Progress Reader

progressreader.go

package main

import (
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"time"
)

type fakeReader struct {
	stopAfter time.Time
}

func newFakeReader(stopAfter time.Time) *fakeReader {
	return &fakeReader{stopAfter}
}

func (fr *fakeReader) Read(p []byte) (n int, err error) {
	if time.Now().After(fr.stopAfter) {
		return 0, io.EOF
	}
	return len(p), nil
}

func main() {
	fr := newFakeReader(time.Now().Add(10 * time.Second))
	pr := newProgressReader(fr, os.Stdout)
	io.Copy(ioutil.Discard, pr)
}

type progressReader struct {
	in    io.Reader
	out   io.Writer
	cnt   int
	cntCh chan int
	errCh chan error
}

func newProgressReader(in io.Reader, out io.Writer) *progressReader {
	pr := &progressReader{
		in:    in,
		out:   out,
		cnt:   0,
		cntCh: make(chan int, 0),
		errCh: make(chan error, 0),
	}
	go func() {
		t := time.Tick(1 * time.Second)
		for {
			select {
			case n := <-pr.cntCh:
				pr.cnt += n
			case err := <-pr.errCh:
				if errors.Is(err, io.EOF) {
					fmt.Fprintln(pr.out, "transfer finished")
				}
				return
			case <-t:
				fmt.Fprintf(pr.out, "wrote %d bytes\n", pr.cnt)
			}
		}
	}()

	return pr
}

func (pg *progressReader) Read(p []byte) (n int, err error) {
	n, err = pg.in.Read(p)
	if err != nil {
		pg.errCh <- err
	} else {
		pg.cntCh <- n
	}
	return
}