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CHAPTER 12 DOING THINGS IN PARALLEL
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The signals emitted from the QProcess class can be used to monitor the progress or failure of a launched process: error( QProcess::ProcessError error ): The process has experienced some sort of internal error. started(): The process has started. finished( int code, QProcess::ExitStatus status ): The process has exited. readyReadStandardError(): There is data to read from the standard error channel. readyReadStandardOutput(): There is data to read from the standard output channel. stateChanged( QProcess::ProcessState newState ): The process has entered a new state. When there is data ready to read, you can read it using the readAllStandardError method or readAllStandardOutput method, depending on the channel in which you are interested. Using the set standardOutputFile and setStandardErrorFile, you can redirect the output from either channel to a file. The process state can change between the three states NotRunning, Starting, and Running. When entering NotRunning, you know that the process has ended or will end very soon. You can receive finished signals after the state is changed to NotRunning, but error signals generally are emitted before the stateChanged signal. Before you can receive any signals at all, you need to start a new process from the runUic slot. You can see the slot implementation in Listing 12-41. The nonhighlighted lines disable the user interface and clear the QTextEdit widget used for showing the application output before creating a new QProcess object and setting up the connections. The highlighted lines show how to initialize and launch a process. First the arguments are assembled in a QStringList object before start is called. The start call takes the executable s name and the arguments as parameters. After the start method call, it is a matter of waiting for the signals to arrive. Listing 12-41. A QProcess object is created, connected, and launched. void ProcessDialog::runUic() { ui.uicButton->setEnabled( false ); ui.textEdit->setText( "" ); if( process ) delete process; process = new QProcess( this ); connect( process, SIGNAL(error(QProcess::ProcessError)), this, SLOT(handleError(QProcess::ProcessError)) ); connect( process, SIGNAL(finished(int,QProcess::ExitStatus)), this, SLOT(handleFinish(int,QProcess::ExitStatus)) ); connect( process, SIGNAL(readyReadStandardError()),
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this, SLOT(handleReadStandardError()) ); connect( process, SIGNAL(readyReadStandardOutput()), this, SLOT(handleReadStandardOutput()) ); connect( process, SIGNAL(started()), this, SLOT(handleStarted()) ); connect( process, SIGNAL(stateChanged(QProcess::ProcessState)), this, SLOT(handleStateChange(QProcess::ProcessState)) ); QStringList arguments; arguments << "-tr" << "MYTR" << "processdialog.ui"; process->start( "uic", arguments ); } When the signals arrive, the slots will make the output visible in the QTextEdit widget used for showing the results of the execution. Because almost all slots look the same, take a look at handleFinish. You can see the source code in Listing 12-42. The slot passes the enumerated type through a switch statement to convert it into a string. It then appends the resulting text to the text edit as a new paragraph in bold. All bold texts are status messages, while the text with normal weight is the actual output from the application. Listing 12-42. The handleFinish slot implementation void ProcessDialog::handleFinish( int code, QProcess::ExitStatus status ) { QString statusText; switch( status ) { case QProcess::NormalExit: statusText = "Normal exit"; break; case QProcess::CrashExit: statusText = "Crash exit"; break; } ui.textEdit->append( QString( "<p><b>%1 (%2)</b><p>" ) .arg( statusText ) .arg( code ) ); } Running the application shows the different signals being emitted in the different phases of the process life. Figure 12-4 shows the result of a successful execution. The signals emitted are the following: 1. stateChanged( Starting ) 2. started()
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3. readyReadStandardOutput() (several times) 4. stateChanged( NotRunning ) 5. finished( 0, NormalExit )
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Figure 12-4. The uic process running and completing succesfully. The top image shows the top of the output text; the bottom image shows the end of the same text.
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Note You add the application s output to QTextEdit using append calls, which leads to each new chunk
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of text being added as a new paragraph. That is why the output looks slightly odd in the screenshots.
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