Draw a point when button is clicked

mitrovicl

So, I created 2 functions: onClick() - function that executes when the button is clicked, and drawPoint(int x,int y).
onClick function calls the drawPoint function with x and y parameters, but when I run the program, and I click the button, it says:
"QPainter::drawPoints: Painter not active".
Here's my mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QPushButton *button = QMainWindow::findChild<QPushButton *>("pushButton");
    connect(button, SIGNAL(released()), SLOT(onClick()));

}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::drawPoint(int x, int y) {

    QPainter painter;
    painter.drawPoint(x,y);

}

void MainWindow::onClick() {
    QPoint point = QCursor::pos();
    int x = point.x();
    int y = point.y();
    drawPoint(x,y);
}

I'm a total beginner in Qt, and have no idea what's going on. I've searched on this forum, but I couldn't find anything.

JonB

@mitrovicl
You can only draw a point in a paint event.

Basically, can you pick something different from this to do? Widget applications are not usually about drawing points. Depends what you are ultimately trying to achieve.

mitrovicl

@JonB Thanks for the answer, I'm trying to create CPS (Clicks per second) test. And I want every click to leave a "trail". I'll look into pain events.

JonB

@mitrovicl
Maybe you might be interested in examples which come with Qt:

Basic Drawing Example

Scribble Example

Search for paintEvent in them.

SGaist

Hi and welcome to devnet,

The first thing: you cannot paint on a widget outside the paint event.

Second, please read the QPainter documentation. It provides a lot of information as well as links to various examples.

Pl45m4

@mitrovicl said in Draw a point when button is clicked:

QPushButton *button = QMainWindow::findChild<QPushButton *>("pushButton");

I assume you have a mainWindow.ui (modified with QtDesigner), then this is not necessary.
Just access you button by using your ui pointer.

ui->pushButton

connect(button, SIGNAL(released()), SLOT(onClick()));

and better use the updated, Qt5 function-based "Signal & Slot"-syntax.

connect(ui->pushButton, &QPushButton::released, this, &MainWindow::onClick);

https://wiki.qt.io/New_Signal_Slot_Syntax