go - 如何扩展标准 Golang 未导出的结构类型？

Question

考虑这个工作示例代码，它扩展(不完全)

type multiWriter struct {
    writers []Writer
}

从 io 包中，添加名为 Remove 和 Append 的两个方法来操作内部 slice writers:

package main

import (
    "io"
    "os"
)

func main() {
    w1, e := os.Create("file1.txt")
    if e != nil {
        panic(e)
    }
    w2, e := os.Create("file2.txt")
    if e != nil {
        panic(e)
    }
    mw := MultiWriter(w1, w2)
    data := []byte("Hello ")
    _, e = mw.Write(data)
    if e != nil {
        panic(e)
    }

    var m *multiWriter = mw.(*multiWriter)
    m.Remove(w2)
    w2.Close()
    w3, e := os.Create("file3.txt")
    if e != nil {
        panic(e)
    }
    m.Append(w3)
    data = []byte("World ")
    _, e = mw.Write(data)
    if e != nil {
        panic(e)
    }
    w3.Close()
    w1.Close()
}
func (t *multiWriter) Remove(writers ...io.Writer) {
    for i := len(t.writers) - 1; i > 0; i-- {
        for _, v := range writers {
            if t.writers[i] == v {
                t.writers = append(t.writers[:i], t.writers[i+1:]...)
                break
            }
        }
    }
}
func (t *multiWriter) Append(writers ...io.Writer) {
    t.writers = append(t.writers, writers...)
}

type multiWriter struct {
    writers []io.Writer
}

func (t *multiWriter) Write(p []byte) (n int, err error) {
    for _, w := range t.writers {
        n, err = w.Write(p)
        if err != nil {
            return
        }
        if n != len(p) {
            err = io.ErrShortWrite
            return
        }
    }
    return len(p), nil
}

var _ stringWriter = (*multiWriter)(nil)

func (t *multiWriter) WriteString(s string) (n int, err error) {
    var p []byte // lazily initialized if/when needed
    for _, w := range t.writers {
        if sw, ok := w.(stringWriter); ok {
            n, err = sw.WriteString(s)
        } else {
            if p == nil {
                p = []byte(s)
            }
            n, err = w.Write(p)
        }
        if err != nil {
            return
        }
        if n != len(s) {
            err = io.ErrShortWrite
            return
        }
    }
    return len(s), nil
}

// MultiWriter creates a writer that duplicates its writes to all the
// provided writers, similar to the Unix tee(1) command.
func MultiWriter(writers ...io.Writer) io.Writer {
    w := make([]io.Writer, len(writers))
    copy(w, writers)
    return &multiWriter{w}
}

// stringWriter is the interface that wraps the WriteString method.
type stringWriter interface {
    WriteString(s string) (n int, err error)
}

有什么简洁的方法可以做到这一点，只添加名为 Remove 和 Append 的两个方法到 io.MultiWriter？

Answer 1

您不能为其他包中的类型定义方法。代码只能为同一包中的类型定义方法。

Spec: Method declarations:

The type denoted by T is called the receiver base type; it must not be a pointer or interface type and it must be declared in the same package as the method.

因此除了复制其完整代码并将方法添加到您自己的类型之外，没有其他方法可以使用方法扩展未导出的类型 io.multiWriter。

注意:作为实现说明，在您的multiWriter.Remove() 方法中，一旦您找到可移动写入器，在重新 slice 后您可以“中断”(从内部loop) 以省略 slice 的其余部分:

// ...
if t.writers[i] == v {
    t.writers = append(t.writers[:i], t.writers[i+1:]...)
    break
}
// ...