When events should be used in SFML?

Question

I am confused about how to get inputs from Mouse or Keyboard. As an example, I want to draw little dots on my Mouse position when I pressed the button of my Mouse. Which implementation should I follow?

I have used window.pollEvent function to catch the mouse pressed event in the code below.

#include <SFML/Graphics.hpp>
#include <iostream>

int main()
{
    sf::RenderWindow window(sf::VideoMode(640,480), "Paint");
    std::vector<sf::CircleShape> dots;

    while (window.isOpen()) {
        sf::Event event;
        if (window.pollEvent(event)) {
            if (event.type == sf::Event::Closed) {
                window.close();
            }
            
            if (event.type == sf::Event::MouseButtonPressed) {
                sf::CircleShape shape(10);
                shape.setFillColor(sf::Color::Black);
                shape.setPosition(event.mouseButton.x, event.mouseButton.y);
                dots.push_back(shape);
            }
        }

        window.clear(sf::Color::White);

        for (auto& i : dots) {
            window.draw(i);
        }

        window.display();
    }
    return 0;
}

or should I do it in the way like this?

#include <SFML/Graphics.hpp>
#include <iostream>

int main()
{
    sf::RenderWindow window(sf::VideoMode(640,480), "Paint");
    std::vector<sf::CircleShape> dots;

    while (window.isOpen()) {
        sf::Event event;
        if (window.pollEvent(event)) {
            if (event.type == sf::Event::Closed) {
                window.close();
            }
        }

        if (sf::Mouse::isButtonPressed(sf::Mouse::Left)) {
            sf::CircleShape shape(10);
            shape.setFillColor(sf::Color::Black);
            shape.setPosition(sf::Mouse::getPosition().x, sf::Mouse::getPosition().y);
            dots.push_back(shape);
        }

        window.clear(sf::Color::White);

        for (auto& i : dots) {
            window.draw(i);
        }

        window.display();
    }

    return 0;
}

If the latter is the appropriate one, then where the if block that checks if the mouse button is pressed should be located, before window.clear() or between window.clear() and window.draw() ? I could not understand the different between them thoroughly. SFML documentation, for example, shows the latter implemenatation for gunfire action but I could not figure out why. Thanks...

Answer 1

You are essentially asking about the two ways for handling user input:

• Events: handling objects that represent an occurrence.
• Real-time input: querying the input device about its real-time state.

Your first approach – calling sf::Window::pollEvent() – relies on events. It is an asynchronous mechanism; the button may not be pressed when your code handles the event. Event handling is usually the way to go if all you are interested in is whether the state of the input device has undergone a change^X, e.g., a button has been pressed or released.

Your second approach – calling sf::Mouse::isButtonPressed() – is based on real-time input. It consists of querying the mouse whether a given button is pressed at the moment of calling the function. This approach for handling user input is usually the way to go if you just want to find out the current state of the input device.

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^XNote, however, that events can be repeated (e.g., if you keep a key pressed for a long time), and therefore they may not necessarily imply a change in the state of the input device. You can disable this with sf::Window::SetKeyRepeatEnabled(), though.