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Forum Update on Monday, May 27th 2025

QListViewDelegate paint issue

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delegatepainterqlistview
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  • R Offline
    R Offline
    Rory_1
    wrote on last edited by Rory_1
    #3

    I tried that with/without the aliasing. The pen width and aliasing does not make any difference. I want to know what is happening when I move the mouse away as it is triggering a refresh of the listview that clears up the problem. I tried creating a signal/slot back to my listview to update the model but that did not help.

    kshegunovK 1 Reply Last reply
    0
    • R Rory_1

      I tried that with/without the aliasing. The pen width and aliasing does not make any difference. I want to know what is happening when I move the mouse away as it is triggering a refresh of the listview that clears up the problem. I tried creating a signal/slot back to my listview to update the model but that did not help.

      kshegunovK Offline
      kshegunovK Offline
      kshegunov
      Moderators
      wrote on last edited by
      #4

      @Rory_1

      I want to know what is happening when I move the mouse away as it is triggering a refresh of the listview that clears up the problem.

      Hello,
      You probably are getting a full repaint request, and that will erase the background, and when your code runs after that it will display properly. I still think that there is some strange problem with the painting over the white line, when the mouse leaves your item. I believe that is the case, because it is partially erased. I suggest using a red line for the inactive items, setting the width of the pen to 2px for the inactive items and disabling antialiasing all at the same time. When you get the partially erased border, take a look for a red line next to the white one. If you see such a thing, that simply means that you're not fully erasing the active item border.

      Kind regards.

      Read and abide by the Qt Code of Conduct

      1 Reply Last reply
      0
      • R Offline
        R Offline
        Rory_1
        wrote on last edited by
        #5

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

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

        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();
        }
        
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        • kshegunovK Offline
          kshegunovK Offline
          kshegunov
          Moderators
          wrote on last edited by kshegunov
          #6

          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.

          Read and abide by the Qt Code of Conduct

          1 Reply Last reply
          0
          • R Offline
            R Offline
            Rory_1
            wrote on last edited by
            #7

            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.

            kshegunovK 1 Reply Last reply
            0
            • R 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.

              kshegunovK Offline
              kshegunovK Offline
              kshegunov
              Moderators
              wrote on last edited by
              #8

              @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.

              Read and abide by the Qt Code of Conduct

              1 Reply Last reply
              0
              • R Offline
                R Offline
                Rory_1
                wrote on last edited by
                #9

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

                1 Reply Last reply
                0
                • R Offline
                  R Offline
                  Rory_1
                  wrote on last edited by
                  #10

                  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();
                  }
                  
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                  • kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by
                    #11

                    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.

                    Read and abide by the Qt Code of Conduct

                    1 Reply Last reply
                    1
                    • R Offline
                      R Offline
                      Rory_1
                      wrote on last edited by
                      #12

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

                      kshegunovK 1 Reply Last reply
                      0
                      • R Rory_1

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

                        kshegunovK Offline
                        kshegunovK Offline
                        kshegunov
                        Moderators
                        wrote on last edited by
                        #13

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

                        Kind regards.

                        Read and abide by the Qt Code of Conduct

                        1 Reply Last reply
                        0
                        • raven-worxR Offline
                          raven-worxR Offline
                          raven-worx
                          Moderators
                          wrote on last edited by
                          #14

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

                          --- SUPPORT REQUESTS VIA CHAT WILL BE IGNORED ---
                          If you have a question please use the forum so others can benefit from the solution in the future

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