Exploration of sf::Event::visit use cases

[vittorioromeo]

Prerequisite Checklist

• I searched for existing issues to prevent duplicates
• I searched for existing discussions on the forum to prevent duplicates
• I understand SFML's scope and believe the feature fits it

Describe your feature request here

In #2990 and event-related PRs, there was some discussion about providing a sf::Event::visit function. Would it actually be worth it?

Here are two examples of what could be achieved with such a function. Note that both examples are based on #2992, which I believe it's the right way forward for events anyway.

First of all, the user defines this convenience function:

void pollEvents(sf::Window& window, auto... lambdas)
{   
    auto visitor = overload(lambdas...);
    while (const std::optional event = window.pollEvent())
        event.visit(visitor);
}

Now, example 1:

//
//
// CURRENT API 
while (window.isOpen())
{
    while (const std::optional event = window.pollEvent())
    {
        if (event->is<sf::Event::Closed>())
        {
            window.close();
            break;
        }

        if (const auto* keyPressed = event->getIf<sf::Event::KeyPressed>();
            keyPressed && keyPressed->code == sf::Keyboard::Key::Escape)
        {
            window.close();
            break;
        }
    }
}

//
//
// WITH VISITATION
while (window.isOpen())
{
    pollEvents(window, 
        [&](sf::Event::Closed)
        { 
            window.close(); 
        },
        [&](sf::Event::KeyPressed e)
        { 
            if (e.code == sf::Keyboard::Key::Escape)
                window.close();
        }
    );
}

Example 2 (from Tennis):

//
//
// CURRENT API
while (const auto event = window.pollEvent())
{
    // Window closed or escape key pressed: exit
    if (event.is<sf::Event::Closed>() || (event.is<sf::Event::KeyPressed>() &&
                                            event.getIf<sf::Event::KeyPressed>()->code == sf::Keyboard::Key::Escape))
    {
        window.close();
        break;
    }

    // Space key pressed: play
    if ((event.is<sf::Event::KeyPressed>() && event.getIf<sf::Event::KeyPressed>()->code == sf::Keyboard::Key::Space) ||
        event.is<sf::Event::TouchBegan>())
    {
        if (!isPlaying)
        {
            // (re)start the game
            isPlaying = true;
            clock.restart();

            // Reset the position of the paddles and ball
            leftPaddle.setPosition({10.f + paddleSize.x / 2.f, gameHeight / 2.f});
            rightPaddle.setPosition({gameWidth - 10.f - paddleSize.x / 2.f, gameHeight / 2.f});
            ball.setPosition({gameWidth / 2.f, gameHeight / 2.f});

            // Reset the ball angle
            do
            {
                // Make sure the ball initial angle is not too much vertical
                ballAngle = sf::degrees(std::uniform_real_distribution<float>(0, 360)(rng));
            } while (std::abs(std::cos(ballAngle.asRadians())) < 0.7f);
        }
    }

    // Window size changed, adjust view appropriately
    if (event.is<sf::Event::Resized>())
    {
        sf::View view;
        view.setSize({gameWidth, gameHeight});
        view.setCenter({gameWidth / 2.f, gameHeight / 2.f});
        window.setView(view);
    }
}

//
//
// WITH VISITATION
auto playAction = [&]
{
    if (!isPlaying)
    {
        // (re)start the game
        isPlaying = true;
        clock.restart();

        // Reset the position of the paddles and ball
        leftPaddle.setPosition({10.f + paddleSize.x / 2.f, gameHeight / 2.f});
        rightPaddle.setPosition({gameWidth - 10.f - paddleSize.x / 2.f, gameHeight / 2.f});
        ball.setPosition({gameWidth / 2.f, gameHeight / 2.f});

        // Reset the ball angle
        do
        {
            // Make sure the ball initial angle is not too much vertical
            ballAngle = sf::degrees(std::uniform_real_distribution<float>(0, 360)(rng));
        } while (std::abs(std::cos(ballAngle.asRadians())) < 0.7f);
    }
}; 

pollEvents(window, 
    [&](sf::Event::Closed)
    { 
        window.close(); 
    },
    [&](sf::Event::KeyPressed e)
    {
        if (e.code == sf::Keyboard::Key::Escape)
            window.close();
        else if (e.code == sf::Keyboard::Key::Space)
            playAction();
    },
    [&](sf::Event::TouchBegan)
    {
        playAction();
    }
    [&](sf::Event::Resized)
    {
        sf::View view;
        view.setSize({gameWidth, gameHeight});
        view.setCenter({gameWidth / 2.f, gameHeight / 2.f});
        window.setView(view);
    }
);

I kinda like it. Thoughts?

Observations:

• pollEvents makes a sf::Event::visit much more compelling.
• Yes, every lambda needs to capture, but [&] is safe in this context.
• Repeated actions need to be extracted such as for playAction. Not a big deal, might actually promote good coding practices.
• All the "empty state" or "optional has_value" BS disappears.

Use Cases

N/A

API Example

No response