bubbletea/tutorials/commands

Commands in Bubble Tea

This is the second tutorial for Bubble Tea covering commands, which deal with I/O. The tutorial assumes you have a working knowlege of Go and a decent understanding of the first tutorial.

You can find the non-annotated version of this program on GitHub.

Let's Go!

For this tutorial we're building a very simple program that makes an HTTP request to a server and reports the status code of the response.

We'll import a few necessary packages and put the URL we're going to check in a const.

    package main

    import (
        "fmt"
        "net/http"
        "os"
        "time"

        tea "github.com/charmbracelet/bubbletea"
    )

    const url = "https://charm.sh/"

The Model

Next we'll define our model. The only things we need to store are the status code of the HTTP response and a possible error.

    type model struct {
        status int
        err    error
    }

Commands and Messages

Cmds are functions that perform some I/O and then return a Msg. Checking the time, ticking a timer, reading from the disk, and network stuff are all I/O and should be run through commands. That might sound harsh, but it will keep your Bubble Tea program staightforward and simple.

Anyway, let's write a Cmd that makes a request to a server and returns the result as a Msg.

    func checkServer() tea.Msg {

        // Create an HTTP client and make a GET request.
        c := &http.Client{Timeout: 10 * time.Second}
        res, err := c.Get(url)

        if err != nil {
            // There was an error making our request. Wrap the error we received
            // in a message and return it.
            return errMsg(err)
        }
        // We received a response from the server. Return the HTTP status code
        // as a message.
        return statusMsg(res.StatusCode)
    }

    type statusMsg int
    type errMsg error

And notice that we've defined two new Msg types. They can be any type, even an empty struct. We'll come back to them later later in our update function. First, let's write our initialization function.

The Initialization Function

The initilization function is very simple. We return an empty model and the the Cmd we made earlier. Note that we don't call the function; the Bubble Tea runtime will do that when the time is right.

    func initialize() (tea.Model, tea.Cmd) {
        return model{}, checkServer
    }

The Update Function

Internally, Cmds run asynchronously in a goroutine. The Msg they return is collected and sent to our update function for handling. Remember those message types we made earlier when we were making the checkServer command? We handle them here. This makes dealing with many asynchronous operations very easy.

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

        switch msg := msg.(type) {

        case statusMsg:
            // The server returned a status message. Save it to our model. Also
            // tell the Bubble Tea runtime we ant to exit because we have
            // nothing else to do. Don't worry, we'll still be able to render
            // a final view with our status message.
            m.status = int(msg)
            return m, tea.Quit

        case errMsg:
            // There was an error. Note it in the model. And tell the runtime
            // we're done and want to quit.
            m.err = msg
            return m, tea.Quit

        case tea.KeyMsg:
            // Ctrl+c exits. Even with short running programs it's good to have
            // a quit key, just incase your logic is off. Users will be very
            // annoyed if they can't exit.
            if msg.Type == tea.KeyCtrlC {
                return m, tea.Quit
            }
        }

        // If we happen to get any other messages, don't do anything.
        return m, nil
    }

The View Function

Our view is very straightforward. We look at the current model and build a string accordingly:

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

        // If there's an error, print it out and don't do anything else.
        if m.err != nil {
            return fmt.Sprintf("\nWe had some trouble: %v\n\n", m.err)
        }

        // Tell the user we're doing something.
        s := fmt.Sprintf("Checking %s ... ", url)

        // When the server responds with a status, add it to the current line.
        if m.status > 0 {
            s += fmt.Sprintf("%d %s!", m.status, http.StatusText(m.status))
        }

        // Send off whatever we came up with above for rendering.
        return "\n" + s + "\n\n"
    }

Run the program

The only thing left to do is run the program, so let's do that!

    func main() {
        if err := tea.NewProgram(initialize, update, view).Start(); err != nil {
            fmt.Printf("Uh oh, there was an error: %v\n", err)
            os.Exit(1)
        }
    }

And that's that. There's one more thing you that is helpful to know about Cmds, though.

One More Thing About Commands

Cmds are defined in Bubble Tea as type Cmd func() Msg. So they're just functions that don't take any arguments and return a Msg, which can be anything. If you need to pass arguments to a command, you just make a function that returns a command. For example:

    func checkSomeUrl(url string) tea.Cmd {
        return func() tea.Msg {
            c := &http.Client{Timeout: 10 * time.Second}
            res, err := c.Get(url)
            if err != nil {
                return errMsg(err)
            }
            return statusMsg(res.StatusCode)
        }
    }

Just make sure you do as much stuff as you can in the innermost function, because that's the one that runs asynchronously.

Anyway, Now What?

After doing this tutorial and the previous one you should be ready to build a Bubble Tea program of your own. We also recommend that you look at the Bubble Tea example programs as well as Bubbles, a component library for Bubble Tea.

And, of course, check out the 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 we've built
• Termenv: Advanced ANSI styling for terminal applications
• Reflow: ANSI-aware methods for reflowing blocks of text
• go-runewidth: Get 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!

• Twitter
• The Fediverse