(seemingly) simple Qt app: creating a progress bar

[goetz]

On a first glance, that looks good. Despite the fact that you might consider using [[Doc:QFile]]/[[Doc:QTextStream]] for the file operations and [[Doc:QFileInfo]] for getting the file size (instead of your method).

Then you must not define the method for the signal yourself, this is done automatically by moc. It is sufficient to just declare the signal in the class header file.

The signal must be emitted in the run method of your thread:

@
// some kind of signal to be emitted here
emit percentChanged(percent);
@

You'll also have to connect that signal to the progress bar:

@
connect(worker, SIGNAL(percentChanged(int)), bar, SLOT(setValue(int)));
@

Also, the worker needs to be moved to a thread, and eventually started:

@
QProgressBar bar(0);
bar.setRange(0, 100);
bar.setValue(0);
bar.show();

WorkerThread worker;
QThread thread;
worker.moveToThread(&thread);

// run the run method of the worker object once the thread has started
connect(&thread, SIGNAL(started())), &worker, SLOT(run()));

// update the progress bar
connect(&worker, SIGNAL(percentChanged(int)), &bar, SLOT(setValue(int)));

thread.start();
@

to make the autostart work, you must declare WorkerThread::run() as public slot.

[mzimmers]

[quote author="Volker" date="1329477431"]On a first glance, that looks good. Despite the fact that you might consider using [[Doc:QFile]]/[[Doc:QTextStream]] for the file operations and [[Doc:QFileInfo]] for getting the file size (instead of your method)[/quote]

Good to know about those...thanks. For now, I'll just leave it as is. The plan is to:

get this working with a minimum of effort on my sample program

transfer the Qt-specific code to my simulator

let the powers-that-be see how cool even a bit of Qt razzle-dazzle is

go whole-hog on the big job that's hopefully coming up this spring

[quote]Then you must not define the method for the signal yourself, this is done automatically by moc. It is sufficient to just declare the signal in the class header file.[/quote]

Oh yeah; I knew that (sort of).

[quote] The signal must be emitted in the run method of your thread:

@
// some kind of signal to be emitted here
emit percentChanged(percent);
@

bq. You'll also have to connect that signal to the progress bar:

@
connect(worker, SIGNAL(percentChanged(int)), bar, SLOT(setValue(int)));
@

bq. Also, the worker needs to be moved to a thread, and eventually started:

@
QProgressBar bar(0);
bar.setRange(0, 100);
bar.setValue(0);
bar.show();

WorkerThread worker;
QThread thread;
worker.moveToThread(&thread);

// run the run method of the worker object once the thread has started
connect(&thread, SIGNAL(started())), &worker, SLOT(run()));

// update the progress bar
connect(&worker, SIGNAL(percentChanged(int)), &bar, SLOT(setValue(int)));

thread.start();
@

bq. to make the autostart work, you must declare WorkerThread::run() as public slot.[/quote]

Done, done and done. I'm geting a compiler error that "'connect' was not declared in this scope." I'm including QObject; isn't that where connect lives?

EDIT: does the connect need object context? There isn't any in the example on the signals/slots doc page.

[mlong]

If you're using connect() outside of a class that's a subclass of QObject, you'll need to use the full name of the static method, "QObject::connect()"

[goetz]

Oh, using QFile and friends is much more easier than the stdlib methods - give it a try! :)

As mark already mentioned, just call QObject::connect(). That method ist static in QObject, so that works without problems.

[mzimmers]

[quote author="Volker" date="1329495814"]Oh, using QFile and friends is much more easier than the stdlib methods - give it a try! :) [/quote]

I believe you, but...the goal for this exercise is not to show off all of Qt's programming features, but to impress upon some reluctant management that it needn't be invasive to a program's design.

[quote]As mark already mentioned, just call QObject::connect(). That method ist static in QObject, so that works without problems.[/quote]

Cool...that's the first time I've seen that done.

OK...now I'm getting a linker error:

@Undefined symbols:
"vtable for WorkerThread", referenced from:
WorkerThread::WorkerThread()in main.o
@

That doesn't look like a coding problem to me; am I missing a library or something?

[goetz]

Some missing moc steps.

I would recommend to put the WokerThread class in a separate .h header file (and probably extract the implementation to a .cpp file).

moc is usually not run on the file that contains the main method.

If you have changed your project, do a full rebuild to catch up for all the changes.

[mzimmers]

Oh...excellent! This is very, very encouraging.

EDIT:

Actually, let's just get to the real thing here: this is the main routine in my application:

@#include <iostream>
#include <string>

#include "modem.h"

using namespace std;

int main ()
{
Modem modem;
bool clockEnable = HIGH;
bool resetFlag = LOW;

int32_t rc = 0;
int32_t i = 0;

while (rc == 0)
{
systemClock.setClock(HIGH);
rc = modem.cycle(clockEnable, resetFlag);

systemClock.setClock(LOW);
modem.cycle(clockEnable, resetFlag); // no need to check rc on low clock

if (++i % 1000 == 0)
cout << "main cycled " << dec << i << " times." << endl;
}

return 0;
}
@

So...do the guts of this become my run() function? And, what's currently in my example main() essentially goes into my app main()? Is that the gist of it?

Also, I'd like to move as much of the progress bar processing out of main(). I know it's only three lines now, but...once I get this working in my app, I'm planning on adding some stuff to the UI. Any reason I shouldn't pursue this?

Thanks...

[goetz]

Yes, the contents of the main method would go into the worker class. Make the Modem and the flags a class variable, and the rest into the run slot. The singal and emit is just like in the dummy example we had before.

[EDIT: the contents of the main method goes to the class, not the method itself, of course :) ]

[mzimmers]

OK, thanks, Volker. On a related subject, I've been experimenting with creating a QMainWindow, but evidently, I don't understand the constructors as well as I thought. The line:

@ QProgressBar bar(0);@

Is creating an object of type QProgressBar named bar, and with no parent, right? So why doesn't this:

@ QMainWindow mainWindow;
QProgressBar bar(*mainWindow);
@

Create a QMainWindow, and then a bar whose parent is the QMainWindow? I'm getting a "no matching function" compiler error.

[mlong]

You'd want @QProgessBar bar(&mainWindow)@

(&foo takes the address of an object, where *foo dereferences a pointer to an object)

But...

That's a bad idea! In this case, QProgressBar is created on the stack. Parenting it to QMainWindow would make QMainWindow attempt to take ownership of it (and, as such, try to delete it upon its own destruction, which would cause all kinds of badness to happen) You never want to call delete() on a stack-based item.

However, the following would be ok:
@
QMainWindow mainWindow;
QProgressBar *bar = new QProgressBar(&mainWindow);
@

[goetz]

mlong is correct here. In general, for real world applications, you should create QWidget based objects on the heap using new. If you set a parent, you do not need to call delete on those, that's handled by Qt automatically.

Also, you should consider using Qt Designer for creating the UI components. I personally find it much more convenient and easier to use than hard coding that stuff manually.

[mzimmers]

Thank you both for that. So, as a follow-up question...if it's taboo to create bar on the stack, why is it OK to create mainWindow on the stack? (And I realize this is main(), but I see the reasoning in general.)

And, Volker...I'll go look more closely at Qt Designer for awhile. I assume that whatever I come up with there can be neatly stitched back into my sample application?

EDIT:

I just tried the suggestion above:

@ QMainWindow mainWindow;
QProgressBar *bar = new QProgressBar(&mainWindow);
@

But now the linker is complaining about my second connect() call, which I modified to look like this:

@ QObject::connect(&worker, SIGNAL(percentChanged(int)), *bar, SLOT(setValue(int)));
@

(It's another "no matching function" error; I can provide the text if desired.) What did I do to cause this?

[goetz]

Ah, you're trapped by the pointer-reference-address-dereferencing confusion.

connect takes a pointer for the first and third argument. Which operator (* or & or nothing) you need to prefix the variable with depends on the type of the variable:

@
// stack based, w1 is an object
QWidget w1;

// pointer to a QWidget
// & gets the address of w1
QWidget w1ptr = &w1;

// reference to a QWidget
// no special operator is needed for the object w1
QWidget &w1ref1 = w1;

// reference to a QWidget
// dereference the w1ptr with * to get the object
QWidget &w1ref2 = *w1ptr;

// call a method:
w1.objectName();
w1ptr->objectName();
w1ref1.objectName();

// ------------------------------

// heap based, w2 is a pointer
QWidget w2 = new QWidget(this);

// pointer to a QWidget
// w2 is a pointer - no conversion needed
QWidget *w2ptr = w2;

// reference to a QWidget
// dereference the w1ptr with * to get the object in both cases
QWidget &w2ref1 = *w2;
QWidget &w2ref2 = *w2ptr;

// call a method:
w2->objectName();
w2ptr->objectName();
w2ref1.objectName();
w2ref2.objectName();
@

So, back to connect:

In our first example, all object were created on the stack, and you had that objects in the variables. To make connect() happy, you need to make them to pointers using operator &.

In your modified code, you already have a pointer to the progress bar, so you do not need an operator at all:

@
QObject::connect(&worker, SIGNAL(percentChanged(int)), bar, SLOT(setValue(int)));
@

Passing *bar would be the very same as passing bar (without operator) in the first version.

I suggest reading some C++ tutorial on that topic. I'm sure they explain this much better than me :-)

[mzimmers]

Hey, Volker...you explained that just fine. That was just a brain-fade on my part. So, back to my earlier question: if bar needs to be on the heap, why doesn't mainWindow?

Also, as long as I'm throwing out random questions: I notice my program no longer terminates. When I get to the end of file, can I signal a dismissal or something from the worker thread?

Alternatively, I can create a "quit" button or something; at that point, a little text and this thing is ready to go.

Thanks.

EDIT: one more random question: I was experimenting with adding text to the progress bar. (I know I can get it to show the precent completed, but right now, I'm being lazy and just going with "xxx." I'm still cracking the code of the documentation syntax, so I'm not sure how to form this: I tried:
@ QString text = "xxx";
bar->text = text;
@
But that didn't work (presumably because the text property is private). So I tried:
@ QString text = "xxx";
bar->text(text);
@
But that didn't work either. The compiler is giving me an error that I'm having trouble interpreting (something about using "const" in the call). What am I doing wrong here?

[mzimmers]

Ah...after looking at the doc a bit more closely, I believe I misunderstood the text() function. It appears to be a getter, not a setter, so I can see how that wouldn't have worked. What's less clear is how I do set the text.

Here's my calls to the bar object:

@ bar->setRange(0, 100);
bar->setValue(0);
bar->move(10, 10);
bar->setTextVisible(true);
bar->show();
@

What am I missing here to get the progress text to display? Thanks.

[goetz]

The "format property":/doc/qt-4.8/qprogressbar.html#format-prop of QProgressBar holds the text template that is displayed. Be aware that it's possible that the text is not displayed at all, e.g. in the Mac style!

For your quit problem:
By default, the application is not terminated once the last window has been closed. You can change this by adding the following line to your main method:

@
QApplication app(argc, argv);

app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit()));
@

A quit button does work too, of course.

[mzimmers]

I did look at that (honestly!)...I figured that since it said:

@The default value is "%p%".@

...that would mean it would show up by default. No big deal; it was more for my education anyway.

Thanks for the info on the quit slot. I do plan on adding a "close" button; just taking on one thing at a time. Right now, I'm fighting a weird Windoze build/debug problem.

More questions to follow soon...thanks.

EDIT:

Regarding the quit slot you suggested above, I now get a rather ominous message in the console window when I close the app window:

bq. QThread: Destroyed while thread is still running

Have I left some loose ends here? For example, am I supposed to write my own quit() to overload whatever the one supplied by QApplication does?

[mzimmers]

OK, I'm back to this. I'm ready to add a "close" button. In fact, I've already added the button object using QPushButton. Before I wade into connecting this up with signals and slots, I have a couple housekeeping questions:

1. I still am not sure I understand why the completion percentage doesn't show up in text format. The way I read the documentation, by default, "%p%" will display, and just to make sure, I added a line:

@ bar->setFormat("%p%");
@

But nothing shows up.

2. I now have a QMainWindow objects that "contains" (if that's the right term) two other objects: QProgressBar and QPushButton. Each of these objects will need some formatting code, as will (I imagine) any other objects I might add. Currently, I have a routine to configure the progress bar:

@void configBar(QProgressBar* bar)
{
QString text;
bar->setRange(0, 100);
bar->setValue(0);
bar->move(50, 50);
bar->setTextVisible(true);
bar->setFormat("%p%");
bar->show();
text = bar->text();
}
@

As you can see, I pass it a pointer to the object as a parameter. It wouldn't seem to make sense that I'd have a separate routine for each object; is there some way to derive the "contained" objects from QMainWindow? That way, I could do all my formatting in one routine. Or, should I create my own class, with all of these objects as member elements, and access them that way?

Thanks.

[mlong]

1. The setFormat as written above should set the text to "0%" through "100%" as the progress bar progresses. But, as Volker said, keep in mind that text may or may not be displayed (depending on the widget style in use.) The %p is the percentage, and the trailing % is just a percent sign. %m would be used for the current step, and %v would be used for the actual value.

2. The widgets in your mainwindow should be declared as private: member variables in the mainwindow header file. (This is basic encapsulation, so I hope I'm not saying anything too obvious to you.) Then you can have one setup routine. Traditionally (though it's a matter of style) I would something such as:

mainwindow.h:
@
class MainWindow : public QMainWindow
{
Q_OBJECT
...
void MainWindow(QWidget *parent = 0);
...
private:
void setupWidgets();

QProgressBar *m_bar;
QPushButton *m_button;
// etc.
};
@

and in the .cpp file:
@
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
m_bar = new QProgressBar(this); // these are children of MainWindow
m_button = new QPushButton(this);

setupWidgets();
}
...
void MainWindow::setupWidgets()
{
// setup ProgressBar
m_bar->setRange(0, 100);
m_bar->setValue(0);
m_bar->move(50, 50);
m_bar->setTextVisible(true);
m_bar->setFormat("%p%");
// since calling MainWindow::show() will call show() on its children, there's no need to use m_bar->show().

// setup PushButton
m_button->setText("Close");

// can also set up any appropriate signals/slots, etc. here

}
@
(Brain to terminal. For illustrative purposes only.)

I'm not sure why you're setting the "text" variable in your code above, but if you needed it somewhere else within the MainWindow class, it would be accessable via "m_bar->text()"

[mzimmers]

Thanks for the clarification on #2 above. I was only putting text in the button as a convenience; I'll probably do it as you suggest above.

I still don't get this:

bq. But, as Volker said, keep in mind that text may or may not be displayed (depending on the widget style in use.)

What is this widget style you're talking about? Do I have control over it?

Thanks.