Config.rs

At the moment, our keys are hard coded into the app.

impl Component for Home {

  fn handle_key_events(&mut self, key: KeyEvent) -> Action {
    match self.mode {
      Mode::Normal | Mode::Processing => {
        match key.code {
          KeyCode::Char('q') => Action::Quit,
          KeyCode::Char('d') if key.modifiers.contains(KeyModifiers::CONTROL) => Action::Quit,
          KeyCode::Char('c') if key.modifiers.contains(KeyModifiers::CONTROL) => Action::Quit,
          KeyCode::Char('z') if key.modifiers.contains(KeyModifiers::CONTROL) => Action::Suspend,
          KeyCode::Char('?') => Action::ToggleShowHelp,
          KeyCode::Char('j') => Action::ScheduleIncrement,
          KeyCode::Char('k') => Action::ScheduleDecrement,
          KeyCode::Char('/') => Action::EnterInsert,
          _ => Action::Tick,
        }
      },
      Mode::Insert => {
        match key.code {
          KeyCode::Esc => Action::EnterNormal,
          KeyCode::Enter => Action::EnterNormal,
          _ => {
            self.input.handle_event(&crossterm::event::Event::Key(key));
            Action::Update
          },
        }
      },
    }
  }

If a user wants to press Up and Down arrow key to ScheduleIncrement and ScheduleDecrement, the only way for them to do it is having to make changes to the source code and recompile the app. It would be better to provide a way for users to set up a configuration file that maps key presses to actions.

For example, assume we want a user to be able to set up a keyevents-to-actions mapping in a config.toml file like below:

[keymap]
"q" = "Quit"
"j" = "ScheduleIncrement"
"k" = "ScheduleDecrement"
"l" = "ToggleShowHelp"
"/" = "EnterInsert"
"ESC" = "EnterNormal"
"Enter" = "EnterNormal"
"Ctrl-d" = "Quit"
"Ctrl-c" = "Quit"
"Ctrl-z" = "Suspend"

We can set up a Config struct using the excellent config crate:

use std::collections::HashMap;

use color_eyre::eyre::Result;
use crossterm::event::KeyEvent;
use serde_derive::Deserialize;

use crate::action::Action;

#[derive(Clone, Debug, Default, Deserialize)]
pub struct Config {
  #[serde(default)]
  pub keymap: KeyMap,
}

#[derive(Clone, Debug, Default, Deserialize)]
pub struct KeyMap(pub HashMap<KeyEvent, Action>);

impl Config {
  pub fn new() -> Result<Self, config::ConfigError> {
    let mut builder = config::Config::builder();
    builder = builder
      .add_source(config::File::from(config_dir.join("config.toml")).format(config::FileFormat::Toml).required(false));
    builder.build()?.try_deserialize()
  }
}

We are using serde to deserialize from a TOML file.

Now the default KeyEvent serialized format is not very user friendly, so let’s implement our own version:

#[derive(Clone, Debug, Default)]
pub struct KeyMap(pub HashMap<KeyEvent, Action>);

impl<'de> Deserialize<'de> for KeyMap {
  fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: Deserializer<'de>,
  {
    struct KeyMapVisitor;
    impl<'de> Visitor<'de> for KeyMapVisitor {
      type Value = KeyMap;
      fn visit_map<M>(self, mut access: M) -> Result<KeyMap, M::Error>
      where
        M: MapAccess<'de>,
      {
        let mut keymap = HashMap::new();
        while let Some((key_str, action)) = access.next_entry::<String, Action>()? {
          let key_event = parse_key_event(&key_str).map_err(de::Error::custom)?;
          keymap.insert(key_event, action);
        }
        Ok(KeyMap(keymap))
      }
    }
    deserializer.deserialize_map(KeyMapVisitor)
  }
}

Now all we need to do is implement a parse_key_event function. You can check the source code for an example of this implementation.

With that implementation complete, we can add a HashMap to store a map of KeyEvents and Action in the Home component:

#[derive(Default)]
pub struct Home {
  ...
  pub keymap: HashMap<KeyEvent, Action>,
}

Now we have to create an instance of Config and pass the keymap to Home:

impl App {
  pub fn new(tick_rate: (u64, u64)) -> Result<Self> {
    let h = Home::new();
    let config = Config::new()?;
    let h = h.keymap(config.keymap.0.clone());
    let home = Arc::new(Mutex::new(h));
    Ok(Self { tick_rate, home, should_quit: false, should_suspend: false, config })
  }
}

And in the handle_key_events we get the Action that should to be performed from the HashMap directly.

impl Component for Home {
  fn handle_key_events(&mut self, key: KeyEvent) -> Action {
    match self.mode {
      Mode::Normal | Mode::Processing => {
        if let Some(action) = self.keymap.get(&key) {
          *action
        } else {
          Action::Tick
        }
      },
      Mode::Insert => {
        match key.code {
          KeyCode::Esc => Action::EnterNormal,
          KeyCode::Enter => Action::EnterNormal,
          _ => {
            self.input.handle_event(&crossterm::event::Event::Key(key));
            Action::Update
          },
        }
      },
    }
  }
}

In the template, it is set up to handle Vec<KeyEvent> mapped to an Action. This allows you to map for example:

<g><j> to Action::GotoBottom
<g><k> to Action::GotoTop

And because we are now using multiple keys as input, you have to update the app.rs main loop accordingly to handle that:

    // -- snip --
    loop {
      if let Some(e) = tui.next().await {
        match e {
          // -- snip --
          tui::Event::Key(key) => {
            if let Some(keymap) = self.config.keybindings.get(&self.mode) {
              // If the key is a single key action
              if let Some(action) = keymap.get(&vec![key.clone()]) {
                log::info!("Got action: {action:?}");
                action_tx.send(action.clone())?;
              } else {
                // If the key was not handled as a single key action,
                // then consider it for multi-key combinations.
                self.last_tick_key_events.push(key);

                // Check for multi-key combinations
                if let Some(action) = keymap.get(&self.last_tick_key_events) {
                  log::info!("Got action: {action:?}");
                  action_tx.send(action.clone())?;
                }
              }
            };
          },
          _ => {},
        }
        // -- snip --
      }
      while let Ok(action) = action_rx.try_recv() {
        // -- snip --
        for component in self.components.iter_mut() {
          if let Some(action) = component.update(action.clone())? {
            action_tx.send(action)?
          };
        }
      }
      // -- snip --
    }
    // -- snip --

Here’s the JSON configuration we use for the counter application:

{
  "keybindings": {
    "Home": {
      "<q>": "Quit", // Quit the application
      "<j>": "ScheduleIncrement",
      "<k>": "ScheduleDecrement",
      "<l>": "ToggleShowHelp",
      "</>": "EnterInsert",
      "<Ctrl-d>": "Quit", // Another way to quit
      "<Ctrl-c>": "Quit", // Yet another way to quit
      "<Ctrl-z>": "Suspend", // Suspend the application
    },
  },
}