lag in GUI created in Qt (verified by QTime) - c++

Question

I have a GUI C++ Class in Qt which has a SLOT which adds an array of Text box's & label's in a QGridLayout. The array is square depending upon a variable say n. i.e. if n == 10 then there are 10x10 QTextbox & QLabel in the QGridLayout. Basically when a user presses the Increase button the value of n is incremented by 1 & accordingly the QTextbox & QLabel are created in a SLOT & added in the QGridLayout. When the value of n is 15 it is taking 1-2 secs for the GUI to be updated. However when I used QTime in that SLOT it showed me that the actual time to execute that SLOT is about 100 ms. While in Debug mode I observed that QDebug used to print the elapsed time even though the GUI wasn't completely updated for higher values of n. I would like to know why is there so much delay & what is running after that SLOT is executed so that I can measure the time for the same. I hope I made myself clear. Please let me know if you could not understand my Question.
Thank You.
Actually I don't have the exact code with me right now. So I have created a sample code highlighting the logic of my code. Please ignore any syntax errors as I have prepared it as quickly as I could. Sorry for that :(
So here's the code snippet:
Class A
{
private:
int n;
QList <QLabel *> *labelList;
QList <QTextEdit *> *textList;
QGridLayout *inp;
public slots:
updateGUI();
}
A::updateGUI()
{
QTime t;
t.start();
for(int i = 0; i < n; i++)
{
for(int j = 0; j < n; j++)
{
int even = j%2;
QLabel *tempLab = new QLabel();
//some stuff for initialisation of the QLabel
QTextEdit *tempText = new QTextEdit();
//some stuff for initialisation of the QTextEdit
labelList->append(tempLab);
textList->append(tempText);
if(even == 0)
inp->AddWidget(tempLab, i, j, 5, 5, Qt::AlignCenter);
else
inp->AddWidget(tempText, i, j, 5, 5, Qt::AlignCenter);
}
}
QDebug("%d", t.elapsed());
}
PS: QTime tobj.start() is the 1st line in the SLOT & QDebug("%d", tobj.elapsed()) is the last line in the SLOT Ofcourse!

Answer 1

When you're doing a bulk update on a widget, turn off the updatesEnabled property. This prevents 21 individual updates when you're adding (11*11-10*10) new buttons.

Answer 2

The 0.1s delay is because you are creating a ton of widgets and then adding them to a grid (every time you add a widget, which obviously has an overhead, the dimensions of the other cells have to be updated as things 'shuffle' round).
So why the further delay after the timer? All the painting and resizing events that were generated by the slot have now been put into the event queue, which was halted whilst your slot was executing. Once your slot completes, all those tasks need to be processed.
Further to what other people have said, there are two fundamental flaws to what you are doing:
Deleting all the existing widgets to add possibly a single one. I didn't think I need to explain how horribly inefficient this is...
If you have managed to get into the situation where 100s of text boxes and labels in a grid are being shown to the user - your UI has failed. You need to come up with a better way of allowing the user to interact with all this data.

Answer 3

It would really help, if you post some code with the same problem. How will your slot react, when you select let's say n=10 and then n=7? Does it removes 3 last items from layout? And if you select n=7 and then n=10, will it try add only last 3 lines?
Is it possible, that you are not removing items from layout and trying to add new items over the existing ones?
If it is so - then you can remove all widgets from layout with this:
QLayoutItem* item;
while ( ( item = %your_layout_name%->takeAt( 0 ) ) != NULL ){
delete item->widget();
}
delete item;
or you can remove widget by it's object name (or by handle, stored in QList QVector or something):
swLayout->removeWidget(myTextEdit);
UPD:
now, that we have some code,
you also need to clear labelList and textList, which store handles if i'm correct.
Then, lines
inp->AddWidget(tempLab, i, j, 5, 5, Qt::AlignCenter);
inp->AddWidget(tempText, i, j, 5, 5, Qt::AlignCenter);
tries to add tempLab widget to layout inp in position (i,j) with 5 height and width, then you try to add another widget tempText to the same location, which is obviously wrong
Next, the loop itself is incorrect:
for(int i = 0; i < n; i++)
{
for(int j = 0; j < n; j++)
{
is you make a loop like this, then you will be adding widgets like this:
inp->AddWidget(tempLab, i, 0, 5, 5, Qt::AlignCenter);
inp->AddWidget(tempLab, i, 1, 5, 5, Qt::AlignCenter);
inp->AddWidget(tempLab, i, n, 5, 5, Qt::AlignCenter);
but you sat widget width and heigth as 5, so you need to add widgets another way or they will be painted over each other:
inp->AddWidget(tempText, i*5, j*5, 5, 5, Qt::AlignCenter);