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diff --git a/doc/src/porting/porting-qsa.qdoc b/doc/src/porting/porting-qsa.qdoc new file mode 100644 index 0000000..e831583 --- /dev/null +++ b/doc/src/porting/porting-qsa.qdoc @@ -0,0 +1,461 @@ +/**************************************************************************** +** +** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies). +** All rights reserved. +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** 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 Technology Preview License Agreement accompanying +** this package. +** +** GNU Free Documentation License +** Alternatively, this file may be used under the terms of the GNU Free +** Documentation License version 1.3 as published by the Free Software +** Foundation and appearing in the file included in the packaging of this +** file. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@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.js 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.js 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.js 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.js 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.js 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.js 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.js 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.js 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.js 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.js 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.cpp 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.cpp 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.cpp 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.cpp 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. +*/ |