QListViewDelegate paint issue

[Rory_1]

I eliminated everything except the border. The top-left item is the previously selected item and the top-right is the currently selected item.

and then after I mouse-over the scrollbar to the right:

Here is the code:

void ThumbViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                           const QModelIndex &index) const
 {
    painter->save();

    QRect itemRect = option.rect;
    if (option.state & QStyle::State_Selected) {
        QPen selectedPen(Qt::white);
        selectedPen.setWidth(2);
        painter->setPen(selectedPen);
    } else {
        QPen activePen(Qt::red);
        activePen.setWidth(2);
        painter->setPen(activePen);
    }
    painter->drawRoundedRect(itemRect, 8, 8);

    painter->restore();
}

[kshegunov]

Hello,
Now this is clearer to diagnose. Try erasing the background before painting, like this:

painter->fillRect(option.rect, option.backgroundBrush);
// ... Rest of your painting code

or

painter->eraseRect(option.rect);

As a side note, I also advise you to use the test function to check for a flag, like this:

if (option.state.testFlag(QStyle::State_Selected))  {
    // ... Flag is set
}

Kind regards.

[Rory_1]

I tried all of the above: erasing rect and testFlag. No change what-so-ever to behavior. The weird thing is this behavior does not always occur. Sometimes the previous and/or currently selected items paint correctly.

I am going to track which events are firing when I mouse-over leaving the listview. Maybe I can get a work-around using a signal/slot.

[kshegunov]

@Rory_1
Take note that your highlighted border is not properly drawn as well. It seems half the drawing occurs at the old item, and the other half at the new item. This is pretty strange. Do you mind sharing a bit more of your code, for example how you set up your delegate and any event overrides you might have in your list view?

Kind regards.

[Rory_1]

Thanks very much for your help! I have to go out for the afternoon, but I will follow-up later.

[Rory_1]

I have created a small project that shows the problem. It contains:

• testdelegate. pro
• main.cpp
• mw.h
• mw.cpp
• testdelegate.h
• testdelegate.cpp

testDelegate. pro:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = testDelegate
TEMPLATE = app


SOURCES += main.cpp\
        mw.cpp \
    thumbviewdelegate.cpp

HEADERS  += mw.h \
    thumbviewdelegate.h

main.cpp:

#include "mw.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MW w;
    w.show();

    return a.exec();
}

mw.h: (mainwindow)

#ifndef MW_H
#define MW_H

#include <QMainWindow>
#include <QtWidgets>
#include "thumbviewdelegate.h"

class MW : public QMainWindow
{
    Q_OBJECT

public:
    explicit MW(QWidget *parent = 0);

private:
    QHBoxLayout *mainLayout;
    QListView *thumbView;
    QStandardItemModel *thumbViewModel;
    QStandardItem *thumbIitem;
    ThumbViewDelegate *thumbviewDelegate;
};

#endif // MW_H

mw.cpp:

#include "mw.h"

MW::MW(QWidget *parent) :
    QMainWindow(parent)
{
    resize(600, 600);
    thumbView = new QListView;
    mainLayout = new QHBoxLayout;
    mainLayout->setContentsMargins(0, 0, 0, 0);
    mainLayout->setSpacing(0);
    mainLayout->addWidget(thumbView);
    QWidget *centralWidget = new QWidget;
    centralWidget->setLayout(mainLayout);
    setCentralWidget(centralWidget);

    thumbView->setSelectionMode(QAbstractItemView::ExtendedSelection);
    thumbView->setViewMode(QListView::IconMode);
    thumbView->setResizeMode(QListView::Adjust);
    thumbView->setWordWrap(true);
    thumbView->setDragEnabled(true);
    thumbView->setEditTriggers(QAbstractItemView::NoEditTriggers);
    thumbView->setUniformItemSizes(false);

    thumbViewModel = new QStandardItemModel();
    thumbView->setModel(thumbViewModel);
    thumbViewModel->clear();
    thumbviewDelegate = new ThumbViewDelegate(this);
    thumbView->setItemDelegate(thumbviewDelegate);
    thumbView->setSpacing(10);

    for (int i = 0; i < 10; ++i) {
        thumbIitem = new QStandardItem;
        thumbViewModel->appendRow(thumbIitem);
    }
}

testdelegate.h:

#ifndef THUMBVIEWDELEGATE_H
#define THUMBVIEWDELEGATE_H

#include <QtWidgets>

class ThumbViewDelegate : public QStyledItemDelegate
{
    Q_OBJECT

public:
    ThumbViewDelegate(QObject *parent = 0);

    void paint(QPainter *painter, const QStyleOptionViewItem &option,
               const QModelIndex &index) const;

    QSize sizeHint(const QStyleOptionViewItem &option,
                   const QModelIndex &index ) const;
};

#endif // THUMBVIEWDELEGATE_H

testdelegate.cpp:

#include "thumbviewdelegate.h"

ThumbViewDelegate::ThumbViewDelegate(QObject *parent)
{
}

QSize ThumbViewDelegate::sizeHint(const QStyleOptionViewItem &  option ,
                              const QModelIndex & index) const
{
    return(QSize(200,200));
}

void ThumbViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                           const QModelIndex &index) const
 {
    painter->save();

    if (option.state.testFlag(QStyle::State_Selected)) {
        QPen selectedPen(Qt::red);
        selectedPen.setWidth(5);
        painter->setPen(selectedPen);
    } else {
        QPen activePen(Qt::black);
        activePen.setWidth(5);
        painter->setPen(activePen);
    }
    painter->drawRect(option.rect);

    painter->restore();
}

[kshegunov]

Hello,
Right, It took me a few minutes, but I've found the problem - you're painting outside of your allotted space. This is why the behavior you're getting is so erratic - you depend on when the list view will (or will not) erase it's contents. Brace yourself ;), here is the problematic fragment:

painter->drawRect(option.rect);

When you draw a stroked rectangle the area you provide is for the insides (the pen requires additional space).
This means that you draw your border in actuality outside of the item rectangle. This is also the reason why the rounded corners actually get erased (they happen to be inside option.rect). The fix is quite simple, here is an example how you could solve your problem:

painter->save();

// Make the rect smaller to accommodate the border.
static const qint32 penSize = 4;
QPoint strokeOffset(penSize, penSize);
QRect borderRect(option.rect.topLeft() + strokeOffset, option.rect.bottomRight() - strokeOffset);

// ... Rest of your code

painter->drawRect(borderRect);    // Draw the border properly

I hope this helps.
Kind regards.

[Rory_1]

Thank you so much! I don't think I would have figured that out. My first Christmas present :*)

[kshegunov]

@Rory_1
You're welcome. Good luck with your application!

Kind regards.

[raven-worx]

just for addition: see this
So i ncase you draw anti-aliased, the border-offset would just be the half of the border-width.