Open QDialog and run the QProcess simultaneously

Question

This is my btconnect.h file

#ifndef BTCONNECT_H
#define BTCONNECT_H

#include "scandialog.h"

namespace Ui {
class BTConnect;
}

class BTConnect : public QWidget
{
    Q_OBJECT

public:
    explicit BTConnect(QWidget *parent = 0);
    ~BTConnect();

private slots:
    void on_ScanButton_clicked();

    void ScanBTDevices();

    //some slots here

    void ScanDialogShow();

    void ScanDialogClose();

public slots:
//some slots here

private:
    Ui::BTConnect *ui;

    QProcess BTscan_Process;

    scanDialog *scan;
};

#endif // BTCONNECT_H

btconnect.cpp

BTConnect::BTConnect(QWidget *parent) :
QWidget(parent),
ui(new Ui::BTConnect)
{
    //set the userinterface as BTConnect.ui
    ui->setupUi(this);

    scan = new scanDialog(this);
}


void BTConnect::ScanDialogShow()
{
    scan->show();
}

void BTConnect::ScanDialogClose()
{
    scan->close();
}

void BTConnect::ScanBTDevices()
{
    ScanDialogShow();

    //Command to scan nearby bluetooth devices
    //"hcitool scan"
    QString cmd("hcitool scan");

    //start the process
    BTscan_Process.start(cmd);

    //Wait for the processs to finish with a timeout of 20 seconds
    if(BTscan_Process.waitForFinished(20000))
    {
        //Clear the list widget
        this->ui->listWidget->clear();

        //Read the command line output and store it in QString out
        QString out(BTscan_Process.readAllStandardOutput());

        //Split the QString every new line and save theve in a QStringList
        QStringList OutSplit = out.split("\n");

        //Parse the QStringList in btCellsParser
        btCellsParser cp(OutSplit);

        for(unsigned int i = 0; i<cp.count(); i++)
        {
           //writing in listwidget
        }

    }

    ScanDialogClose();
}

void BTConnect::on_ScanButton_clicked()
{
    //Scan for available nearby bluetooth devices
    ScanBTDevices();
}

if I use the above code, the qdialog scandialog does open when the process begins and closes when the data is loaded in qlistwidget, but the contents of qdialog scandialog are not displayed. If I were to change show() to exec(), the contents will be shown but the QProcess does not run until the dialog is closed.

I want the dialog to open when the Qprocess starts and close when the qlistwidget is loaded with data from the scan. And I want the contents of scandialog to be displayed. It has two labels. One with .GIF file and another with text saying scanning.

Any help is appreciated.

Answer 1

you never return to the event loop when you do show (because of waitForFinished) and you never continue to the processing code when you do exec

instead of the waitForFinished you should connect to the finished signal and handle it there and use a single shot timer that will cancel it:

void BTConnect::on_BTscanFinished()//new slot
{
   //Clear the list widget
    this->ui->listWidget->clear();

    //Read the command line output and store it in QString out
    QString out(BTscan_Process.readAllStandardOutput());

    //Split the QString every new line and save theve in a QStringList
    QStringList OutSplit = out.split("\n");

    //Parse the QStringList in btCellsParser
    btCellsParser cp(OutSplit);

    for(unsigned int i = 0; i<cp.count(); i++)
    {
       //writing in listwidget
    }
    ScanDialogClose();
}

void BTConnect::ScanBTDevices()
{
    ScanDialogShow();

    //Command to scan nearby bluetooth devices
    //"hcitool scan"
    QString cmd("hcitool scan");

    //start the process
    connect(BTscan_Process, SIGNAL(finished()), this, SLOT(on_BTscanFinished()));
    BTscan_Process.start(cmd);
    QTimer::singleShot(20000, scan, SLOT(close()));
}

Answer 2

The problem is that QDialog::exec and QProcess::waitForFinished functions block event loop. Never ever block event loop. So you just need to do things more asynchronously.

QProcess class can be handled asynchronously using signals like readReadStandardOutput. And QDialog can be shown asynchronously using open slot.

The example:

void ScanBTDevices() {
    // Open dialog when process is started
    connect(process, SIGNAL(started()), dialog, SLOT(open()));

    // Read standard output asynchronously
    connect(process, SIGNAL(readyReadStandardOutput()), SLOT(onReadyRead()));

    // Start process asynchronously (for example I use recursive ls)
    process->start("ls -R /");
}

void onReadyRead() {
    // Write to list widget
    dialog->appendText(QString(process->readAllStandardOutput()));
}

The data will be appended to the dialog during generating by process. Also using QProcess::finished signal and you can close the dialog.