FAQ

Why am I getting duplicate key events on Windows?

A lot of examples out there in the wild might use the following code for sending key presses:

  CrosstermEvent::Key(e) => tx.send(Event::Key(e)),

However, on Windows, when using Crossterm, this will send the same Event::Key(e) twice; one for when you press the key, i.e. KeyEventKind::Press and one for when you release the key, i.e. KeyEventKind::Release. On MacOS and Linux only KeyEventKind::Press kinds of key event is generated.

To make the code work as expected across all platforms, you can do this instead:

  CrosstermEvent::Key(key) => {
    if key.kind == KeyEventKind::Press {
      event_tx.send(Event::Key(key)).unwrap();
    }
  },

When should I use tokio and async/await?

ratatui isn’t a native async library. So is it beneficial to use tokio or async/await?

As a user of rataui, there really is only one point of interface with the ratatui library and that’s the terminal.draw(|f| ui(f)) functionality (the creation of widgets provided by ratatui typically happens in ui(f)). Everything else in your code is your own to do as you wish.

Should terminal.draw(|f| ui(f)) be async? Possibly. Rendering to the terminal buffer is relatively fast, especially using the double buffer technique that only renders diffs that ratatui uses. Creating of the widgets can also be done quite efficiently.

So one question you may ask is can we make terminal.draw(|f| ui(f)) async ourselves? Yes, we can. Check out https://github.com/ratatui/async-template/tree/v0.1.0 for an example.

The only other part related to ratatui that is beneficial to being async is reading the key event inputs from stdin, and that can be made async with crossterm’s event-stream.

So the real question is what other parts of your app require async or benefit from being async? If the answer is not much, maybe it is simpler to not use async and avoiding tokio.

Another way to think about it is, do you think your app would work better with 1 thread like this?

Note

Even with the above architecture, you can use tokio to spawn tasks during Update State, and follow up on the work done by those tasks in the next iteration.

Or would it work with 3 threads / tokio tasks like this:

In your main thread or tokio task, do you expect to be spawning more tokio tasks? How many more tasks do you plan to be spawning?

The former can be done without any async code and the latter is the approach showcased in async-template#v1.0 with tokio.

The simple-async uses this architecture instead with tokio:

tui.rs history

This project was forked from tui-rs in February 2023, with the blessing of the original author, Florian Dehau (@fdehau).

The original repository contains all the issues, PRs and discussion that were raised originally, and it is useful to refer to when contributing code, documentation, or issues with Ratatui.

We imported all the PRs from the original repository and implemented many of the smaller ones and made notes on the leftovers. These are marked as draft PRs and labelled as imported from tui. We have documented the current state of those PRs, and anyone is welcome to pick them up and continue the work on them.

We have not imported all issues opened on the previous repository. For that reason, anyone wanting to work on or discuss an issue will have to follow the following workflow:

• Recreate the issue
• Start by referencing the original issue: Referencing issue #[<issue number>](<original issue link>)
• Then, paste the original issues opening text

You can then resume the conversation by replying to the new issue you have created.

What is the difference between a library and a framework?

The terms library and framework are often used interchangeably in software development, but they serve different purposes and have distinct characteristics.

	Library	Framework
Usage	A library is a collection of functions and procedures that a programmer can call in their application. The library provides specific functionality, but it’s the developer’s responsibility to explicitly call and use it.	A framework is a pre-built structure or scaffold that developers build their application within. It provides a foundation, enforcing a particular way of creating an application.
Control Flow	In the case of a library, the control flow remains with the developer’s application. The developer chooses when and where to use the library.	With a framework, the control flow is inverted. The framework decides the flow of control by providing places for the developer to plug in their own logic (often referred to as “Inversion of Control” or IoC).
Nature	Libraries are passive in nature. They wait for the application’s code to invoke their methods.	Frameworks are active and have a predefined flow of their own. The developer fills in specific pieces of the framework with their own code.
Example	Imagine you’re building a house. A library would be like a toolbox with tools (functions) that you can use at will. You decide when and where to use each tool.	Using the house-building analogy, a framework would be like a prefabricated house where the main structure is already built. You’re tasked with filling in the interiors and decor, but you have to follow the design and architecture already provided by the prefabricated design.

What is the difference between ratatui (a library) and tui-realm (a framework)?

While ratatui provides tools (widgets) for building terminal UIs, it doesn’t dictate or enforce a specific way to structure your application. You need to decide how to best use the library in your particular context, giving you more flexibility.

In contrast, tui-realm might provide more guidelines and enforcements about how your application should be structured or how data flows through it. And, for the price of that freedom, you get more features out of the box with tui-realm and potentially lesser code in your application to do the same thing that you would with ratatui.

What is the difference between ratatui and cursive?

Cursive and Ratatui are both libraries that make TUIs easier to write. Both libraries are great! Both also work on linux, macOS and windows.

Cursive

Cursive uses a more declarative UI: the user defines the layout, then cursive handles the event loop. Cursive also handles most input (including mouse clicks), and forwards events to the currently focused view. User-code is more focused on “events” than on keyboard input. Cursive also supports different backends like ncurses, pancurses, termion, and crossterm.

One of cursive’s main features is its built-in event loop. You can easily attach callbacks to events like clicks or key presses, making it straightforward to handle user interactions.

use cursive::views::{Dialog, TextView};

fn main() {
    // Creates the cursive root - required for every application.
    let mut siv = cursive::default();

    // Creates a dialog with a single "Quit" button
    siv.add_layer(Dialog::around(TextView::new("Hello World!"))
                         .title("Cursive")
                         .button("Quit", |s| s.quit()));

    // Starts the event loop.
    siv.run();
}

Ratatui

In Ratatui, the user handles the event loop, the application state, and re-draws the entire UI on each iteration. It does not handle input and users have use another library (like crossterm). Ratatui supports Crossterm, termion, wezterm as backends.

use ratatui::{prelude::*, widgets::*};

fn init() -> Result<(), Box<dyn std::error::Error>> {
  crossterm::terminal::enable_raw_mode()?;
  crossterm::execute!(std::io::stderr(), crossterm::terminal::EnterAlternateScreen)?;
  Ok(())
}

fn exit() -> Result<(), Box<dyn std::error::Error>> {
  crossterm::execute!(std::io::stderr(), crossterm::terminal::LeaveAlternateScreen)?;
  crossterm::terminal::disable_raw_mode()?;
  Ok(())
}

fn centered_rect(r: Rect, percent_x: u16, percent_y: u16) -> Rect {
  let popup_layout = Layout::default()
    .direction(Direction::Vertical)
    .constraints([
      Constraint::Percentage((100 - percent_y) / 2),
      Constraint::Percentage(percent_y),
      Constraint::Percentage((100 - percent_y) / 2),
    ])
    .split(r);

  Layout::default()
    .direction(Direction::Horizontal)
    .constraints([
      Constraint::Percentage((100 - percent_x) / 2),
      Constraint::Percentage(percent_x),
      Constraint::Percentage((100 - percent_x) / 2),
    ])
    .split(popup_layout[1])[1]
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
  init()?;

  let mut terminal = Terminal::new(CrosstermBackend::new(std::io::stderr()))?;

  loop {
    terminal.draw(|f| {
      let rect = centered_rect(f.size(), 35, 35);
      f.render_widget(
        Paragraph::new("Hello World!\n\n\n'q' to quit")
          .block(
            Block::default().title(block::Title::from("Ratatui").alignment(Alignment::Center)).borders(Borders::all()),
          )
          .alignment(Alignment::Center),
        rect,
      );
    })?;

    if crossterm::event::poll(std::time::Duration::from_millis(250))? {
      if let crossterm::event::Event::Key(key) = crossterm::event::read()? {
        if key.code == crossterm::event::KeyCode::Char('q') {
          break;
        }
      }
    }
  }
  exit()?;

  Ok(())
}

You may have to write more code but you get precise control over exact UI you want to display with Ratatui.

Can you change font size in a terminal using ratatui?

ratatui itself doesn’t control the terminal’s font size. ratatui renders content based on the size and capabilities of the terminal it’s running in. If you want to change the font size, you’ll need to adjust the settings of your terminal emulator.

However, changing this setting in your terminal emulator will only change the font size for you while you are developing your ratatui based application.

When a user zooms in and out using terminal shortcuts, that will typically change the font size in their terminal. You typically will not know what the terminal font size is ahead of time.

However, you can know the current terminal size (i.e. columns and rows). Additionally, when zooming in and out ratatui applications will see a terminal resize event that will contain the new columns and rows. You should ensure your ratatui application can handle these changes gracefully.

You can detect changes in the terminal’s size by listening for terminal resize events from the backend of your choice and you can adjust your application layout as needed.

For example, here’s how you might do it in crossterm:

    match crossterm::terminal::read() {
        Ok(evt) => {
            match evt {
                crossterm::event::Event::Resize(x, y) => {
                    // handle resize event here
                },
                _ => {}
            }
        }
    }

Tip

Since this can happen on the user end without your control, this means that you’ll have to design your ratatui based terminal user interface application to display content well in a number of different terminal sizes.

ratatui does support various styles, including bold, italic, underline, and more, and while this doesn’t change the font size, it does provide you with the capability to emphasize or de-emphasize text content in your application.

Additionally you can use figlet or tui-big-text to display text content across multiple lines. Here’s an example using tui-big-text:

Some characters appear to be missing / look weird

ratatui, and TUIs in general, use special drawing characters like box-drawing characters, braille characters or even icons. If your font doesn’t support such features, it may display a white square (□) or a replacement character (�).

To fix this, we recommend you use a nerd font, this is generally what works the best. Kreative Square is also a good alternative. Though note that some characters may render a bit differently from font to font.

In addition, some terminals such as Alacritty (see builtin_box_drawing) or ITerm2 can use a special, separate font to render box drawing characters correctly.

Can you use multiple terminal.draw() calls consequently?

You cannot use terminal.draw() multiple times in the same main loop.

Because Ratatui uses a double buffer rendering technique, writing code like this will NOT render all three widgets:

  loop {
    terminal.draw(|f| {
      f.render_widget(widget1, f.size());
    })?;
    terminal.draw(|f| {
      f.render_widget(widget2, f.size());
    })?;
    terminal.draw(|f| {
      f.render_widget(widget3, f.size());
    })?;
    // handle events
    // manage state
  }

You want to write the code like this instead:

  loop {
    terminal.draw(|f| {
      f.render_widget(widget1, f.size());
      f.render_widget(widget2, f.size());
      f.render_widget(widget3, f.size());
    })?;
    // handle events
    // manage state
  }

Should I use stdout or stderr?

When using crossterm, application developers have the option of rendering to stdout or stderr.

let mut t = Terminal::new(CrosstermBackend::new(std::io::stdout()))?;
// OR
let mut t = Terminal::new(CrosstermBackend::new(std::io::stderr()))?;

Both of these will work fine for normal purposes. The question you have to ask is how would you like your application to behave in non-TTY environments.

For example, if you run ratatui-application | grep foo with stdout, your application won’t render anything to the screen and there would be no indication of anything going wrong. With stderr the application will still render a TUI.

With stdout:

• Every app needs to add code to check if the output is a TTY and do something different based on the result
• App can’t write a result to the user that can be passed in a pipeline, e.g. my-select-some-value-app | grep foo
• Tends to be what most command line applications do by default.

With stderr:

• No special setup necessary in order to run in a pipe command
• Unconventional and that might subvert users expectations

Out of the box, stdout will be faster than stderr because it is buffered. However you can very easily make stderr buffered too by wrapping it in a BufWriter like so:

let mut terminal = Terminal::new(CrosstermBackend::new(BufWriter::new(std::io::stderr())))?;

Our recommendation is to use stdout. If you really need stderr, then accept the performance loss (which is unnoticeable in most applications) or make it buffered.

If you want to know more, we recommend reading this excellent article by @orhun.

How do I avoid panics due to out of range calls on the Buffer?

In general, most of the code in Ratatui was not designed around using Results to prevent panics. We have an Issue to address the panics, but for now there are some easy approaches that can help avoid them.

A one liner that will mostly fix this for pretty much any widget is to avoid rendering stuff outside of the buffer is to reassign the area to the intersection of the area and the buffer’s area. If you are calling a widget that has an out of bounds problem, you can call this code just before calling a that widget.

fn render_ref(&self, area: Rect, buf: &mut Buffer) {
    let area = area.intersection(buf.area);

    // -- snip --
}

This breaks down on edge cases where you want to handle this inside the implementation logic, but that should be fairly rare (I don’t know of any cases where widgets would need this edge case).

Another general rule to follow if you’re calculating layouts, is to make sure the math can’t generate locations outside of the buffer. Any manual calculations on coordinates should be suspect. The code in Layout is programmed to always lie within the input constraints, but we know that there’s a few places in the Ratatui code bases where this isn’t quite as robust. (Please report these as bugs if you do encounter them!).

Make sure to use methods built-in to the std library (e.g. u16::min() and u16::clamp() when doing calculations on coordinates). We have also provided Rect::intersection() and Rect::clamp() methods that can help ensure similar constraints.

Additionally, for widgets that work in terms of rows or columns of text we have created iterators: Rect::columns() and Rect::rows() that help ensure that can be used instead of incremental offsets and calculations that may cause out of bounds values. It can be useful to combine these with the Iterator::zip() method to generate tuples of valid data and areas. E.g. this code in Text::render():

for (line, row) in self.iter().zip(area.rows()) {
    // -- snip
}