docs: add another progress bar example

examples/progress-download/README.md

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+# Download Progress
+This example was built based on [this](https://github.com/charmbracelet/bubbles/discussions/127) discussion.
+This example demonstrates how to download a file from a given URL, and show its progress with a [progress Bubble](https://github.com/charmbracelet/bubbles/).
+The status of the download is updated with [`io.TeeReader`](https://pkg.go.dev/io#TeeReader).
+This calls `Write` which is where we send the updated status with `Program#Send()` 
+
+## How to Run
+`go build .` in this directory on your machine (in examples/download-progress)
+then run `./download-progress --url="https://download.blender.org/demo/color_vortex.blend"` this can be whatever file you'd like to download. 
+Note: the current version will not show a TUI for downloads that do not provide the ContentLength header field.
+

examples/progress-download/main.go

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+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"github.com/charmbracelet/bubbles/progress"
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+var p *tea.Program
+
+type progressWriter struct {
+	total      int
+	downloaded int
+	file       *os.File
+	reader     io.Reader
+	onProgress func(float64)
+}
+
+func (pw *progressWriter) Start() {
+	// TeeReader calls pw.Write() each time a new response is received
+	_, err := io.Copy(pw.file, io.TeeReader(pw.reader, pw))
+	if err != nil {
+		if p != nil {
+			p.Send(progressErrMsg{err})
+		}
+	}
+}
+
+func (pw *progressWriter) Write(p []byte) (int, error) {
+	pw.downloaded += len(p)
+	if pw.total > 0 && pw.onProgress != nil {
+		pw.onProgress(float64(pw.downloaded) / float64(pw.total))
+	}
+	return len(p), nil
+}
+
+func getResponse(url string) (*http.Response, error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("receiving status of %d for url: %s", resp.StatusCode, url)
+	}
+	return resp, nil
+}
+
+func main() {
+	url := flag.String("url", "", "url for the file to download")
+	flag.Parse()
+
+	if *url == "" {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	resp, err := getResponse(*url)
+	if err != nil {
+		fmt.Println("could not get response", err)
+		os.Exit(1)
+	}
+	defer resp.Body.Close()
+
+	filename := filepath.Base(*url)
+	file, err := os.Create(filename)
+	if err != nil {
+		fmt.Println("could not create file: ", err)
+		os.Exit(1)
+	}
+	defer file.Close()
+
+	pw := &progressWriter{
+		total:  int(resp.ContentLength),
+		file:   file,
+		reader: resp.Body,
+		onProgress: func(ratio float64) {
+			if p != nil {
+				p.Send(progressMsg(ratio))
+			}
+		},
+	}
+
+	m := model{
+		pw:       pw,
+		progress: progress.New(progress.WithDefaultGradient()),
+	}
+
+	// Start the download
+	go pw.Start()
+
+	// Don't add TUI if the header doesn't include content size
+	// it's impossible see progress without total
+	if resp.ContentLength > 0 {
+		// Start Bubble Tea
+		p = tea.NewProgram(m)
+		if err := p.Start(); err != nil {
+			fmt.Println("error running program:", err)
+			os.Exit(1)
+		}
+	}
+}

examples/progress-download/tui.go

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+package main
+
+import (
+	"strings"
+	"time"
+
+	"github.com/charmbracelet/bubbles/progress"
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
+)
+
+var helpStyle = lipgloss.NewStyle().Foreground(lipgloss.Color("#626262")).Render
+
+const (
+	padding  = 2
+	maxWidth = 80
+)
+
+type progressMsg float64
+
+type progressErrMsg struct{ err error }
+
+func finalPause() tea.Cmd {
+	return tea.Tick(time.Millisecond*750, func(_ time.Time) tea.Msg {
+		return nil
+	})
+}
+
+type model struct {
+	url, path string
+	pw        *progressWriter
+	progress  progress.Model
+	err       error
+}
+
+func (m model) Init() tea.Cmd {
+	return nil
+}
+
+func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	switch msg := msg.(type) {
+	case tea.KeyMsg:
+		return m, tea.Quit
+
+	case tea.WindowSizeMsg:
+		m.progress.Width = msg.Width - padding*2 - 4
+		if m.progress.Width > maxWidth {
+			m.progress.Width = maxWidth
+		}
+		return m, nil
+
+	case progressErrMsg:
+		m.err = msg.err
+		return m, tea.Quit
+
+	case progressMsg:
+		var cmds []tea.Cmd
+
+		if msg >= 1.0 {
+			cmds = append(cmds, tea.Sequentially(finalPause(), tea.Quit))
+		}
+
+		cmds = append(cmds, m.progress.SetPercent(float64(msg)))
+		return m, tea.Batch(cmds...)
+
+	// FrameMsg is sent when the progress bar wants to animate itself
+	case progress.FrameMsg:
+		progressModel, cmd := m.progress.Update(msg)
+		m.progress = progressModel.(progress.Model)
+		return m, cmd
+
+	default:
+		return m, nil
+	}
+}
+
+func (m model) View() string {
+	if m.err != nil {
+		return "Error downloading: " + m.err.Error() + "\n"
+	}
+
+	pad := strings.Repeat(" ", padding)
+	return "\n" +
+		pad + m.progress.View() + "\n\n" +
+		pad + helpStyle("Press any key to quit")
+}