Go: 属性存在但 Go 编译器说它不存在？

Question

filelogger.go

package logger

import (
    "io"
)

type FileLogger struct{
    File io.Writer
}

func NewFileLogger(file io.Writer) *FileLogger{
    return &FileLogger{file}
}

func (this *FileLogger) Log(message string) error {
    _, err := this.File.Write([]byte(appendNewLine(message)))

    return err
}

filelogger_test.go:

package logger

import (
    "testing"

    "github.com/stretchr/testify/assert"
)

type WriterMock struct{
    data []byte
}

func (this WriterMock) Write(b []byte) (n int, err error) {
    this.data = append(this.data, b ...)

    return len(this.data), nil
}

func NewMockedFileLogger() *FileLogger{
    writer := WriterMock{}

    fileLogger := FileLogger{writer}

    return &fileLogger
}

func TestLog(t *testing.T) {
    fileLogger := NewMockedFileLogger()

    fileLogger.Log("Hello World!")

    assert.Equal(t, "Hello World!", string(fileLogger.File.data))
}

我的问题：

我在运行进行测试时收到此错误消息：

fileLogger.File.data undefined (type io.Writer has no field or method data)

file.Logger.File 确实属于 type io.Writer，但是这个字段 data 存在，我知道 Go 是一种强类型语言，所以它不接受这个。

如何解决这个问题？

Answer 1

FileLogger 中的作者 File 是 interface (io.Writer)，而不是 struct。

您需要 type assertion 才能访问 data of WriterMock:

fileLooger.File.(*WriterMock).data

（注意：如果 File 不是 *WriterMock，那将失败：下面有更多内容）

见this simplified example:

主要包

import "fmt"
import "io"

type WriterMock struct {
    data []byte
}

func (this WriterMock) Write(b []byte) (n int, err error) {
    this.data = append(this.data, b...)

    return len(this.data), nil
}

func main() {
    var w io.Writer = &WriterMock{}
    fmt.Printf("Hello, playground '%+v'", w.(*WriterMock).data)
}

输出：

Hello, playground '[]'

----

由于类型断言汽车错误，您应该始终检查错误，请考虑“Interface conversions and type assertions”部分：

But if it turns out that the value does not contain a string, the program will crash with a run-time error.
To guard against that, use the "comma, ok" idiom to test, safely, whether the value is a string:

str, ok := value.(string)
if ok {
    fmt.Printf("string value is: %q\n", str)
} else {
    fmt.Printf("value is not a string\n")
}

Go: 属性存在但 Go 编译器说它不存在？

Go: Property exists but Go compiler says it doesn't?

go

tdd

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Go: 属性 存在但 Go 编译器说它不存在？

Go: Property exists but Go compiler says it doesn't?

go

tdd

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Go: 属性存在但 Go 编译器说它不存在？