ChunkedReader is an io.Reader which runs a callback after each read chunk

chunked_reader.go

package chunked_reader

import "io"

// ChunkedReader implements io.Reader interface while allowing the source to be
// read in set increments and running a callback function after each increment.
type ChunkedReader struct {
	readSoFar int64
	Reader    io.Reader          // The underlying io.Reader.
	ChunkSize int                // Maximum bytes to read on each Read call.
	Callback  func(int64) error  // Callback to run on each non-zero read.
}

// Read implements implements io.Reader interface. It resizes the buffer to the
// ChunkedReader's maximum chunk size and if the number of bytes read is greater
// than zero it also runs a provided callback. This method returns a non-nil
// error either if the underlying Read operation OR the callback return one.
func (c *ChunkedReader) Read(p []byte) (n int, err error) {
	if len(p) > c.ChunkSize {
		p = p[:c.ChunkSize]
	}
	n, err = c.Reader.Read(p)
	if n == 0 {
		return
	}
	c.readSoFar += int64(n)
	err = c.Callback(c.readSoFar)
	return
}