Custom widget is flickering and clipping while moving - c++

Question

I'll give you a minimal reproduceable example that was a part of the more complex widget.
Here we just have a custom widget(called MovableItem) with a simple paintEvent. Widget is created, placed onto the central widget and moved to mouse position on MainWindow::mousePressEvent-s.
When moving, it seems like the widget is getting clipped at the side it's moving towards.
mainwindow.h
#include <QMainWindow>
#include <QMouseEvent>
#include <QPropertyAnimation>
#include "movableitem.h"
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
void mousePressEvent(QMouseEvent *event) override;
QWidget* mItem;
};
mainwindow.cpp
#include "mainwindow.h"
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
resize(640, 480);
QWidget* central_widget = new QWidget(this);
mItem = new MovableItem(central_widget);
mItem->move(20, 20);
setCentralWidget(central_widget);
}
void MainWindow::mousePressEvent(QMouseEvent *event) {
QPoint pos = event->pos();
QPropertyAnimation* anim = new QPropertyAnimation(mItem, "geometry");
anim->setDuration(750);
anim->setStartValue(QRect(mItem->x(), mItem->y(), mItem->width(), mItem->height()));
anim->setEndValue(QRect(pos.x(), pos.y(), mItem->width(), mItem->height()));
anim->start();
}
MainWindow::~MainWindow() {}
movableitem.h
#include <QWidget>
#include <QPainter>
#include <QPainterPath>
class MovableItem : public QWidget
{
Q_OBJECT
public:
MovableItem(QWidget *parent = nullptr);
QSize sizeHint() const override;
void paintEvent(QPaintEvent *event) override;
};
movableitem.cpp
#include "movableitem.h"
MovableItem::MovableItem(QWidget *parent) : QWidget(parent)
{
setParent(parent);
}
QSize MovableItem::sizeHint() const {
return QSize(150, 40);
}
void MovableItem::paintEvent(QPaintEvent *event) {
QRect r = rect();
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
QPainterPath path;
path.addRoundedRect(r, 5, 5);
QBrush brush(QColor(217, 217, 217));
painter.fillPath(path, brush);
painter.drawPath(path);
}
Example
As you can see, movement is not fluid, but choppy. I have no idea what is happening. Am I doing something completely wrong ? Do I need to implement some additional functions, is double buffering needed, is this because of Qt's automatic clipping ? Should I rewrite it in QGraphicsView ?

Answer 1

Add mItem->repaint(); and mItem->update(); in mousePressEvent function
void MainWindow::mousePressEvent(QMouseEvent *event)
{
QPoint pos = event->pos();
QPropertyAnimation* anim = new QPropertyAnimation(mItem, "geometry");
anim->setDuration(750);
anim->setStartValue(QRect(mItem->x(), mItem->y(), mItem->width(), mItem->height()));
anim->setEndValue(QRect(pos.x(), pos.y(), mItem->width(), mItem->height()));
anim->start();
mItem->repaint();
mItem->update();
}
my out put is here :