docs: add another progress bar example (#270)

* docs: add another progress bar example

* chore: copy edits

Co-authored-by: Christian Rocha <christian@rocha.is>

examples/progress-download/main.go

@@ -0,0 +1,108 @@
+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)
+		}
+	}
+}