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diff --git a/doc/src/datainputoutput.qdoc b/doc/src/datainputoutput.qdoc new file mode 100644 index 00000000..649c0ba1 --- /dev/null +++ b/doc/src/datainputoutput.qdoc @@ -0,0 +1,272 @@ +// Copyright (C) 2022 The Qt Company Ltd. +// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only + +/*! +\page topics-data-io.html +\title Data Input Output +\brief Qt's data input output + +This page provides an overview of various ways of retrieving and saving data using Qt. + +\section1 Saving and Loading Data + +The QIODevice class is the base class for all file and data storage devices +in \l {Qt Core}. All classes that are used for reading and writing data +inherit from it. + +Examples of devices are QFile, QBuffer, QTcpSocket, and QProcess. +QFile is used for reading and writing text, binary files, and \l resources, +using the QIODevice interface. A QFile may be used by itself or, more +conveniently, with a QTextStream or QDataStream. + +The QBuffer class provides a QIODevice interface for a QByteArray. +QTcpSocket enables the developer to establish a TCP connection and transfer +streams of data. QProcess is used to start external programs, and to read +from and write to that process. + +\list +\li \l {Input/Output and Networking} (list of I/O related classes) +\li \l {File and Datastream Functions} +\endlist + +\section1 Serialized data +The Qt API provides support for \l {qtserialization} {Data serialization} for +several use cases. + +\list +\li \l JSON support in Qt provides an easy to use C++ API to parse, modify + and save JSON data +\li \l QDataStream class provides serialization of binary data to a QIODevice +\li \l {Qt XML C++ Classes} provide C++ implementations of the \l XML Streaming + and DOM standards for XML +\endlist + +\section1 Database + +The \l {Qt SQL} module uses driver plugins to communicate with several database +APIs. Qt has drivers for SQLite, MySQL, DB2, Borland InterBase, Oracle, ODBC, +and PostgreSQL. It is also possible to develop your own driver if Qt does not +provide the driver needed. + +Qt's SQL classes can be divided in 3 layers: +\table + \header + \li Layer + \li Purpose + \li Example class + \row + \li + \list + \li Driver layer + \li SQL API layer + \li User Interface layer + \endlist + \li + \list + \li Low-level communication between database and the SQL API layer + \li Provide access to databases + \li Link data from a database to data-aware widgets + \endlist + \li + \list + \li QSqlDriver, QSqlDriverCreator + \li QSqlDatabase, QSqlQuery + \li QSqlQueryModel (readonly), QSqlTableModel (read/write), QSqlRelationalTableModel (read/write with foreign-key support) + \endlist +\endtable + +With all the SQL drivers, with the exception of SQLite, you can connect to a +server that is hosting your database system. If you use the embedded MySQL Server, +you do not need a MySQL server in order to use that database system. + +For instructions on how to build the SQL database drivers, see \l{SQL Database Drivers}. + +The Qt SQLite plugin is very suitable for local storage. SQLite is a relational +database management system contained in a small (~350 KiB) C library. +In contrast to other database management systems, SQLite is not a separate process +that is accessed from the client application, but an integral part of it. SQLite +operates on a single file, which must be set as the database name when opening +a connection. If the file does not exist, SQLite will try to create it. + +SQLite has some restrictions regarding multiple users and multiple transactions. +If you are reading or writing on a file from different transactions, your +application might freeze until one transaction commits or rolls back. + +Once the driver part is set up, the data is accessible using the classes, +QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel. QSqlTableModel +and QSqlRelationalTableModel provide editable models that can used with Qt's +item views. QSqlTableModel has read/write access to a single table, whereas +QSqlRelationalTableModel has read/write access to the main table (not to the +table with the foreign key). + +The following pages contain information about incorporating SQL into +applications: +\list +\li \l {SQL Programming} + \list + \li \l{Database Classes} + \li \l{Executing SQL Statements} + \li \l{Using the SQL Model Classes} + \li \l{Presenting Data in a Table View} + \li \l{Creating Data-Aware Forms} + \endlist +\endlist + +\section1 XML Support in Qt + +Qt provides APIs to read and parse XML streams, and also to write to these +streams. The following key classes facilitate these actions by providing the +necessary infrastructure: +\list + \li QDomDocument class represents the entire XML document as a document tree and + provides primary access to the document's data. + \list + \li It has several functions for creating document data, for example, + \l {QDomDocument::createElement()} {createElement()}, \l {QDomDocument::createTextNode()} + {createTextNode()}, \l {QDomDocument::createComment()} {createComment()}, + \l {QDomDocument::createCDATASection()} {createCDATASection()}, + \l {QDomDocument::createProcessingInstruction()} {createProcessingInstruction()}, + \l {QDomDocument::createAttribute()} {createAttribute()}, + and \l {QDomDocument::createEntityReference()} {createEntityReference()}. + \endlist + \li QXmlStreamReader class provides a parser to read XML. It is a well-formed + XML 1.0 parser that does not include external parsed entities. + \list + \li It understands and resolves XML namespaces. For example, in + case of a StartElement, namespaceUri() returns the namespace the element + is in, and name() returns the element's local name. The combination of + namespaceUri() and name() uniquely identifies an element. + \li It is not CPU-intensive, as it doesn't store the entire XML + document tree in memory. It only stores the current token at the time + it is reported. + \endlist + \li The QXmlStreamWriter class provides an XML writer with a simple streaming + API. It is the counterpart to QXmlStreamReader for writing XML, and it + operates on a QIODevice specified with setDevice(). + \list + \li It is a simple API that provides a dedicated function for every XML + token or event you want to write. + \li It takes care of prefixing namespaces based on the namespaceUri + specified while writing elements or attributes. If you have to use + certain standardized prefixes, you can force the writer to use them + by declaring the namespaces manually with either writeNamespace() + or writeDefaultNamespace(). + \li It can automatically format the generated XML data by adding + line-breaks and indentation, making it readable. This feature can be + turned on with the auto-formatting property. + \endlist +\endlist + +The following topics provide more insight into Qt XML support: +\list +\li \l {An Introduction to Namespaces} +\li \l {XML Streaming} +\endlist + +\section1 JSON in Qt + +JSON is a text-based open standard for data interchange that is easy to read +and parse. It is used for representing simple data structures and associative +arrays, called objects. It is related to JavaScript, but is a language-independent +notation form. + +An object can take 2 forms: + +\table + \header + \list + \li Collection of name/value pairs + \li Ordered list of values + \endlist + \row + \li + \list + \li + { + "last_name": "Routledge", + "first_name": "Ronald", + "birth_date": 1960 + } + \li "colours": ["green", "blue", "yellow"] + \endlist + \li +\endtable + +\list +\li \l {JSON Support in Qt} +\li \l {JSON Save Game Example} +\endlist + +\section1 Local Storage + +The Local Storage API provides the ability to access local offline storage in an + SQL database from QML and JavaScript. + +These databases are user-specific and QML-specific, but accessible to all QML +applications. They are stored in the Databases subdirectory of +QDeclarativeEngine::offlineStoragePath() as SQLite databases +(\l {SQL Database Drivers}). + +The API conforms to the Synchronous API of the HTML5 Web Database API, +W3C Working Draft 29 October 2009 (\l {HTML5 Web Database API}). + +See \l {Qt Quick Examples - Local Storage} for a basic demonstration of using the Local +Storage API. + +\list +\li \l {Qt Quick Local Storage QML Types} +\endlist + +\section1 QSettings Class + +The QSettings class provides persistent storage of application settings. +An application usually remembers its settings from the previous session. + +Settings are stored differently on different platforms. For example, +on Windows they are stored in the registry, whereas on \macos they are +stored in XML files. + +QSettings enable you to save and restore application settings in a portable +manner. Constructing and destroying a QSettings object is lightweight and +fast. While creating an object of QSettings, it is a good practice to specify +not only the name of the application, but also the name of your organization. +For example: + +\code + QSettings settings("MyCompany", "Accountancy"); +\endcode + +\section1 Resources + +\l {The Qt Resource System} is a platform-independent mechanism for storing binary +files in the application's executable. This is handy if your application +frequently needs a certain file, or set of files. It also protects against +loss of that particular file . + +Resource data can either be compiled into the binary and accessed +immediately in the application code, or a binary resource can be created +dynamically and registered with the resource system by the application. + +By default, resources are accessible from the application code by the same file +name as they are stored in the source tree, with a \c{:/} prefix, or by a URL +with a qrc scheme. + +\section1 Connectivity +\list + \li \l {Qt Serial Port} + \li \l {Qt Modbus} + \li \l {Qt CAN Bus} + \li \l {Qt Bluetooth} + \li \l {Qt NFC} +\endlist + +\section1 File Archiving + +An archive file is a collection of files or directories which are generally +compressed in order to reduce the space they would otherwise consume on a +drive. Examples of archive files are ZIP, TAR, RAR and 7z. + +Qt has support for archives produced by zlib (see +\l{How to compress data with Qt?}{qCompress() and qUncompress()}). + +*/ |