Using QPainter to draw a line between QWidgets - c++

Question

I'm working on an application where I need to be able to draw a line between two QWidget objects. I have tried quite a few things, but my current attempt (which I think is in the right direction I just think I'm missing something) is to have the containing widget (which I called DrawWidget and which holds the QGridLayout that the QWidget objects are added to) override the paintEvent method and call the QPainter::drawLine() function.
The issues I'm having are that:
No matter how I try to get the position of the widgets, the endpoints of the line are always in the wrong place
Whenever I try to draw a second line, the first line that I drew gets erased.
Here is the paintEvent function of the containing widget:
void paintEvent(QPaintEvent *)
{
if (!drewSinceUpdate){
drewSinceUpdate = true;
QPainter painter(this);
painter.setPen(QPen(Qt::black));
painter.drawLine(start->geometry().center(), end->geometry().center());
}
}
I have tried many different ways to get the correct position of the widgets in the last line of paintEvent, which I will post some of the ways (I can't remember all of them):
painter.drawLine(start->pos(), end->pos());
painter.drawLine(start->mapToGlobal(start->geometry().center()), end->mapToGlobal(end->geometry().center()));
painter.drawLine(this->mapToGlobal(start->geometry().center()), this->mapToGlobal(end->geometry().center()));
painter.drawLine(start->mapTo(this, start->pos()), end->mapTo(this, end->pos()));
painter.drawLine(this->mapFrom(start, start->pos()), this->mapFrom(end, end->pos()));
And just to make my question clear, here is an example of what I am looking for, taken from QT Diagram Scene Example:
But this is what I end up getting:
Thank you for any help you can provide.
NOTE:
-start and end are both QWidget objects which I passed in using another method
-The hierarchy relevant to DrawWidget is:
QMainWindow
->QScrollArea
->DrawWidget
->QGridLayout
->Items <-- These are the things I want to connect
EDIT: To make a Complete and Verifiable example, here is the entirety of the relevant code.
MainWindow.cpp:
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QTextEdit>
#include <QPushButton>
#include <QScrollBar>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
// Setting up the relevant hierarchy
ui->setupUi(this);
scrollArea = new QScrollArea();
setCentralWidget(scrollArea);
drawWidget = new DrawWidget();
gridLayout = new QGridLayout();
gridLayout->setSpacing(300);
drawWidget->setLayout(gridLayout);
scrollArea->setWidget(drawWidget);
scrollArea->setWidgetResizable(true);
AddItemSlot();
QApplication::connect(scrollArea->horizontalScrollBar(), SIGNAL(rangeChanged(int,int)), this, SLOT(scrollHorizontal()));
}
// This is just creating a single one of the example widgets which I want to connect
QWidget* MainWindow::CreateNewItem(){
QWidget* itemWidget = new QWidget();
itemWidget->setStyleSheet("background-color: lightgray");
QHBoxLayout* singleItemLayout = new QHBoxLayout();
itemWidget->setLayout(singleItemLayout);
QTextEdit* textEdit = new QTextEdit(std::to_string(counter++).c_str());
textEdit->setStyleSheet("background-color:white;");
singleItemLayout->addWidget(textEdit);
QVBoxLayout* rightSidePanel = new QVBoxLayout();
rightSidePanel->setAlignment(Qt::AlignTop);
QPushButton* button1 = new QPushButton("Top Button");
QApplication::connect(button1, SIGNAL(clicked(bool)), this, SLOT(AddItemSlot()));
rightSidePanel->addWidget(button1);
QWidget* rightPanelWidget = new QWidget();
rightSidePanel->setMargin(0);
rightPanelWidget->setLayout(rightSidePanel);
singleItemLayout->addWidget(rightPanelWidget);
itemWidget->setLayout(singleItemLayout);
itemWidget->setMinimumWidth(400);
itemWidget->setFixedSize(400,200);
return itemWidget;
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::scrollHorizontal()
{
scrollArea->ensureWidgetVisible(noteItems.back());
}
void MainWindow::AddItemSlot()
{
QWidget* w = CreateNewItem();
gridLayout->addWidget(w,currRow, currCol++);
if (!noteItems.empty()){
drawWidget->updateEndpoints(noteItems.back(), w);
}
noteItems.push_back(w);
}
MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QGridLayout>
#include <QWidget>
#include <QMainWindow>
#include <QScrollArea>
#include <drawwidget.h>
#include "drawscrollarea.h"
#include <vector>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
public slots:
void scrollHorizontal();
void AddItemSlot();
private:
Ui::MainWindow *ui;
QWidget* CreateNewItem();
int counter = 0, currCol = 0, currRow = 0;
std::vector<QWidget*> noteItems;
QScrollArea* scrollArea;
DrawWidget* drawWidget;
QGridLayout* gridLayout;
};
#endif // MAINWINDOW_H
DrawWidget.cpp:
#include "drawwidget.h"
#include <QDebug>
#include <QRect>
DrawWidget::DrawWidget(QWidget *parent) : QWidget(parent)
{
}
void DrawWidget::paintEvent(QPaintEvent *)
{
if (!drewSinceUpdate){
drewSinceUpdate = true;
QPainter painter(this);
painter.setPen(QPen(Qt::black));
for (ConnectedPair pair : items){
const QWidget* from = pair.from;
const QWidget* to =pair.to;
QPoint start = from->mapToGlobal(from->rect().topRight() + QPoint(0, from->height()/2));
QPoint end = to->mapToGlobal(to->rect().topLeft() + QPoint(0, to->height()/2));
painter.drawLine(mapFromGlobal(start), mapFromGlobal(end));
}
}
}
void DrawWidget::updateEndpoints(QWidget* startIn, QWidget* endIn){
drewSinceUpdate = false;
items.push_back(ConnectedPair{startIn, endIn});
}
DrawWidget.h
#ifndef DRAWWIDGET_H
#define DRAWWIDGET_H
#include <QWidget>
#include <QPainter>
#include <QtCore>
#include <vector>
class DrawWidget : public QWidget
{
Q_OBJECT
public:
explicit DrawWidget(QWidget *parent = nullptr);
void updateEndpoints(QWidget* startIn, QWidget* endIn);
virtual void paintEvent(QPaintEvent *);
signals:
private:
struct ConnectedPair {
const QWidget* from;
const QWidget* to;
};
std::vector<ConnectedPair> items;
bool drewSinceUpdate = true;
};
#endif // DRAWWIDGET_H

Answer 1

For this case we use the function mapToGlobal() and mapfromGlobal(), since pos() returns a position with respect to the parent and this can cause problems if the widget has different parents.
drawwidget.h
#ifndef DRAWWIDGET_H
#define DRAWWIDGET_H
#include <QWidget>
class DrawWidget : public QWidget
{
Q_OBJECT
public:
explicit DrawWidget(QWidget *parent = nullptr);
void addWidgets(const QWidget *from, const QWidget *to);
protected:
void paintEvent(QPaintEvent *);
private:
struct WidgetsConnected {
const QWidget* from;
const QWidget* to;
};
QList<WidgetsConnected> list;
};
#endif // DRAWWIDGET_H
drawwidget.cpp
#include "drawwidget.h"
#include <QPainter>
DrawWidget::DrawWidget(QWidget *parent) : QWidget(parent)
{
}
void DrawWidget::addWidgets(const QWidget * from, const QWidget * to)
{
list.append(WidgetsConnected{from , to});
update();
}
void DrawWidget::paintEvent(QPaintEvent *)
{
QPainter painter(this);
for(const WidgetsConnected el: list){
const QWidget* from = el.from;
const QWidget* to = el.to;
QPoint start = from->mapToGlobal(from->rect().topRight() + QPoint(0, from->height()/2));
QPoint end = to->mapToGlobal(to->rect().topLeft() + QPoint(0, to->height()/2));
painter.drawLine(mapFromGlobal(start), mapFromGlobal(end));
}
}
The complete example can be found here.