qt-everywhere-src-5.15.1/qtserialport/examples/serialport/doc/blockingslave.qdoc

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/*!
    \example blockingslave
    \title Blocking Slave Example
    \ingroup qtserialport-examples
    \brief Shows how to use the synchronous API of QSerialPort in a non-GUI thread.

    \e{Blocking Slave} shows how to create an application for a serial interface
    using QSerialPort's synchronous API in a non-GUI thread.

    \image blockingslave-example.png

    QSerialPort supports two general programming approaches:

    \list

    \li \e{The asynchronous (non-blocking) approach.} Operations are scheduled
    and performed when the control returns to Qt's event loop. QSerialPort emits
    a signal when the operation is finished. For example, QSerialPort::write()
    returns immediately. When the data is sent to the serial port, QSerialPort
    emits \l{QIODevice::bytesWritten()}{bytesWritten()}.

    \li \e{The synchronous (blocking) approach.} In non-GUI and multithreaded
    applications, the \c waitFor...() functions can be called (i.e.
    QSerialPort::waitForReadyRead()) to suspend the calling thread until the
    operation has completed.

    \endlist

    In this example, the synchronous approach is demonstrated. The
    \l{terminal}{Terminal} example illustrates the
    asynchronous approach.

    The purpose of this example is to demonstrate a pattern that you can use
    to simplify your serial programming code, without losing responsiveness
    in your user interface. Use of Qt's blocking serial programming API often
    leads to simpler code, but because of its blocking behavior, it should only
    be used in non-GUI threads to prevent the user interface from freezing.
    But contrary to what many think, using threads with QThread does not
    necessarily add unmanageable complexity to your application.

    This application is a Slave, that demonstrate the work paired with Master
    application \l{Blocking Master Example}.

    The Slave application is receives the request via serial port from
    the Master application and send a response to it.

    We will start with the SlaveThread class, which handles the serial
    programming code.

    \snippet blockingslave/slavethread.h 0

    SlaveThread is a QThread subclass that provides an API for receive requests
    from Master, and it has signals for delivering responses and reporting
    errors.

    You should be call startSlave() to startup Slave application. This method
    transfers to the SlaveThread desired parameters for configure and startup
    the serial interface. When SlaveThread received from Master any request then
    emitted the request() signal. If any error occurs, the error() or timeout()
    signals is emitted.

    It's important to notice that startSlave() is called from the main, GUI
    thread, but the response data and other parameters will be accessed from
    SlaveThread's thread. SlaveThread's data members are read and written
    from different threads concurrently, so it is advisable to use QMutex to
    synchronize access.

    \snippet blockingslave/slavethread.cpp 2

    The startSlave() function stores the serial port name, timeout and response
    data, and QMutexLocker locks the mutex to protect these data. We then
    start the thread, unless it is already running. QWaitCondition::wakeOne()
    will be discussed later.

    \snippet blockingslave/slavethread.cpp 4
    \snippet blockingslave/slavethread.cpp 5

    In the run() function, start by acquiring the mutex lock, fetch the
    serial port name, timeout and response data from the member data, and then
    release the lock again. Under no circumstance should the method \c startSlave()
    be called simultaneously with a process fetching these data. QString is reentrant
    but not thread-safe, and it is not recommended to read the serial port name
    from one startup, call and timeout or response data of another.  SlaveThread
    can only handle one startup at a time.

    The QSerialPort object we construct on stack into run() function before loop
    enter:

    \snippet blockingslave/slavethread.cpp 6

    This allows us once to create an object, while running loop, and also means
    that all the methods of the object will be executed in the context of the
    run() thread.

    In the loop, check whether the name of the serial port for the current
    startup has changed or not. If it has, re-open and reconfigure the serial port.

    \snippet blockingslave/slavethread.cpp 7

    The loop will continue waiting for request data:

    \snippet blockingslave/slavethread.cpp 8

    \warning The method waitForReadyRead() should be used before each read()
    call for the blocking approach, because it processes all the I/O routines
    instead of Qt event-loop.

    The timeout() signal is emitted if error occurs when reading data.

    \snippet blockingslave/slavethread.cpp 9

    After a successful read, try to send a response and wait for completion of the
    transfer:

    \snippet blockingslave/slavethread.cpp 10

    \warning The method waitForBytesWritten() should be used after each write()
    call for the blocking approach, because it processes all the I/O routines
    instead of Qt event-loop.

    The timeout() signal is emitted if an error occurs when writing data.

    \snippet blockingslave/slavethread.cpp 11

    After a successful writing is emitted, request() signal containing the
    data received from the Master application:

    \snippet blockingslave/slavethread.cpp 12

    Next, the thread switches to reading the current parameters for the serial
    interface, because they can already have been updated, and run the loop
    from the beginning.

    \snippet blockingslave/slavethread.cpp 13

    \sa {Terminal Example}, {Blocking Master Example}

    \include examples-run.qdocinc
*/