Handling User Input

enums and match

Yeah, it's kind of hard to close the window. This is because we have not told our program how to deal with events, so it continues to run and we have to force it to quit. That's uncomfortable, let's change that.

Go to the sdl2 documentation. Go to the event module, find the Event enum. An enum in Rust is a type that represents data that is one of several possible variants. Each variant in the enum can optionally have data associated with it. Look at the possible event variants in the declaration of Event. We want to end the program by pushing esc. What variant are we looking for?

In our game loop, we iterate over events. Depending on what kind of event is happening, we want the program to react in different ways. Pushing space means something different than pushing esc. We will match the event to the variants of the enum Event. match is a way to control the flow of the program when there are several possible options, similar to if else statements.

1. To be able to use the enum and work with keyboard input, we need to add the following dependencies:

#![allow(unused_variables)]
fn main() {
use sdl2::event::Event;
use sdl2::keyboard::Keycode;
}

2. Enter the following lines into the for loop. match compares the value of the event-variable to the variants of the enum Event. If the value of the event-variable is a pressed esc-key, the 'game-loop breaks. If the value is something else, the loop continues. The last part of a match always needs to be _ =>, the case that covers all cases that are not explicitly defined.

#![allow(unused_variables)]
fn main() {
match event {
    Event::KeyDown {
        keycode: Some(Keycode::Escape),
        ..
    } => break 'game,
    _ => continue 'game,
}
}

3. Run the program, try if the user input you implemented works!
4. To be able to close the window with a mouse click, change the first line of the match to include Event::Quit { .. }. The 'game -loop breaks now if either the esc-key is pressed, or the quit-button is clicked with the cursor.

#![allow(unused_variables)]
fn main() {
match event {
    Event::Quit {
        ..
    } | Event::KeyDown {
        keycode: Some(Keycode::Escape),
        ..
    } => break 'game,
    _ => continue 'game,
}
}