Stretching a QLabel in a QGridLayout - c++

Question

I am manually creating a set of QLabels that are being put into a QGridLayout, and should be distributed evenly. When I create a test form using the QT Designer and add a series of labels, and put them in a QGridLayout, the labels occupy the full size of their cells in the grid. When I do this manually in c++, the labels don't expand and say the minimum size for the text. I am able to get these labels to expand vertically, but not horizontally.
Here is how I'm creating the QGridLayout:
m_layout = new QGridLayout;
m_layout->setHorizontalSpacing( 1 );
m_layout->setVerticalSpacing( 1 );
m_layout->setContentsMargins(0,0,0,0);
m_layout->setMargin(0);
m_layout->setSizeConstraint( QGridLayout::SetDefaultConstraint );
//m_layout->setSizeConstraint( QGridLayout::SetMaximumSize );
When I change the size constraint, it does not affect the size of the labels at all, so then I assumed that the issue may lie on how I'm creating the labels, which happens like this:
QLabel *label = new QLabel;
label->setAlignment( Qt::AlignCenter );
label->setFrameStyle( QFrame::Raised );
label->setMinimumSize( QSize(0,0) );
label->setMaximumSize( QSize(16777215, 16777215) );
label->setSizePolicy( QSizePolicy::Preferred, QSizePolicy::Preferred );
Right now, all the QLabels are the same size, but eventually they will have differing sizes, which is why I'm using a packed-bin algorithm. I designed the algorithm around classes like this:
struct ButtonFittingNode
{
ButtonFittingNode( QRect rectangle, QGridLayout *layout )
: used( false )
, node( rectangle )
, down( NULL )
, right( NULL )
, m_layout( layout )
{ }
~ButtonFittingNode()
{
if ( down ) delete down;
if ( right ) delete right;
}
ButtonFittingNode *findNode( QLabel *btn )
{
int w = 1;
int h = 1;
if ( this->used )
{
ButtonFittingNode *bfn = NULL;
if ( this->right ) bfn = this->right->findNode( btn );
if ( this->down && !bfn ) bfn = this->down->findNode( btn );
return bfn;
}
else if ( ( w <= node.width() ) && ( h <= node.height() ) )
{
qDebug() << "Placing at " << node.x() << node.y();
m_layout->addWidget( btn, node.y(), node.x(), w, h, Qt::AlignJustify );
this->used = true;
this->splitNode( QSize( w, h ) );
return this;
}
return NULL;
}
void splitNode( QSize sz )
{
int w = 0, h = 0;
// Create a down node
w = this->node.width();
h = this->node.height() - sz.height();
if ( h > 0 )
{
QRect n( this->node.x(), this->node.y() + sz.height(), w, h );
this->down = new ButtonFittingNode( n, m_layout );
}
// Create a right node
w = this->node.size().width() - sz.width();
h = sz.height();
if ( w > 0 )
{
QRect n( this->node.x() + sz.width(), this->node.y(), w, h );
this->right = new ButtonFittingNode( n, m_layout );
}
}
bool used;
QRect node;
ButtonFittingNode *down;
ButtonFittingNode *right;
private:
QGridLayout *m_layout;
};
Using a 3x2 grid, I get the following output:
Sorry, I had to blur the text, since this is a work related project, but grey is background, white is the label's background. As you can see, they are tall, but skinny. I want these labels to be tall and fat. Am I setting some of the attributes wrong?

Answer 1

This is all really simple. The size constraint of the layout has got nothing to do with what you're seeing. You're also including a bunch of useless boilerplate code. Get rid of explicit setting of minimum and maximum sizes unless you have some non-default sizes in mind. This is completely useless:
label->setMinimumSize( QSize(0,0) );
label->setMaximumSize( QSize(16777215, 16777215) );
You need to set the label's sizePolicy to Expanding in the horizontal direction. That's all there's to it:
label->setSizePolicy( QSizePolicy::Expanding, QSizePolicy::Preferred );
I see what you want from your bin packing system, but it won't work very well as-is, since the column/row sizes in the grid will vary as the grid gets resized. Your packer will choose some column/row spans based on then current row/column sizes. Upon resizing, the packer's decisions won't be adequate anymore.
What you really want is a custom layout that does all the packing on the fly. The flow layout example may be very close to what you want.