Qt apps basic functionality

[mrjj]

@tomy
Hi
Yes there is.
The Qtimer puts on the event queue and will be called later meaning
the execution continues to the next line.

Calling the requestNewFortune will happen right now and the next line is not processed before this function ends.

[JKSH]

@tomy said in Qt apps basic functionality:

what is the difference between QTimer::singleShot(0, this, &Client::requestNewFortune); and calling requestNewFortune() directly without the timer?

Here is a simple illustration:

void triggeredFunc()
{
    qDebug("World");
}

void f1()
{
    QTimer::singleShot(0, &triggeredFunc);
    qDebug("Hello");
}

void f2()
{
    triggeredFunc();
    qDebug("Hello");
}

Assuming that your application has an event loop,

• If you call f1(), your debug output will print "Hello" before "World".
• If you call f2(), your debug output will print "World" before "Hello".

[KroMignon]

@tomy As try to explain to you calling QTimer::singleShot(0, this, &Client::requestNewFortune); will post event into the QEventLoop of the QThread used by this.
This means requestNewFortune() will be executed some time later, when all other previous event in the QEventLoop have been executed.
This also ensure, requestNewFortune() will be executed in the thread on which this is running.
And, to be exhaustive, requestNewFortune() will be executed, when attached Thread has an QEventQueue and the thread is running.

[tomy]

@mrjj
I'm confused by what you said.
You firstly say QTimer will be called later. Then you say, calling the function will happen right now!

@JKSH

If you call f1(), your debug output will print "Hello" before "World".

So, apparently my first assumption mentioned here more than one time was correct. That is, that QTimer postpones the execution of the method inside it (requestNewFortune), until all next lines inside the function (void Client::readFortune()), are executed. And then at the end when there is no line to execute, QTimer calls its function. Right?

@KroMignon

This means requestNewFortune() will be executed some time later, when all other previous event in the QEventLoop have been executed.

This is on the contrary to the code @JKSH offered above, because there, the function inside the QTimer is called when all other next lines are called.

The subject presumably is not that convoluted but I don't know why I understand your posts differently! :(

[mrjj]

@tomy said in Qt apps basic functionality:

You firstly say QTimer will be called later. Then you say, calling the function will happen right now!

Ok let me make it clear.

QTimer::singleShot(0, this, &Client::requestNewFortune); ---> later called

---

requestNewFortune(); ---> called right here. right now.

And in this context. Later means when event loop gets around to it.

[KroMignon]

@tomy said in Qt apps basic functionality:

This is on the contrary to the code @JKSH offered above, because there, the function inside the QTimer is called when all other next lines are called.
The subject presumably is not that convoluted but I don't know why I understand your posts differently! :(

Dear @tomy, it seems my explanation are not that clear as I supposed. So I do another try:

1. QThread owns a QEventLoop
2. When starting a QThread, the thread will wait until a new event is stored in the event loop, take it and execute the function with the attached parameters
3. When calling QTimer::singleShot(0, this, &Client::requestNewFortune);, what happen is that a temporary QTimer instance is created and started. When the passed delay is elapsed (in this case 0 milliseconds), the function/slots (in this case &Client::requestNewFortune) is stored in the event loop from the thread used by the receiver QObject instance (in this case this)

I think I can't explain it more clearly and hope now you have understand the difference between QTimer::singleShot(0, this, &Client::requestNewFortune); and calling directly requestNewFortune().

[JKSH]

@tomy said in Qt apps basic functionality:

QTimer postpones the execution of the method inside it (requestNewFortune), until all next lines inside the function (void Client::readFortune()), are executed. And then at the end when there is no line to execute, QTimer calls its function. Right?

Almost right.

QTimer does not call requestNewFortune(). QTimer puts requestNewFortune() in the event loop.

The event loop calls the requestNewFortune() after readFortune() finishes all its lines and returns.

@tomy said in Qt apps basic functionality:

This means requestNewFortune() will be executed some time later, when all other previous event in the QEventLoop have been executed.

This is on the contrary to the code @JKSH offered above, because there, the function inside the QTimer is called when all other next lines are called.

@KroMignon's description matches the code that I wrote. It is not contrary.

The sequence is this:

1. The event loop calls f1(). f1() starts running, so it blocks the event loop.
2. The QTimer in f1() puts triggeredFunc() in the event loop queue.
3. f1() prints "Hello".
4. f1() returns. The event loop can now proceed to the next task.
5. The event loop checks its queue. It sees triggeredFunc() in the queue, so the event loop calls triggeredFunc(). triggeredFunc() starts running, so it blocks the event loop.
6. triggeredFunc() prints "World".
7. triggeredFunc() returns. The event loop can now proceed to the next task.
8. The event loop checks its queue. If there is nothing in the queue, the event loop waits for the next event.

---

With f2(), the sequence is this:

1. The event loop calls f2(). f2() starts running, so it blocks the event loop.
2. f2() calls triggeredFunc() directly. triggeredFunc() starts running, so it blocks f2().
3. triggeredFunc() prints "World".
4. triggeredFunc() returns. f2() can now proceed to the next line.
5. f2() prints "World".
6. f2() returns. The event loop can now proceed to the next task.
7. The event loop checks its queue. If there is nothing in the queue, the event loop waits for the next event.

[tomy]

@JKSH

This is a clear answer, explained step-by-step and omits nothing for ambiguity. Thanks.
I also must thank other members, who dedicated time to help me. thanks.
The thread is closed and the issue is solved. :)

[ODБOï]

This post is deleted!

[JKSH]

@tomy You're most welcome. Happy coding!