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  1. 25th June 2010, 10:09 #1
    gen_mass
    gen_mass is offline Novice
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    Default QFileSystemWatcher used for a Qwt "Real_time" plot

    Hi,

    I work on Windows XP, and I want to plot a signal coming from a device in real-time(approx.). This device is connected to my PC by a RS232 serial port. I can't access directly to the data coming in my PC by the serial port, but only from a log text file.

    I know that new data enters the text file every second, but because I want to be sure of the synchronisation between the entering of data and the plotting, I try to implement a QFileSystemWatcher: Everytime the file changes, a fileChanged signal is emitted and received by a function who opens the text file, read the new data just written and close the file.

    Here's the class I use to get the data in my text file, with the QSystemFileWatcher:

    data.cpp:

    Qt Code: 
    1. //////////////////////////////////////////////////////////////////////////////////////////////////////////////
    2. //The following class (data) is used as a sampling thread with the QFileSystemWatcher.
    3. // It puts the significant values extracted from each "length 13" line of the text file in a data buffer. This data buffer 
    4. //is passed to another class (plot) who take cares of displaying these datas on a qwt plot.
    5.  
    6. #include "data.h" 
    7. Data::Data(double * &_voies, QObject*p):       
    9. {     
    10.   _voies = voies= new double[2];     
    11.   file_name = "Portmon.log";    
    12.   m_position =0;    
    13.   m_num = 0;
    14.  
    15.  //Find the last length 13 in the text file, so we can find the 
    16.  //start position of the text file cursor (m_position)
    17.  
    18.   ifstream file("Portmon.log");   
    19.  
    20.  while(getline(file,ligne))           
    21.  {               
    22.      string::size_type loc = line.find("Length 13:",0);                
    23.          if (loc != string::npos)               
    24.          m_position=file.tellg();
    25.  
    26.   }
    27. file.close();
    28.  
    29.  
    30.  
    31.  }
    32.  
    33. //--------------------------------------------------------------------------------
    34. //void Data::on_watcher()
    35. // Implement a QFileSystemWatcher with a fileChanged signal
    36.  
    37. void Data::on_watcher()
    38. {   
    39.     m_watcher = new QFileSystemWatcher();     
    40.     m_watcher->addPath(file_name);    
    41.     QObject::connect(m_watcher, SIGNAL(fileChanged(const QString&)), this, SLOT(onFileChanged(const QString&)));
    42. } 
    43.  
    44.  
    45. //-----------------------------------------------------------------------------
    46. //void Data::onFileChanged( const QString& file_name)  
    47. //When the file is detected as changed, 
    48. //the text file is opened, the data forming the signal/
    49. // is extracted, and the file is closed.
    50.  
    51.  void Data::onFileChanged( const QString& file_name)
    52. {    
    53.   int nb_data =0;    
    54.   double perSec=1;      
    55.   ifstream file("Portmon.log");   
    56.   file.clear();    
    57.   file.seekg(m_position,ios::beg);    
    58.   while(getline(file,ligne))           
    59.  {               
    60.       string::size_type loc = line.find("Length 13:",0);
    61.  
    62.     if (loc != string::npos)               
    63.    {      
    64.      // Data collecting                              
    65.       contractions_hex = ligne.substr (loc+14,2); 
    66.      valeur_cardio_connecte = ligne.substr(loc+26,2);  
    67.     //position of the file cursor uptaded          
    68.      m_position = file.tellg(); 
    69.     //Conversion from hexadecimal to int.                     
    70.     std::istringstream iss (contractions_hex);          
    71.     iss >> std::hex >> m_num;               
    72.     double y1 =m_num/2;                 
    73.     double time = nb_data*perSec;  
    74.     //Adding data to the data buffer, another class is in charge of displaying the data on plot:          
    75.     voies[0]=time;                 
    76.     voies[1]=y1;                
    77.     nb_data++;               
    78.      }   
    79.   }
    80.  file.close();
    81.  }
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    data.h

    Qt Code: 
    1. #ifndef DATA_H
    2. #define DATA_H 
    3. #include <QtGui>
    4. #include <QFileSystemWatcher>
    5. #include <QString>
    6. #include <QFile>
    7. #include <string>
    8. #include <iostream>
    9. #include <sstream>
    10. #include <fstream>
    11. #include <QFileInfo>  
    12.  
    13. using namespace::std; 
    14.  
    15. class Data: 
    16.  
    17. public QObject
    18.  
    19. {  
    20.  
    21.   Q_OBJECT 
    22.  
    23. public:    Data(double* &_voies,QObject*p=NULL);    
    24. void on_watcher();
    25.  
    26. private:    
    27. QFileSystemWatcher *m_watcher;    
    28. QFile file;    
    29. QString file_name;    
    30. double *voies;   
    31. int m_position;    
    32. int m_num;   
    33. std::string line;    
    34. std::string contractions_hex;    
    35. std::string valeur_cardio_connecte;
    36.  
    37. public slots:      
    38. void onFileChanged(const QString& file_name);
    39.  }; 
    40.  
    41. #endif  DATA_H
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    Here's an example at of a text file (Portmon.log):

    Qt Code: 
    1. [\\ISTILPORT06]
    2. 1179  10:06:58  SUCCESS  
    3. 1180  10:06:59  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    4. 1180  10:06:59  SUCCESS  Length 14: 10 02 43 80 02 00 00 00 00 00 00 00 00 00
    5. 1181  10:06:59  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0  
    6. 1181  10:06:59  SUCCESS  
    7. 1182  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0 
    8. 1182  10:06:59  SUCCESS  
    9. 1183  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    10. 1183  10:06:59  SUCCESS  
    11. 1184  10:06:59  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    12. 1184  10:06:59  SUCCESS  Length 14: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
    13. 1185  10:06:59  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0  
    14. 1185  10:06:59  SUCCESS  
    15. 1186  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    16. 1186  10:06:59  SUCCESS  
    17. 1187  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    18. 1187  10:06:59  SUCCESS  
    19. 1188  10:06:59  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    20. 1188  10:06:59  SUCCESS  Length 13: 00 0A 0B 0C 0A 00 00 08 00 10 03 92 38 
    21. 1189  10:06:59  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0  
    22. 1189  10:06:59  SUCCESS  
    23. 1190  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0 
    24. 1190  10:06:59  SUCCESS  
    25. 1191  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    26. 1191  10:06:59  SUCCESS  
    27. 1192  10:06:59  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    28. 1192  10:06:59  SUCCESS  Length 14: 10 02 43 80 02 00 00 00 00 00 00 00 00 00 
    29. 1193  10:06:59  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0  
    30. 1193  10:06:59  SUCCESS  
    31. 1194  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    32. 1194  10:06:59  SUCCESS  
    33. 1195  10:06:59  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    34. 1195  10:06:59  SUCCESS  
    35. 1196  10:07:00  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    36. 1196  10:07:00  SUCCESS  Length 14: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
    37. 1197  10:07:00  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0  
    38. 1197  10:07:00  SUCCESS  
    39. 1198  10:07:00  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    40. 1198  10:07:00  SUCCESS  
    41. 1199  10:07:00  dacspy.exe  IOCTL_SERIAL_WAIT_ON_MASK  Serial0  
    42. 1199  10:07:00  SUCCESS  
    43. 1200  10:07:00  dacspy.exe  IRP_MJ_READ  Serial0  Length 100
    44. 1200  10:07:00  SUCCESS  Length 13: 00 07 08 08 08 00 00 08 00 10 03 58 C9 
    45. 1201  10:07:00  dacspy.exe  IOCTL_SERIAL_GET_COMMSTATUS  Serial0
    46.  1201  10:07:00  SUCCESS  1202
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    When I try to run this code, no data is added to the plot, and I get an empty plot.

    I know that the problem is with the QFileSystemWatcher because everything else has been tested and debugged before.
    I have put my text file in the debug file of my project (I'm in debug mode) and my whole project including Qt Creator implementation is on an external disk (:G), I don't know if that changes something..

    Thanks for your help !!
    Last edited by gen_mass; 25th June 2010 at 12:05.
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  2. 28th June 2010, 14:43 #2
    amoswood's Avatar
    amoswood
    amoswood is offline Novice
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    Default Re: QFileSystemWatcher used for a Qwt "Real_time" plot

    I don't think that QFileSystemWatcher is going to get you where you need to go.

    Instead of your current approach, you could try:

    1. Just open the file using QFile as read-only and them simply check when the file is bigger. If you keep a position of your last read, you can simply start reading from where you last read and read to the bottom of the file. If something new is read in, then output that to the plot. You can do this in a while loop.

    2. A better programming approach is to output the data to a local network socket or pipe which will be easier to use built-in Qt class like QTcpSocket or QSocketNotifier.

    Hope that helps.
    Amos
    Qt Programmer Extraordinaire

    Current Work:
    Ripxx Sports Measurement Device - www.ripxx.com
    (Featured in MYTHBUSTERS on 2010-05-19 in S08E08)
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