在QGridLayout中拉伸QLabel

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.