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Diffstat (limited to 'examples/corelib/serialization/savegame/doc/src/savegame.qdoc')
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1 files changed, 109 insertions, 73 deletions
diff --git a/examples/corelib/serialization/savegame/doc/src/savegame.qdoc b/examples/corelib/serialization/savegame/doc/src/savegame.qdoc index 72dd393cb0..46fca15b62 100644 --- a/examples/corelib/serialization/savegame/doc/src/savegame.qdoc +++ b/examples/corelib/serialization/savegame/doc/src/savegame.qdoc @@ -1,44 +1,21 @@ -/**************************************************************************** -** -** Copyright (C) 2016 The Qt Company Ltd. -** Contact: https://www.qt.io/licensing/ -** -** This file is part of the documentation of the Qt Toolkit. -** -** $QT_BEGIN_LICENSE:FDL$ -** Commercial License Usage -** Licensees holding valid commercial Qt licenses may use this file in -** accordance with the commercial license agreement provided with the -** Software or, alternatively, in accordance with the terms contained in -** a written agreement between you and The Qt Company. For licensing terms -** and conditions see https://www.qt.io/terms-conditions. For further -** information use the contact form at https://www.qt.io/contact-us. -** -** GNU Free Documentation License Usage -** 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. Please review the following information to ensure -** the GNU Free Documentation License version 1.3 requirements -** will be met: https://www.gnu.org/licenses/fdl-1.3.html. -** $QT_END_LICENSE$ -** -****************************************************************************/ +// Copyright (C) 2016 The Qt Company Ltd. +// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only /*! \example serialization/savegame - \title JSON Save Game Example + \examplecategory {Data Processing & I/O} + \title Saving and Loading a Game - \brief The JSON Save Game example demonstrates how to save and load a - small game using QJsonDocument, QJsonObject and QJsonArray. + \brief How to save and load a game using Qt's JSON or CBOR classes. Many games provide save functionality, so that the player's progress through the game can be saved and loaded at a later time. The process of saving a - game generally involves serializing each game object's member variables - to a file. Many formats can be used for this purpose, one of which is JSON. - With QJsonDocument, you also have the ability to serialize a document in a - \l {RFC 7049} {CBOR} format, which is great if you - don't want the save file to be readable, or if you need to keep the file size down. + game generally involves serializing each game object's member variables to a + file. Many formats can be used for this purpose, one of which is JSON. With + QJsonDocument, you also have the ability to serialize a document in a \l + {RFC 7049} {CBOR} format, which is great if you don't want the save file to + be easy to read (but see \l {Parsing and displaying CBOR data} for how it \e + can be read), or if you need to keep the file size down. In this example, we'll demonstrate how to save and load a simple game to and from JSON and binary formats. @@ -48,45 +25,83 @@ The Character class represents a non-player character (NPC) in our game, and stores the player's name, level, and class type. - It provides read() and write() functions to serialise its member variables. + It provides static fromJson() and non-static toJson() functions to + serialise itself. + + \note This pattern (fromJson()/toJson()) works because QJsonObjects can be + constructed independent of an owning QJsonDocument, and because the data + types being (de)serialized here are value types, so can be copied. When + serializing to another format — for example XML or QDataStream, which require passing + a document-like object — or when the object identity is important (QObject + subclasses, for example), other patterns may be more suitable. See the + \l{dombookmarks} example for XML, and the implementation of + \l QListWidgetItem::read() and \l QListWidgetItem::write() + for idiomatic QDataStream serialization. The \c{print()} functions in this example + are good examples of QTextStream serialization, even though they, of course, lack + the deserialization side. \snippet serialization/savegame/character.h 0 - Of particular interest to us are the read and write function + Of particular interest to us are the fromJson() and toJson() function implementations: - \snippet serialization/savegame/character.cpp 0 + \snippet serialization/savegame/character.cpp fromJson - In the read() function, we assign Character's members values from the - QJsonObject argument. You can use either \l QJsonObject::operator[]() or - QJsonObject::value() to access values within the JSON object; both are - const functions and return QJsonValue::Undefined if the key is invalid. We - check if the keys are valid before attempting to read them with - QJsonObject::contains(). + In the fromJson() function, we construct a local \c result Character object + and assign \c{result}'s members values from the QJsonObject argument. You + can use either \l QJsonObject::operator[]() or QJsonObject::value() to + access values within the JSON object; both are const functions and return + QJsonValue::Undefined if the key is invalid. In particular, the \c{is...} + functions (for example \l QJsonValue::isString(), \l + QJsonValue::isDouble()) return \c false for QJsonValue::Undefined, so we + can check for existence as well as the correct type in a single lookup. - \snippet serialization/savegame/character.cpp 1 + If a value does not exist in the JSON object, or has the wrong type, we + don't write to the corresponding \c result member, either, thereby + preserving any values the default constructor may have set. This means + default values are centrally defined in one location (the default + constructor) and need not be repeated in serialisation code + (\l{https://en.wikipedia.org/wiki/Don%27t_repeat_yourself}{DRY}). - In the write() function, we do the reverse of the read() function; assign - values from the Character object to the JSON object. As with accessing - values, there are two ways to set values on a QJsonObject: - \l QJsonObject::operator[]() and QJsonObject::insert(). Both will override - any existing value at the given key. + Observe the use of + \l{https://en.cppreference.com/w/cpp/language/if#If_statements_with_initializer} + {C++17 if-with-initializer} to separate scoping and checking of the variable \c v. + This means we can keep the variable name short, because its scope is limited. - Next up is the Level class: + Compare that to the naïve approach using \c QJsonObject::contains(): + + \badcode + if (json.contains("name") && json["name"].isString()) + result.mName = json["name"].toString(); + \endcode + + which, beside being less readable, requires a total of three lookups (no, + the compiler will \e not optimize these into one), so is three times + slower and repeats \c{"name"} three times (violating the DRY principle). + + \snippet serialization/savegame/character.cpp toJson + + In the toJson() function, we do the reverse of the fromJson() function; + assign values from the Character object to a new JSON object we then + return. As with accessing values, there are two ways to set values on a + QJsonObject: \l QJsonObject::operator[]() and \l QJsonObject::insert(). + Both will override any existing value at the given key. + + \section1 The Level Class \snippet serialization/savegame/level.h 0 - We want to have several levels in our game, each with several NPCs, so we - keep a QList of Character objects. We also provide the familiar read() and - write() functions. + We want the levels in our game to each each have several NPCs, so we keep a QList + of Character objects. We also provide the familiar fromJson() and toJson() + functions. - \snippet serialization/savegame/level.cpp 0 + \snippet serialization/savegame/level.cpp fromJson - Containers can be written and read to and from JSON using QJsonArray. In our + Containers can be written to and read from JSON using QJsonArray. In our case, we construct a QJsonArray from the value associated with the key \c "npcs". Then, for each QJsonValue element in the array, we call - toObject() to get the Character's JSON object. The Character object can then - read their JSON and be appended to our NPC array. + toObject() to get the Character's JSON object. Character::fromJson() can + then turn that QJSonObject into a Character object to append to our NPC array. \note \l{Container Classes}{Associate containers} can be written by storing the key in each value object (if it's not already). With this approach, the @@ -94,11 +109,13 @@ element is used as the key to construct the container when reading it back in. - \snippet serialization/savegame/level.cpp 1 + \snippet serialization/savegame/level.cpp toJson - Again, the write() function is similar to the read() function, except + Again, the toJson() function is similar to the fromJson() function, except reversed. + \section1 The Game Class + Having established the Character and Level classes, we can move on to the Game class: @@ -110,26 +127,43 @@ Next, we provide accessors for the player and levels. We then expose three functions: newGame(), saveGame() and loadGame(). - The read() and write() functions are used by saveGame() and loadGame(). + The read() and toJson() functions are used by saveGame() and loadGame(). + + \div{class="admonition note"}\b{Note:} + Despite \c Game being a value class, we assume that the author wants a game to have + identity, much like your main window would have. We therefore don't use a + static fromJson() function, which would create a new object, but a read() + function we can call on existing objects. There's a 1:1 correspondence + between read() and fromJson(), in that one can be implemented in terms of + the other: + + \code + void read(const QJsonObject &json) { *this = fromJson(json); } + static Game fromObject(const QJsonObject &json) { Game g; g.read(json); return g; } + \endcode - \snippet serialization/savegame/game.cpp 0 + We just use what's more convenient for callers of the functions. + \enddiv + + \snippet serialization/savegame/game.cpp newGame To setup a new game, we create the player and populate the levels and their NPCs. - \snippet serialization/savegame/game.cpp 1 + \snippet serialization/savegame/game.cpp read - The first thing we do in the read() function is tell the player to read - itself. We then clear the level array so that calling loadGame() on the - same Game object twice doesn't result in old levels hanging around. + The read() function starts by replacing the player with the + one read from JSON. We then clear() the level array so that calling + loadGame() on the same Game object twice doesn't result in old levels + hanging around. We then populate the level array by reading each Level from a QJsonArray. - \snippet serialization/savegame/game.cpp 2 + \snippet serialization/savegame/game.cpp toJson - We write the game to JSON similarly to how we write Level. + Writing the game to JSON is similar to writing a level. - \snippet serialization/savegame/game.cpp 3 + \snippet serialization/savegame/game.cpp loadGame When loading a saved game in loadGame(), the first thing we do is open the save file based on which format it was saved to; \c "save.json" for JSON, @@ -143,14 +177,16 @@ After constructing the QJsonDocument, we instruct the Game object to read itself and then return \c true to indicate success. - \snippet serialization/savegame/game.cpp 4 + \snippet serialization/savegame/game.cpp saveGame Not surprisingly, saveGame() looks very much like loadGame(). We determine the file extension based on the format, print a warning and return \c false if the opening of the file fails. We then write the Game object to a - QJsonDocument, and call either QJsonDocument::toJson() or to - QJsonDocument::toBinaryData() to save the game, depending on which format - was specified. + QJsonObject. To save the game in the format that was specified, we + convert the JSON object into either a QJsonDocument for a subsequent + QJsonDocument::toJson() call, or a QCborValue for QCborValue::toCbor(). + + \section1 Tying It All Together We are now ready to enter main(): @@ -182,5 +218,5 @@ human-readable JSON files, but you also have the option to use a binary format if it's required, \e without rewriting any code. - \sa {JSON Support in Qt}, {Data Storage} + \sa {JSON Support in Qt}, {CBOR Support in Qt}, {Data Input Output} */ |