+ \row \o String.endsWith()
+ \o This function is not available in Qt Script. Use lastIndexOf() instead.
+
+ \row \o String.find()
+ \o This function is not available in Qt Script. Use indexOf() instead.
+
+ \row \o String.findRev()
+ \o This function is not available in Qt Script. Use lastIndexOf() and length instead.
+
+ \row \o String.isEmpty()
+ \o This function is not available in Qt Script. Use length == 0 instead.
+
+ \row \o String.left()
+ \o This function is not available in Qt Script. Use substring() instead.
+
+ \row \o String.lower()
+ \o This function is not available in Qt Script. Use toLowerCase() instead.
+
+ \row \o String.mid()
+ \o This function is not available in Qt Script. Use substring() instead.
+
+ \row \o String.right()
+ \o This function is not available in Qt Script. Use substring() instead.
+
+ \row \o String.searchRev()
+ \o This function is not available in Qt Script. Use search() / match() instead.
+
+ \row \o String.startsWith()
+ \o This function is not available in Qt Script. Use indexOf() == 0 instead.
+
+ \row \o String.upper()
+ \o This function is not available in Qt Script. Use toUpperCase() instead.
+
+ \row \o RegExp.valid
+ \o This property is not available in Qt Script because it is not
+ required; a \c SyntaxError exception is thrown for bad \c RegExp objects.
+
+ \row \o RegExp.empty
+ \o This property is not available in Qt Script. Use \c{toString().length == 0} instead.
+
+ \row \o RegExp.matchedLength
+ \o This property is not available in Qt Script. RegExp.exec() returns an
+ array whose size is the matched length.
+
+ \row \o RegExp.capturedTexts
+ \o This property is not available in Qt Script. RegExp.exec() returns an
+ array of captured texts.
+
+ \row \o RegExp.search()
+ \o This function is not available in Qt Script. Use RegExp.exec() instead.
+
+ \row \o RegExp.searchRev()
+ \o This function is not available in Qt Script. Use RegExp.exec() or
+ String.search()/match() instead.
+
+ \row \o RegExp.exactMatch()
+ \o This function is not available in Qt Script. Use RegExp.exec() instead.
+
+ \row \o RegExp.pos()
+ \o This function is not available in Qt Script. Use String.match() instead.
+
+ \row \o RegExp.cap()
+ \o This function is not available in Qt Script. RegExp.exec() returns an
+ array of captured texts.
+ \endtable
+
+ QSA also defined some internal Qt API which is not present in Qt
+ Script. The types provided by QSA which are not provided by Qt Script are:
+
+ \list
+ \o Rect
+ \o Point
+ \o Size
+ \o Color
+ \o Palette
+ \o ColorGroup
+ \o Font
+ \o Pixmap
+ \o ByteArray
+ \endlist
+
+
+ \section1 The C++ API of QSA vs Qt Script
+
+ QSA is more than just a scripting engine. It provides project
+ management, an editor with completion and a minimalistic IDE to edit
+ scriptable projects. Qt Script on the other hand is just a scripting
+ engine. This means that equivalents to the classes \c QSEditor,
+ \c QSScript, \c QSProject and \c QSWorkbench do not exist in Qt Script.
+ QSA also provides some extension APIs through the \c QSUtilFactory and
+ \c QSInputDialogFactory. There is also no equivalent to these classes
+ in the Qt Script API.
+
+
+ \section2 Making QObjects Accessible from Scripts
+
+ There are two different ways of making \l{QObject}s accessible from
+ scripts in QSA. The first method is via the
+ \c QSInterpreter::addTransientObject() and \c QSProject::addObject()
+ functions. In this case objects are added to the global namespace of
+ the interpreter using their object names as the names of the
+ variables.
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 16
+
+ The code above adds the button to the global namespace under the name
+ "button". One obvious limitation here is that there is potential for
+ either unnamed \l{QObject}s or objects whose names conflict. Qt Script
+ provides a more flexible way of adding QObjects to the scripting
+ environment.
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 17
+
+ In the code above we create a QPushButton and wrap it in a script
+ value using the function, QScriptEngine::newQObject(). This gives us
+ a script value that we put into the global object using the name
+ "button". The concept of objects and properties discussed above is
+ quite visible here in the public C++ API as well. We have no
+ dependency on the object's name and we can also resolve name conflicts
+ more gracefully. Here, we operate directly on QScriptValue objects.
+ This is the actual object that is being passed around inside
+ the script engine, so we actually have low-level access to the
+ internal script data structures, far beyond that which is possible
+ in QSA. Properties, signals and slots of the QObject are accessible
+ to the scripter in Qt Script, just like in QSA.
+
+ The other way to expose \l{QObject}s in QSA was to create a
+ \c QSObjectFactory that made it possible to instantiate QObjects from
+ scripts.
+
+ Below is listed some code from the filter example in the QSA
+ package.
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 18
+
+ The equivalent in Qt Script is written in much the same way as
+ constructors are written in scripts. We register a callback C++
+ function under the name "ImageSource" in the global namespace and
+ return the QObject from this function:
+
+ \snippet doc/src/snippets/code/doc_src_porting-qsa.qdoc 19
+
+ In the Qt Script case we use the same approach that we use to expose
+ a QObject, namely via QScriptEngine::newQObject(). This function also
+ has the benefit that it is possible to specify if the QObject should
+ expose properties and slots of its base class. It is also possible to
+ specify custom ownership rules.
+
+ The reader might question why we don't add the constructor function
+ directly into the namespace, but create a meta-object script value for
+ it in addition. The plain function would certainly be good enough,
+ but by creating a QMetaObject based constructor we get the enums on
+ QPushButton for free in the QPushButton function object. Exposing
+ enums in QSA is rather painful in comparison.
+
+ If we want to add more "static" data to the QPushButton type in Qt
+ Script, we're free to add properties, similar to how we did for
+ the script. It is also possible to add custom functions to a Qt Script
+ QPushButton instance by setting more properties on it, such as making
+ the \l{QPushButton::}{setText()} C++ function available. It is also
+ possible to acheive this by installing a custom prototype, and be
+ memory efficient, as discussed in the script example above.
+
+
+ \section2 Accessing Non-QObjects
+
+ In QSA, it was possible to expose non-QObjects to QSA by wrapping them
+ in a QObject and using either \c QSWrapperFactory or \c QSObjectFactory
+ to expose them. Deciding when to use each of these classes could be
+ confusing, as one was used for script based construction and the other
+ for wrapping function parameters and return values, but in essence they
+ did exactly the same thing.
+
+ In Qt Script, providing access to QObjects and non-QObjects is done in
+ the same way as shown above, by creating a constructor function, and
+ by adding properties or a custom prototype to the constructed object.
+
+
+ \section2 Data Mapping
+
+ QSA supported a hardcoded set of type mappings which covered most
+ of the QVariant types, QObjects and primitives. For more complex type
+ signatures, such as the template-based tool classes, it had rather
+ limited support. Qt Script is significantly better at type mapping
+ and will convert lists of template types into arrays of the
+ appropriate types, given that all the types are declared to the
+ meta-type system.
+*/