nfc-overview.qdoc

Go to the documentation of this file.

// Copyright (C) 2013 Aaron McCarthy <mccarthy.aaron@gmail.com>
// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\ingroup technology-apis
\title Qt NFC Overview
\page qtnfc-overview.html
\brief Provides access to NFC enabled devices.
 
\tableofcontents
 
With the Qt NFC API typical use cases are:
 
\list
    \li Detecting NFC tags.
    \li Reading and writing NDEF messages.
    \li Registering NDEF message handlers.
    \li Sharing files and messages.
\endlist
 
The following sections describe how to use Qt NFC C++ classes and QML types for the above use cases.
 
\note On Android, the detection of new NFC tags only works in foreground applications. Android
services do not support this because of API limitations on the Android side. The only way to use a
\l{https://developer.android.com/reference/android/nfc/Tag}{Tag} in a service is to provide an
\l{https://developer.android.com/guide/components/aidl}{AIDL} interface accepting the Tag and forward
it to Qt as shown in the following example.
 
\code
    public void setTag(Tag pTag) {
        Intent newIntent = new Intent();
        newIntent.putExtra(NfcAdapter.EXTRA_TAG, pTag);
        QtNative.onNewIntent(newIntent);
    }
\endcode
 
\section1 C++ Overview
 
The C++ API provides access to the full feature set of the Qt NFC API. This section introduces the
major features available to developers.
 
\section2 Detecting NFC Tags
 
The \l QNearFieldManager class is responsible for the detection of new NFC tags coming
into range of the device. The \l QNearFieldManager::targetDetected() and
\l QNearFieldManager::targetLost() signals are emitted when
a tag comes into or leaves the range. The passed \l QNearFieldTarget parameter acts
as primary interaction point for each detected tag. The detection does not actually start though until
\l QNearFieldManager::startTargetDetection() has been called.
 
\code
m_manager = new QNearFieldManager(this);
connect(m_manager, &QNearFieldManager::targetDetected,
        this, &MainWindow::targetDetected);
connect(m_manager, &QNearFieldManager::targetLost,
        this, &MainWindow::targetLost);
m_manager->startTargetDetection(QNearFieldTarget::NdefAccess);
\endcode
 
Finally the detection can be stopped:
 
\code
m_manager->stopTargetDetection();
\endcode
 
Although each \l QNearFieldTarget instance is owned by its related \l QNearFieldManager
instance it can be beneficial to manually delete each instance. Otherwise they would continue to
exist until the \l QNearFieldManager instance is deleted. The best way to do that would be in response
to the \l QNearFieldManager::targetLost() signal:
 
\code
void MainWindow::targetLost(QNearFieldTarget *target)
{
    target->deleteLater();
}
\endcode
 
\note The target object should only be deleted via deleteLater() if it is deleted inside the slot.
 
\section2 Connecting NFC Tags
 
All functions of \l QNearFieldTarget that require a connection will create one by its own.
An active connection will prevent other instances to create a connection because only one
connection at the time is allowed.
 
Qt 5 disconnected the tag at the end of the functions to allow other instances to connect.
QNearFieldManager::setKeepConnection() allowed to change this behavior.
 
Since Qt 6, \l QNearFieldTarget keeps the connection by default. The connection is only closed
when the \l QNearFieldTarget is destroyed or \l QNearFieldManager::disconnect() is called.
 
\section2 Reading and Writing NDEF Messages
 
The \l QNearFieldTarget instance returned by \l QNearFieldManager::targetDetected() signal
is used to interact with the tag. Reading and writing a message is an asynchronous operation.
The \l QNearFieldTarget::RequestId class associates individual operations and their results.
 
\code
void MainWindow::targetDetected(QNearFieldTarget *target)
{
    switch (m_touchAction) {
    case NoAction:
        break;
    case ReadNdef:
        connect(target, &QNearFieldTarget::ndefMessageRead, this, &MainWindow::ndefMessageRead);
        connect(target, &QNearFieldTarget::error, this, &MainWindow::targetError);
 
        m_request = target->readNdefMessages();
        if (!m_request.isValid()) // cannot read messages
            targetError(QNearFieldTarget::NdefReadError, m_request);
        break;
    case WriteNdef:
        connect(target, &QNearFieldTarget::requestCompleted, this, &MainWindow::ndefMessageWritten);
        connect(target, &QNearFieldTarget::error, this, &MainWindow::targetError);
 
        m_request = target->writeNdefMessages(QList<QNdefMessage>() << ndefMessage());
        if (!m_request.isValid()) // cannot write messages
            targetError(QNearFieldTarget::NdefWriteError, m_request);
        break;
    }
}
\endcode
 
Once the \l QNearFieldTarget::readNdefMessages() request was successfully processed, the
\l QNearFieldTarget::ndefMessageRead() signal is emitted. Each returned \l QNdefMessage
may consist of zero or more \l QNdefRecord entries, which can be identified by their type.
For more information about processing of records, see the \l QNdefRecord class documentation.
As the above code demonstrates, writing of NDEF messages is triggered via
\l QNearFieldTarget::writeNdefMessages(). The successful completion of the write operation
is indicated by the emission of the \l QNearFieldTarget::requestCompleted() signal with the
corresponding request id. Any type of error during read or write is indicated via
\l QNearFieldTarget::error().
*/