QFilesystemwatcher in another thread

[RenanBS]

Hello,

I'm trying to monitor a file deletion from a certain path.
While this file is not deleted, my application shouldn't do anything.
Of course, if something goes wrong and the file is not deleted, there will be a timeout.

I have a 3 signals depending on what happens with the file, removed, modified or created. When the file is removed, for example, the signal removed set a variable "finished" to break the loop on the file main.cpp, but it looks like it isn't. Any help on this?

My test code is attached here: teste-wait.zip

If this is needed, my OS is Windows 7 Professional.

Thanks,
Renan

[anda_skoa]

Your problem is that you are doing a cross-thread signal/slot connection but do not run an event loop in the receiver object's thread (main thread).

So when the object in the worker thread emits its signals, the event that would call the MainClass' slots cannot be delivered.

However, the main question is why do you need a thread at all?

Cheers,
Kevin

[RenanBS]

I need to know when a particular file has been deleted in order to proceed with my application.
I need to block the main thread of my application waiting for it to be deleted. There is no need at all to have another thread...
Do you have any suggestions on how to do it?

Thanks,
Renan

[wysota]

You don't need to block your main thread while the file is not deleted. You need to do something when the file is deleted. You don't need any extra threads for that.

[RenanBS]

We have a application here were I work, that when we write a file ("close.txt") in some specific path it closes itself. But this operation takes some time. It depends on what it was doing... Sometimes it takes more than 200 seconds to do it.
My application will write this file to close our other application and will wait until it closes. My signal is when the file is deleted.
The problem is that I need to wait for it to be deleted. I thought of blocking the main thread, but like you said, I don't need to do it. So how should I proceed with this?

Renan

[wysota]

You just return to the event loop and let the filesystem watcher call your slot you connected to one of its signals. Since your example doesn't run an event loop at all, you need to adjust it for the loop to be running.

[RenanBS]

I did it using the class QEventLoop. Would it be the recommended way to run this event loop?
This is what I did:

main.cpp:

#include <QCoreApplication>
#include <QThread>
 
#include <iostream>
 
#include "filewatcher.h"
 
int main (int argc, char *argv[])
{
	QCoreApplication a (argc, argv);
	QEventLoop loop;
	FileWatcher *close = new FileWatcher ("c:/home/rs94036/");
 
	QObject::connect (close, SIGNAL (removed ()), close, SLOT (removedSlot ()));
	QObject::connect (close, SIGNAL (modified ()), close, SLOT (modifiedSlot ()));
	QObject::connect (close, SIGNAL (created ()), close, SLOT (createdSlot ()));
	QObject::connect (close, SIGNAL (exitLoop ()), &loop, SLOT (quit ()));
	close->closeSepApp ();
 
	loop.exec ();
 
	std::cout << "AFTER LOOP" << std::endl;
 
	return 0;
}

#include <QCoreApplication>
#include <QThread>

#include <iostream>

#include "filewatcher.h"

int main (int argc, char *argv[])
{
	QCoreApplication a (argc, argv);
	QEventLoop loop;
	FileWatcher *close = new FileWatcher ("c:/home/rs94036/");

	QObject::connect (close, SIGNAL (removed ()), close, SLOT (removedSlot ()));
	QObject::connect (close, SIGNAL (modified ()), close, SLOT (modifiedSlot ()));
	QObject::connect (close, SIGNAL (created ()), close, SLOT (createdSlot ()));
	QObject::connect (close, SIGNAL (exitLoop ()), &loop, SLOT (quit ()));
	close->closeSepApp ();

	loop.exec ();

	std::cout << "AFTER LOOP" << std::endl;

	return 0;
}

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FileWatcher.cpp:

#include <QFileInfo>
#include <QTextStream>
 
#include "FileWatcher.h"
 
#include <iostream>
 
FileWatcher::FileWatcher (const QString &path)
{
	monitoringPath = path;
	status = FILE_TIMEOUT;
}
 
bool FileWatcher::closeSepApp ()
{
	monitoringFileName = "close.txt";
	QFile *close = new QFile (monitoringPath + monitoringFileName);
	if (!close->exists ())
	{
		if (close->open (QIODevice::WriteOnly))
		{
			// Write something on it
			QTextStream *text = new QTextStream (close);
			*text << "1";
			close->close ();
		}
		else
			return false;
	}
	else
		return false;
 
	startMonitoring ();
	return true;
}
 
void FileWatcher::startMonitoring ()
{
	existsBeforeEvent = QFileInfo (monitoringPath + monitoringFileName).exists();
	status = existsBeforeEvent ? FILE_CHANGED : FILE_TIMEOUT;
 
	mon = new QFileSystemWatcher ();
	connect (mon, SIGNAL (fileChanged (QString)), this, SLOT (onFileChanged (QString)));
	mon->addPath (monitoringPath + monitoringFileName);
}
 
void FileWatcher::onFileChanged (const QString& path)
{
	bool existsAfterEvent = QFileInfo (path).exists();
 
	if (existsBeforeEvent && !existsAfterEvent)
		emit removed ();
	else if (!existsBeforeEvent && existsAfterEvent)
		emit created ();
	else
		emit modified ();
}
 
void FileWatcher::removedSlot ()
{
	std::cout << "FILE_REMOVED" << std::endl;
	emit exitLoop ();
}
 
void FileWatcher::modifiedSlot ()
{
	std::cout << "FILE_CHANGED" << std::endl;
	emit exitLoop ();
}
 
void FileWatcher::createdSlot ()
{
	std::cout << "FILE_CREATED" << std::endl;
	emit exitLoop ();
}

#include <QFileInfo>
#include <QTextStream>

#include "FileWatcher.h"

#include <iostream>

FileWatcher::FileWatcher (const QString &path)
{
	monitoringPath = path;
	status = FILE_TIMEOUT;
}

bool FileWatcher::closeSepApp ()
{
	monitoringFileName = "close.txt";
	QFile *close = new QFile (monitoringPath + monitoringFileName);
	if (!close->exists ())
	{
		if (close->open (QIODevice::WriteOnly))
		{
			// Write something on it
			QTextStream *text = new QTextStream (close);
			*text << "1";
			close->close ();
		}
		else
			return false;
	}
	else
		return false;

	startMonitoring ();
	return true;
}

void FileWatcher::startMonitoring ()
{
	existsBeforeEvent = QFileInfo (monitoringPath + monitoringFileName).exists();
	status = existsBeforeEvent ? FILE_CHANGED : FILE_TIMEOUT;

	mon = new QFileSystemWatcher ();
	connect (mon, SIGNAL (fileChanged (QString)), this, SLOT (onFileChanged (QString)));
	mon->addPath (monitoringPath + monitoringFileName);
}

void FileWatcher::onFileChanged (const QString& path)
{
	bool existsAfterEvent = QFileInfo (path).exists();

	if (existsBeforeEvent && !existsAfterEvent)
		emit removed ();
	else if (!existsBeforeEvent && existsAfterEvent)
		emit created ();
	else
		emit modified ();
}

void FileWatcher::removedSlot ()
{
	std::cout << "FILE_REMOVED" << std::endl;
	emit exitLoop ();
}

void FileWatcher::modifiedSlot ()
{
	std::cout << "FILE_CHANGED" << std::endl;
	emit exitLoop ();
}

void FileWatcher::createdSlot ()
{
	std::cout << "FILE_CREATED" << std::endl;
	emit exitLoop ();
}

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FileWatcher.h:

#ifndef FileWatcher_H
#define FileWatcher_H
 
#include <QFileSystemWatcher>
#include <QObject>
#include <QEventLoop>
 
enum FileStatus
{
	FILE_CREATED,
	FILE_CHANGED,
	FILE_REMOVED,
	FILE_TIMEOUT
};
 
class FileWatcher : public QObject
{
	Q_OBJECT
	public:
		FileWatcher (const QString& path);
		bool closeSepApp ();
 
	private:
		QString monitoringPath;
		QString monitoringFileName;
		bool existsBeforeEvent;
		FileStatus status;
		QFileSystemWatcher *mon;
 
	public slots:
		void startMonitoring ();
 
	protected slots:
		void onFileChanged (const QString& path);
 
	signals:
		void removed ();
		void modified ();
		void created ();
		void exitLoop ();
 
	public slots:
		void removedSlot ();
		void modifiedSlot ();
		void createdSlot ();
 
};
 
#endif // FileWatcher_H

#ifndef FileWatcher_H
#define FileWatcher_H

#include <QFileSystemWatcher>
#include <QObject>
#include <QEventLoop>

enum FileStatus
{
	FILE_CREATED,
	FILE_CHANGED,
	FILE_REMOVED,
	FILE_TIMEOUT
};

class FileWatcher : public QObject
{
	Q_OBJECT
	public:
		FileWatcher (const QString& path);
		bool closeSepApp ();

	private:
		QString monitoringPath;
		QString monitoringFileName;
		bool existsBeforeEvent;
		FileStatus status;
		QFileSystemWatcher *mon;

	public slots:
		void startMonitoring ();

	protected slots:
		void onFileChanged (const QString& path);

	signals:
		void removed ();
		void modified ();
		void created ();
		void exitLoop ();

	public slots:
		void removedSlot ();
		void modifiedSlot ();
		void createdSlot ();

};

#endif // FileWatcher_H

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Renan

[RenanBS]

Since this method is working fine, I'm assuming it is correct.

Thanks.

[wysota]

You might have just called a.exec() instead.

[RenanBS]

I thought about that.
In the example I could have done this. But in my application I don't have access to the QCoreApplication a anymore...

Thanks.

[wysota]

Sure you did -- QCoreApplication::instance() is a static method that returns a pointer to the application object.