io 包中接口的优势

package main

import (
	"bytes"
	"fmt"
	"io"
	"strings"
)

func main() {
	// strings.Builder 主要用于构建字符串，实现的接口如下
	builder := new(strings.Builder)
	_ = interface{}(builder).(io.Writer)
	_ = interface{}(builder).(io.ByteWriter)
	_ = interface{}(builder).(fmt.Stringer)

	// strings.Reader 主要用于读取字符串，实现的接口如下
	reader := strings.NewReader("")
	_ = interface{}(reader).(io.Reader)
	_ = interface{}(reader).(io.ReaderAt)
	_ = interface{}(reader).(io.ByteReader)  
	_ = interface{}(reader).(io.RuneReader)
	_ = interface{}(reader).(io.Seeker)
	_ = interface{}(reader).(io.ByteScanner) // io.ByteReader 的扩展接口
	_ = interface{}(reader).(io.RuneScanner) // io.RuneReader 的扩展接口
	_ = interface{}(reader).(io.WriterTo)

	// bytes.Buffer 集读、写功能于一身，适合作为字节序列的缓冲区，实现的接口如下
	buffer := bytes.NewBuffer([]byte{})
	// 读相关
	_ = interface{}(buffer).(io.Reader)
	_ = interface{}(buffer).(io.ByteReader)
	_ = interface{}(buffer).(io.RuneReader)
	_ = interface{}(buffer).(io.ByteScanner)
	_ = interface{}(buffer).(io.RuneScanner)
	_ = interface{}(buffer).(io.WriterTo)
	// 写相关
	_ = interface{}(buffer).(io.Writer)
	_ = interface{}(buffer).(io.ByteWriter)
	_ = interface{}(buffer).(io.ReaderFrom)
	// 导出相关
	_ = interface{}(buffer).(fmt.Stringer)

	// 实现众多接口的好处：可提高不同程序实体之间的互操作性
	src := strings.NewReader(
		"CopyN copies n bytes (or until an error) from src to dst." +
		"It returns the  number of bytes copied and " +
		"the earliest error encountered while copying.")
	dst := new(strings.Builder)
	// 第一个参数只要实现 io.Writer 接口即可
	// 第二个参数只有实现 io.Reader 接口即可
	// 面向接口编程扩大了它的应用场景
	written, err := io.CopyN(dst, src, 58)
	if err != nil {
		fmt.Printf("error: %v\n", err)
	} else {
		fmt.Printf("Written(%d): %q\n", written, dst.String())
	}
}	

io.Reader 接口的实现类型

package main

import (
	"fmt"
	"io"
	"strings"
	"sync"
	"time"
)

func executeIfNoErr(err error, f func()) {
	if err != nil {
		fmt.Printf("error: %v\n", err)
		return
	}
	f()
}

func main() {
	comment := "Package io provides basic interfaces to I/O primitives. " +
		"Its primary job is to wrap existing implementations of such primitives, " +
		"such as those in package os, " +
		"into shared public interfaces that abstract the functionality, " +
		"plus some other related primitives."

	fmt.Println("New a string reader and name it \"reader1\" ...")
	reader1 := strings.NewReader(comment)
	buf1 := make([]byte, 7)
	n, err := reader1.Read(buf1)
	var offset1, index1 int64
	executeIfNoErr(err, func() {
		fmt.Printf("Read(%d): %q\n", n, buf1[:n])
		offset1 = int64(53)
		index1, err = reader1.Seek(offset1, io.SeekCurrent)
	})
	executeIfNoErr(err, func() {
		fmt.Printf("The new index after seeking from current with offset %d: %d\n",
			offset1, index1)
		n, err = reader1.Read(buf1)
	})
	executeIfNoErr(err, func() {
		fmt.Printf("Read(%d): %q\n", n, buf1[:n])
	})
	fmt.Println()

	// *io.LimitedReader
	// 无论 Read 方法被调用多少次，总数据量会受到限制，这里为 7
	reader1.Reset(comment)
	num1 := int64(7)
	fmt.Printf("New a limited reader with reader1 and number %d ...\n", num1)
	reader2 := io.LimitReader(reader1, 7)
	buf2 := make([]byte, 10)
	for i := 0; i < 3; i++ {
		n, err = reader2.Read(buf2)
		executeIfNoErr(err, func() {
			fmt.Printf("Read(%d): %q\n", n, buf2[:n])
		})
	}
	fmt.Println()

	// *io.SectionReader
	// 只能够读取原始数据的某段，数据段的起始位置和末尾位置需在初始化时指明，之后无法更改
	reader1.Reset(comment)
	offset2 := int64(56)
	num2 := int64(72)
	fmt.Printf("New a section reader with reader1, offset %d and number %d ...\n", offset2, num2)
	reader3 := io.NewSectionReader(reader1, offset2, num2)
	buf3 := make([]byte, 20)
	for i := 0; i < 5; i++ {
		n, err = reader3.Read(buf3)
		executeIfNoErr(err, func() {
			fmt.Printf("Read(%d): %q\n", n, buf3[:n])
		})
	}
	fmt.Println()

	// *io.teeReader
	// 两个参数 r 和 w，类型分别为 io.Reader 和 io.Writer
	// 结果值的 Read 方法会把 r 中的数据经过作为方法参数的字节切片 p 写入到 w
	reader1.Reset(comment)
	writer1 := new(strings.Builder)
	fmt.Println("New a tee reader with reader1 and writer1 ...")
	reader4 := io.TeeReader(reader1, writer1)
	buf4 := make([]byte, 40)
	for i := 0; i < 8; i++ {
		n, err = reader4.Read(buf4)
		executeIfNoErr(err, func() {
			fmt.Printf("Read(%d): %q\n", n, buf4[:n])
		})
	}
	fmt.Println()
	
	// io.multiReader
	// MultiReader 方法可以接受若干个 io.Reader 类型的参数值
	// 并返回一个实际类型为 io.multiReader 的结果值
	// 当该结果值的 Read 方法被调用时，它会顺序地从前面的 io.Reader 参数中读取数据
	reader5a := strings.NewReader( 
		"MultiReader returns a Reader that's the logical concatenation of " +
		"the provided input readers.")
	reader5b := strings.NewReader("They're read sequentially.")
	reader5c := strings.NewReader("Once all inputs have returend EOF, Read will return EOF.")
	reader5d := strings.NewReader("If any of the readers return a  non-nil, " +
 			"non-EOF error, Read will return that error.")
	fmt.Println("New a multi-reader with 4 readers ...")
	reader5 := io.MultiReader(reader5a, reader5b, reader5c, reader5d)
	buf5 := make([]byte, 50)
	for i := 0; i < 8; i++ {
		n, err = reader5.Read(buf5)
		executeIfNoErr(err, func() {
			fmt.Printf("Read(%d): %q\n", n, buf5[:n])
		})
	}
	fmt.Println()

	// io.pipe
	// 实现了 io.Reader 接口和 io.Writer 接口
	// io.Pipe() 会返回那两个类型的指针值，并分别作为内存管道的两端
	fmt.Println("New a synchronous in-memory pipe ...")
	pReader, pWriter := io.Pipe()
	_ = interface{}(pReader).(io.ReadCloser)
	_ = interface{}(pWriter).(io.WriteCloser)

	comments := [][]byte{
		[]byte("Pipe creates a synchronous in-memory pipe."),
		[]byte("It can be used to connect code expecting an io.Reader "),
		[]byte("with code expecting an io.Writer."),

	}

	var wg sync.WaitGroup
	wg.Add(2)

	go func() {
		defer wg.Done()
		for _, d := range comments {
			time.Sleep(time.Millisecond * 500)
			n, err := pWriter.Write(d)
			if err != nil {
				fmt.Printf("write error: %v\n", err)
				break
			}
			fmt.Printf("Written(%d): %q\n", n, d)
		}
		pWriter.Close()
	}()

	go func() {
		defer wg.Done()
		wBuf := make([]byte, 55)
		for {
			n, err := pReader.Read(wBuf)
			if err != nil {
				fmt.Printf("read error: %v\n", err)
				break
			}
			fmt.Printf("Read(%d): %q\n", n, wBuf[:n])
		}
		pReader.Close()
	}()

	wg.Wait()
}	

io 包中的接口体系