add: io composition ifaces example

interfaces/16-io-compose/.gitignore

@@ -0,0 +1,2 @@
+rosie.unknown
+rosie.png

interfaces/16-io-compose/main.go

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+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+// For more tutorials  : https://learngoprogramming.com
+// In-person training  : https://www.linkedin.com/in/inancgumus/
+// Follow me on twitter: https://twitter.com/inancgumus
+
+package main
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+)
+
+func main() {
+	resp, err := http.Get("https://inancgumus.github.com/x/rosie.unknown")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	// creates a file (or truncates if the file exists)
+	file, err := os.Create("rosie.png")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer file.Close()
+
+	n, err := transfer(resp.Body, file)
+
+	// check whether the error was due to the PNG signature.
+	if errors.Is(err, errNotPNG) {
+		log.Fatalln("Please provide a PNG image:", err)
+	}
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%d bytes transferred.\n", n)
+
+	// multiple deferred funcs are run in reverse order:
+	// file.Close()
+	// resp.Body.Close()
+}
+
+// create an error at the package-level.
+// so the other funcs in the package can check.
+var errNotPNG = errors.New("not a png image")
+
+// transfer streams data from a reader to a writer.
+// if the reader doesn't contain a PNG signature, transfer returns errNotPNG.
+// otherwise, it transfers the data to the writer.
+func transfer(r io.Reader, w io.Writer) (n int64, err error) {
+	const pngSign = "\x89PNG\r\n\x1a\n"
+	const pngSignLen = 8
+
+	var memory bytes.Buffer
+
+	// limit what copy() reads.
+	// lr := io.LimitReader(r, pngSignLen)
+	// if n, err = io.Copy(&memory, lr); err != nil {
+	// return n, err
+	// }
+
+	// same as above. behind the scenes CopyN() calls LimitReader.
+	if n, err = io.CopyN(&memory, r, pngSignLen); err != nil {
+		return n, err
+	}
+
+	// check the png signature.
+	if !bytes.HasPrefix(memory.Bytes(), []byte(pngSign)) {
+		return n, errNotPNG
+	}
+
+	// stitch the PNG signature (memory) and the response body reader together.
+	// then copy them successively to the writer (*os.File).
+	return io.Copy(w, io.MultiReader(&memory, r))
+}