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Events explained

Introduction

This tutorial is a detailed list of window events. It describes them, and shows how to (and how not to) use them.

The sf::Event type

Before dealing with events, it is important to understand what the sf::Event type is, and how to correctly use it. sf::Event is a union, which means that only one of its members is valid at a time (remember your C++ lesson: all the members of a union share the same memory space). The valid member is the one that matches the event type, for example event.key for a KeyPressed event. Trying to read any other member will result in an undefined behaviour (most likely: random or invalid values). It is important to never try to use an event member that doesn't match its type.

sf::Event instances are filled by the pollEvent (or waitEvent) function of the sf::Window class. Only these two functions can produce valid events, any attempt to use an sf::Event which was not returned by successful call to pollEvent (or waitEvent) will result in the same undefined behaviour that was mentioned above.

To be clear, here is what a typical event loop looks like:

sf::Event event;

// while there are pending events...
while (window.pollEvent(event))
{
    // check the type of the event...
    switch (event.type)
    {
        // window closed
        case sf::Event::Closed:
            window.close();
            break;

        // key pressed
        case sf::Event::KeyPressed:
            ...
            break;

        // we don't process other types of events
        default:
            break;
    }
}

Read the above paragraph once again and make sure that you fully understand it, the sf::Event union is the cause of many problems for inexperienced programmers.

Alright, now we can see what events SFML supports, what they mean and how to use them properly.

The Closed event

The sf::Event::Closed event is triggered when the user wants to close the window, through any of the possible methods the window manager provides ("close" button, keyboard shortcut, etc.). This event only represents a close request, the window is not yet closed when the event is received.

Typical code will just call window.close() in reaction to this event, to actually close the window. However, you may also want to do something else first, like saving the current application state or asking the user what to do. If you don't do anything, the window remains open.

There's no member associated with this event in the sf::Event union.

if (event.type == sf::Event::Closed)
    window.close();

The Resized event

The sf::Event::Resized event is triggered when the window is resized, either through user action or programmatically by calling window.setSize.

You can use this event to adjust the rendering settings: the viewport if you use OpenGL directly, or the current view if you use sfml-graphics.

The member associated with this event is event.size, it contains the new size of the window.

if (event.type == sf::Event::Resized)
{
    std::cout << "new width: " << event.size.width << std::endl;
    std::cout << "new height: " << event.size.height << std::endl;
}

The LostFocus and GainedFocus events

The sf::Event::LostFocus and sf::Event::GainedFocus events are triggered when the window loses/gains focus, which happens when the user switches the currently active window. When the window is out of focus, it doesn't receive keyboard events.

This event can be used e.g. if you want to pause your game when the window is inactive.

There's no member associated with these events in the sf::Event union.

if (event.type == sf::Event::LostFocus)
    myGame.pause();

if (event.type == sf::Event::GainedFocus)
    myGame.resume();

The TextEntered event

The sf::Event::TextEntered event is triggered when a character is typed. This must not be confused with the KeyPressed event: TextEntered interprets the user input and produces the appropriate printable character. For example, pressing '^' then 'e' on a French keyboard will produce two KeyPressed events, but a single TextEntered event containing the 'ê' character. It works with all the input methods provided by the operating system, even the most specific or complex ones.

This event is typically used to catch user input in a text field.

The member associated with this event is event.text, it contains the Unicode value of the entered character. You can either put it directly in a sf::String, or cast it to a char after making sure that it is in the ASCII range (0 - 127).

if (event.type == sf::Event::TextEntered)
{
    if (event.text.unicode < 128)
        std::cout << "ASCII character typed: " << static_cast<char>(event.text.unicode) << std::endl;
}

Note that, since they are part of the Unicode standard, some non-printable characters such as backspace are generated by this event. In most cases you'll need to filter them out.

Many programmers use the KeyPressed event to get user input, and start to implement crazy algorithms that try to interpret all the possible key combinations to produce correct characters. Don't do that!

The KeyPressed and KeyReleased events

The sf::Event::KeyPressed and sf::Event::KeyReleased events are triggered when a keyboard key is pressed/released.

If a key is held, multiple KeyPressed events will be generated, at the default operating system delay (ie. the same delay that applies when you hold a letter in a text editor). To disable repeated KeyPressed events, you can call window.setKeyRepeatEnabled(false). On the flip side, it is obvious that KeyReleased events can never be repeated.

This event is the one to use if you want to trigger an action exactly once when a key is pressed or released, like making a character jump with space, or exiting something with escape.

Sometimes, people try to react to KeyPressed events directly to implement smooth movement. Doing so will not produce the expected effect, because when you hold a key you only get a few events (remember, the repeat delay). To achieve smooth movement with events, you must use a boolean that you set on KeyPressed and clear on KeyReleased; you can then move (independently of events) as long as the boolean is set.
The other (easier) solution to produce smooth movement is to use real-time keyboard input with sf::Keyboard (see the dedicated tutorial).

The member associated with these events is event.key, it contains the code of the pressed/released key, as well as the current state of the modifier keys (alt, control, shift, system).

if (event.type == sf::Event::KeyPressed)
{
    if (event.key.code == sf::Keyboard::Escape)
    {
        std::cout << "the escape key was pressed" << std::endl;
        std::cout << "control:" << event.key.control << std::endl;
        std::cout << "alt:" << event.key.alt << std::endl;
        std::cout << "shift:" << event.key.shift << std::endl;
        std::cout << "system:" << event.key.system << std::endl;
    }
}

Note that some keys have a special meaning for the operating system, and will lead to unexpected behavior. An example is the F10 key on Windows, which "steals" the focus, or the F12 key which starts the debugger when using Visual Studio. This will probably be solved in a future version of SFML.

The MouseWheelMoved event

The sf::Event::MouseWheelMoved event is triggered when the mouse wheel moves up or down.

The member associated with this event is event.mouseWheel, it contains the number of ticks the wheel has moved, as well as the current position of the mouse cursor.

if (event.type == sf::Event::MouseWheelMoved)
{
    std::cout << "wheel movement: " << event.mouseWheel.delta << std::endl;
    std::cout << "mouse x: " << event.mouseWheel.x << std::endl;
    std::cout << "mouse y: " << event.mouseWheel.y << std::endl;
}

The MouseButtonPressed and MouseButtonReleased events

The sf::Event::MouseButtonPressed and sf::Event::MouseButtonReleased events are triggered when a mouse button is pressed/released.

SFML supports 5 mouse buttons: left, right, middle (wheel), extra #1 and extra #2 (side buttons).

The member associated with these events is event.mouseButton, it contains the code of the pressed/released button, as well as the current position of the mouse cursor.

if (event.type == sf::Event::MouseButtonPressed)
{
    if (event.mouseButton.button == sf::Mouse::Right)
    {
        std::cout << "the right button was pressed" << std::endl;
        std::cout << "mouse x: " << event.mouseButton.x << std::endl;
        std::cout << "mouse y: " << event.mouseButton.y << std::endl;
    }
}

The MouseMoved event

The sf::Event::MouseMoved event is triggered when the mouse moves within the window.

This event is triggered even if the window isn't focused. However, it is triggered only when the mouse moves within the inner area of the window, not when it moves over the title bar or borders.

The member associated with this event is event.mouseMove, it contains the current position of the mouse cursor relative to the window.

if (event.type == sf::Event::MouseMoved)
{
    std::cout << "new mouse x: " << event.mouseMove.x << std::endl;
    std::cout << "new mouse y: " << event.mouseMove.y << std::endl;
}

The MouseEntered and MouseLeft event

The sf::Event::MouseEntered and sf::Event::MouseLeft events are triggered when the mouse cursor enters/leaves the window.

There's no member associated with these events in the sf::Event union.

if (event.type == sf::Event::MouseEntered)
    std::cout << "the mouse cursor has entered the window" << std::endl;

if (event.type == sf::Event::MouseLeft)
    std::cout << "the mouse cursor has left the window" << std::endl;

The JoystickButtonPressed and JoystickButtonReleased events

The sf::Event::JoystickButtonPressed and sf::Event::JoystickButtonReleased events are triggered when a joystick button is pressed/released.

SFML supports up to 8 joysticks and 32 buttons.

The member associated with these events is event.joystickButton, it contains the identifier of the joystick and the index of the pressed/released button.

if (event.type == sf::Event::JoystickButtonPressed)
{
    std::cout << "joystick button pressed!" << std::endl;
    std::cout << "joystick id: " << event.joystickButton.joystickId << std::endl;
    std::cout << "button: " << event.joystickButton.button << std::endl;
}

The JoystickMoved event

The sf::Event::JoystickMoved event is triggered when a joystick axis moves.

Joystick axes are typically very sensitive, that's why SFML uses a detection threshold to avoid spamming your event loop with tons of JoystickMoved events. This threshold can be changed with the Window::setJoystickThreshold function, in case you want to receive more or less joystick move events.

SFML supports 8 joystick axes: X, Y, Z, R, U, V, POV X and POV Y. How they map to your joystick depends on its driver.

The member associated with this event is event.joystickMove, it contains the identifier of the joystick, the name of the axis, and its current position (in the range [-100, 100]).

if (event.type == sf::Event::JoystickMoved)
{
    if (event.joystickMove.axis == sf::Joystick::X)
    {
        std::cout << "X axis moved!" << std::endl;
        std::cout << "joystick id: " << event.joystickMove.joystickId << std::endl;
        std::cout << "new position: " << event.joystickMove.position << std::endl;
    }
}

The JoystickConnected and JoystickDisconnected events

The sf::Event::JoystickConnected and sf::Event::JoystickDisconnected events are triggered when a joystick is connected/disconnected.

The member associated with this event is event.joystickConnect, it contains the identifier of the connected/disconnected joystick.

if (event.type == sf::Event::JoystickConnected)
    std::cout << "joystick connected: " << event.joystickConnect.joystickId << std::endl;

if (event.type == sf::Event::JoystickDisconnected)
    std::cout << "joystick disconnected: " << event.joystickConnect.joystickId << std::endl;

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Copyright © Laurent Gomila