Function keyboard_input_system 

pub fn keyboard_input_system(
    keycode_input: ResMut<'_, ButtonInput<KeyCode>>,
    key_input: ResMut<'_, ButtonInput<Key>>,
    keyboard_input_reader: MessageReader<'_, '_, KeyboardInput>,
    keyboard_focus_lost_reader: MessageReader<'_, '_, KeyboardFocusLost>,
)

Updates the ButtonInput<KeyCode> and ButtonInput<Key> resources with the latest KeyboardInput events.

Differences

The main difference between the KeyboardInput event and the ButtonInput resources are that the latter has convenient functions such as ButtonInput::pressed, ButtonInput::just_pressed and ButtonInput::just_released and is window id agnostic.

There is a ButtonInput for both KeyCode and Key as they are both useful in different situations, see their documentation for the details.