/**************************************************************************** ** ** Copyright (C) 2011 - 2012 Denis Shienkov ** Copyright (C) 2015 The Qt Company Ltd. ** Contact: http://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 http://www.qt.io/terms-conditions. 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Please review the following information to ensure ** the GNU Free Documentation License version 1.3 requirements ** will be met: http://www.gnu.org/copyleft/fdl.html. ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \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 */