Go模板无法在字段上调用方法

Question

我有一个围绕 net/mail.Address 的包装器，它提供了一些编组逻辑。我试图在模板中使用它，但我一直收到 can't evaluate field String in type EmailAddress。模板文档说：

The name of a niladic method of the data, preceded by a period, such as

.Method

The result is the value of invoking the method with dot as the receiver, dot.Method().

和

Method invocations may be chained and combined with fields and keys to any depth:

.Field1.Key1.Method1.Field2.Key2.Method2

考虑到这一点，我写了这篇文章：

package main

import (
    "bytes"
    "fmt"
    "html/template"
    "net/mail"
    "os"
)

type EmailAddress struct{ mail.Address }

type emailFormatter struct {
    From         EmailAddress
    To           EmailAddress
}

var tmpl = template.Must(template.New("Sample Text").Parse("From: {{.From.String}}\r" + `
To: {{.To.String}}` + "\r" + `
Content-Type: text/html` + "\r" + `
Subject: Sample Text` + "\r\n\r" + `
<!DOCTYPE html>
<html lang="en">
<head>
    <title>Sample Text</title>
    <meta charset="utf-8"/>
</head>
<body>
    Sample Text
</body>
</html>
`));

func main() {
    to := EmailAddress{
        mail.Address{
            Address: "em@i.l",
            Name: "",
        },
    }
    from := EmailAddress{
        mail.Address{
            Address: "no-reply@test.quest",
            Name: "",
        },
    }

    fmt.Println(to.String()) //outputs (as expected) "<em@i.l>"
    fmt.Println(from.String()) //outputs (as expected) "<no-reply@test.quest>"

    f := emailFormatter{
        To: to,
        From: from,
    }

    var buff bytes.Buffer
    if err := tmpl.Execute(&buff, f); err != nil {
        fmt.Printf("Error: %v\n", err)
        os.Exit(1)
    }
    fmt.Println(buff.String())
}

我已经单独验证调用EmailAddress.String是完全合法的，所以我不明白为什么只有这样的输出：

Error: template: Sample Text:1:13: executing "Sample Text" at <.From.String>: can't evaluate field String in type main.EmailAddress

编辑

根据评论者的建议，我将调用从 .From.String 和 .To.String 更改为 .From.Address.String 和 .To.Address.String，因为

"String isn't defined on EmailAddress, it's defined on EmailAddress.Address"

但结果是一样的：

Error: template: Sample Text:1:13: executing "Sample Text" at <.From.Address.String>: can't evaluate field String in type mail.Address

Answer 1

由于 String 是使用指针接收器定义的，因此您需要将 mail.Address 的 "addressable" 实例传递给模板才能执行该方法。

您可以通过传递指向 f 的指针来完成此操作。

if err := tmpl.Execute(&buff, &f); err != nil {
    panic(err)
}

或者您可以通过传递指向 EmailAddress.

的指针来实现

type emailFormatter struct {
    From *EmailAddress
    To   *EmailAddress
}

// ...

f := emailFormatter{
    To:   &to,
    From: &from,
}

// ...

if err := tmpl.Execute(&buff, f); err != nil {
    panic(err)
}

或者传入指向 mail.Address 的指针。

type EmailAddress struct{ *mail.Address }

// ...

to := EmailAddress{
    &mail.Address{
        Address: "em@i.l",
        Name: "",
    },
}
from := EmailAddress{
    &mail.Address{
        Address: "no-reply@test.quest",
        Name: "",
    },
}

f := emailFormatter{
    To:   to,
    From: from,
}

// ...

if err := tmpl.Execute(&buff, f); err != nil {
    panic(err)
}

请注意，您不需要在 Go 代码中执行此操作的原因是编译器会为您执行此操作。

例如：

fmt.Println(to.String())

变为：

fmt.Println((&to).String())

A method call x.m() is valid if the method set of (the type of) x contains m and the argument list can be assigned to the parameter list of m. If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m()

Go模板无法在字段上调用方法

Go template can't call method on field

templates

go

go-templates

编辑