How to know thread is running from different button ?

[Hiloshi]

Dear @jsulm , @SGaist

Sorry to confuse you, I didn't explain clearly.

If I use "new" and "T3Thread->start" in the same area, then it will no error(process killed by signal),
as below:
https://www.dropbox.com/s/0488sqlhgn9u4wf/Screenshot from 2017-04-06 09-28-32.png?dl=0

However, if I put them in different place, application will crash and return "process killed by signal",
as below:
https://www.dropbox.com/s/3dfists06z3c5q9/Screenshot from 2017-04-06 09-33-10.png?dl=0

Moreover, if I put "new" just after the "ui->setupUi(this)" and "T3Thread in the on_pushButton_clicked,
it will show a warning message "unused variable ‘T3Thread’ [-Wunused-variable]".

@SGaist , I think maybe this is what you said : "uninitialised pointer " .

But, I am sorry, I don't know how to correct it.

[SGaist]

Remove MyThread * from your constructor.

[jsulm]

@Hiloshi You should really read more carefully what others are writing. What @SGaist said is:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);   
    T3Thread = new MyThread; // Remove MyThread* - else you are declaring a local variable which is completely unrelated to the one in your header!
    T3Thread->start();
}

And after this change think about where to start the thread: in the code you posted you're doing it twice.

[Hiloshi]

Very happy to learn from here. Thanks everyone.

[Hiloshi]

I want to study in depth in this question. In the original question, I have a running T3Thread, mainwindow cannot reach T3Thread, thanks for @a-normal-worker 's suggestion, we can make T3Thread as a class member of mainwindow then mainwindow have ability to access T3Thread.

Can we use signal/slot instead of using class member ?

[SGaist]

What exactly do you want to do with your thread ?

[Hiloshi]

Dear @SGaist ,

I am thinking to create a breathing LED in my thread, "run" function is keep turn on/off LED.
I want to put one button on mainwindow to start the thread.

I do not want this button to be pressed twice ( or thread will create twice), so in the button I put:

void MainWindow::on_pushButton_3_clicked()
{
    if(T3Thread->isRunning()==false)
    {
        MyThread  *T3Thread = new MyThread;  // local variable
        T3Thread->start;
    }
    else
        qDebug()<<"T3 thread is running";
}

If I do not want to use class member, can I use signal/slot to know T3Thread is running or not ?

Thanks.

[J.Hilk]

@Hiloshi
ok, I think we talk at cross here.

Here's a basic setup to check if your thread is running when you press a Button, if its not running the button will start the thread:

//in your header:
example.h

#include <QThread>
#include <qDebug>

private:
QThread *thread;

//in your source file

//Constructor
{
    ui->setupUi(this);   
    thread = new QThread;
....
}


void on_pushButton_clicked(){
   if(thread->isRunning())
      qDebug() << "Thread is running";
   else{
      qDebug() << "Thread started";
      thread->start(); 
   }
}

//Destructor
{
  if(thread->isRunning()){
   thread->quit();
   thread-> wait();
  }
...
  delete ui;
}

[SGaist]

What is the problem with using a member variable to handle that thread object properly ?

[Hiloshi]

@SGaist , There is no problem using member variable. I just thinking if it possible to use signal/slot to reach the same purpose.

[SGaist]

There could be ways however you are talking about creating an object somewhere without any reference to it and still want to interact with it. That's bad by design.

[Hiloshi]

@SGaist , Understood, thanks for the help.