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Bubble Tea Basics

Bubble Tea is based on the functional design paradigms of The Elm Architecture. It might not seem very Go-like at first, but once you get used to the general structure you'll find that most of the idomatic Go things you know and love are still relevant and useful here.

By the way, the non-annotated source code for this program is available on GitHub.

This tutorial assumes you have a working knowledge of Go.

Enough! Let's get to it.

For this tutorial we're making a to-do list.

To start we'll define our package and import some libraries. Our only external import will be the Bubble Tea library, which we'll call tea for short.

    package main

    import (
        "fmt"
        "os"

        tea "github.com/charmbracelet/bubbletea"
    )

Bubble Tea programs are comprised of a model that describes the application state and three simple functions that are centered around that model:

Initialize, a function that returns the model's initial state.
Update, a function that handles incoming events and updates the model accordingly.
View, a function that renders the UI based on the data in the model.

The Model

So let's start by defining our model which will store our application's state. It can be any type, but a struct usually makes the most sense.

    type model struct {
        choices  []string           // items on the to-do list
        cursor   int                // which to-do list item our cursor is pointing at
        selected map[int]struct{}   // which to-do items are selected
    }

The Initialization Function

Next we'll define a function that will initialize our application. An initialize function returns a model representing our application's initial state, as well as a Cmd that could perform some initial I/O. For now, we don't need to do any I/O, so for the command we'll just return nil, which translates to "no command."

    func initialize() (tea.Model, tea.Cmd) {
        m := model{

            // Our to-do list is just a grocery list
            choices:  []string{"Buy carrots", "Buy celery", "Buy kohlrabi"},

            // A map which indicates which choices are selected. We're using
            // the  map like a mathematical set. The keys refer to the indexes
            // of the `choices` slice, above.
            selected: make(map[int]struct{}),
        }

        // Return the model and `nil`, which means "no I/O right now, please."
        return m, nil
    }

The Update Function

Next we'll define the update function. The update function is called when "things happen." Its job is to look at what has happened and return an updated model in response to whatever happened. It can also return a Cmd and make more things happen, but for now don't worry about that part.

In our case, when a user presses the down arrow, update's job is to notice that the down arrow was pressed and move the cursor accordingly (or not).

The "something happened" comes in the form of a Msg, which can be any type. Messages are the result of some I/O that took place, such as a keypress, timer tick, or a response from a server.

We usually figure out which type of Msg we received with a type switch, but you could also use a type assertion.

For now, we'll just deal with tea.KeyMsg messages, which are automatically sent to the update function when keys are pressed.

    func update(msg tea.Msg, mdl tea.Model) (tea.Model, tea.Cmd) {
        m, _ := mdl.(model)

        switch msg := msg.(type) {

        // Is it a key press?
        case tea.KeyMsg:

            // Cool, what was the actual key pressed?
            switch msg.String() {

            // These keys should exit the program.
            case "ctrl+c", "q":
                return m, tea.Quit

            // The "up" and "k" keys move the cursor up
            case "up", "k":
                if m.cursor > 0 {
                    m.cursor--
                }

            // The "down" and "j" keys move the cursor down
            case "down", "j":
                if m.cursor < len(m.choices)-1 {
                    m.cursor++
                }

            // The "enter" key and the spacebar (a literal space) toggle
            // the selected state for the item that the cursor is pointing at.
            case "enter", " ":
                _, ok := m.selected[m.cursor]
                if ok {
                    delete(m.selected, m.cursor)
                } else {
                    m.selected[m.cursor] = struct{}{}
                }
            }
        }

        // Return the updated model to the Bubble Tea runtime for processing.
        // Note that we're not returning a command.
        return m, nil
    }

You may have noticed that "ctrl+c" and "q" above return a tea.Quit command with the model. That's a special command which instructs the Bubble Tea runtime to quit, exiting the program.

The View Function

At last, it's time to render our UI. Of all the functions, the view is the simplest. A model, in it's current state, comes in and a string comes out. That string is our UI!

Because the view describes the entire UI of your application, you don't have to worry about redraw logic and stuff like that. Bubble Tea takes care of it for you.

    func view(mdl tea.Model) string {
        m, _ := mdl.(model)

        // The header
        s := "What should we buy at the market?\n\n"

        // Iterate over our choices
        for i, choice := range m.choices {

            // Is the cursor pointing at this choice?
            cursor := " " // no cursor
            if m.cursor == i {
                cursor = ">" // cursor!
            }

            // Is this choice selected?
            checked := " " // not selected
            if _, ok := m.selected[i]; ok {
                checked = "x" // selected!
            }

            // Render the row
            s += fmt.Sprintf("%s [%s] %s\n", cursor, checked, choice)
        }

        // The footer
        s += "\nPress q to quit.\n"

        // Send the UI for rendering
        return s
    }

All Together Now

The last step is to simply run our program. We pass our functions to tea.NewProgram and let it rip:

    func main() {
        p := tea.NewProgram(initialize, update, view)
        if err := p.Start(); err != nil {
            fmt.Printf("Alas, there's been an error: %v", err)
            os.Exit(1)
        }
    }

What's Next?

This tutorial covers the basics of building an interactive terminal UI, but in the real world you'll also need to perform I/O. To learn about that have a look at the Command Tutorial. It's pretty simple.

There are also several Bubble Tea examples available and, of course, there are Go Docs.

Bubble Tea in the Wild

For some Bubble Tea programs in production, see:

Glow: a markdown reader, browser and online markdown stash
The Charm Tool: the Charm user account manager

Libraries we use with Bubble Tea

Bubbles various Bubble Tea components
Termenv: Advanced ANSI styling for terminal applications
Reflow: ANSI-aware methods for reflowing blocks of text
go-runewidth: Measure the physical width of strings in terms of terminal cells. Many runes, such as East Asian charcters and emojis, are two cells wide, so measuring a layout with len() often won't cut it.

Feedback

We'd love to hear your thoughts on this tutorial. Feel free to drop us a note!

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