Reflections

Package reflections provides high level abstractions above the golang reflect library.

Reflect library is very low-level and can be quite complex when it comes to do simple things like accessing a structure field value, a field tag...

The purpose of reflections package is to make developers life easier when it comes to introspect structures at runtime. Its API is inspired from python language (getattr, setattr, hasattr...) and provides a simplified access to structure fields and tags.

Reflections is an open source library under the MIT license. Any hackers are welcome to supply ideas, features requests, patches, pull requests and so on: see Contribute

Documentation

Documentation is available at http://godoc.org/github.com/oleiade/reflections

Installation

Into the gopath

    go get gopkg.in/oleiade/reflections.v1

Import it in your code

    import (
        "gopkg.in/oleiade/reflections.v1"
    )

Usage

Accessing structure fields

GetField

GetField returns the content of a structure field. It can be very usefull when you'd wanna iterate over a struct specific fields values for example. You can whether provide GetField a structure or a pointer to structure as first argument.

    s := MyStruct {
        FirstField: "first value",
        SecondField: 2,
        ThirdField: "third value",
    }

    fieldsToExtract := []string{"FirstField", "ThirdField"}

    for _, fieldName := range fieldsToExtract {
        value, err := reflections.GetField(s, fieldName)
        DoWhatEverWithThatValue(value)
    }

GetFieldKind

GetFieldKind returns the reflect.Kind of a structure field. It can be used to operate type assertion over a structure fields at runtime. You can whether provide GetFieldKind a structure or a pointer to structure as first argument.

    s := MyStruct{
        FirstField:  "first value",
        SecondField: 2,
        ThirdField:  "third value",
    }

    var firstFieldKind reflect.String
    var secondFieldKind reflect.Int
    var err error

    firstFieldKind, err = GetFieldKind(s, "FirstField")
    if err != nil {
        log.Fatal(err)
    }

    secondFieldKind, err = GetFieldKind(s, "SecondField")
    if err != nil {
        log.Fatal(err)
    }

GetFieldType

GetFieldType returns the string literal of a structure field type. It can be used to operate type assertion over a structure fields at runtime. You can whether provide GetFieldType a structure or a pointer to structure as first argument.

    s := MyStruct{
        FirstField:  "first value",
        SecondField: 2,
        ThirdField:  "third value",
    }

    var firstFieldKind string
    var secondFieldKind string
    var err error

    firstFieldKind, err = GetFieldType(s, "FirstField")
    if err != nil {
        log.Fatal(err)
    }

    secondFieldKind, err = GetFieldType(s, "SecondField")
    if err != nil {
        log.Fatal(err)
    }

GetFieldTag

GetFieldTag extracts a specific structure field tag. You can whether provide GetFieldTag a structure or a pointer to structure as first argument.

    s := MyStruct{}

    tag, err := reflections.GetFieldTag(s, "FirstField", "matched")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(tag)

    tag, err = reflections.GetFieldTag(s, "ThirdField", "unmatched")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(tag)

HasField

HasField asserts a field exists through structure. You can whether provide HasField a structure or a pointer to structure as first argument.

    s := MyStruct {
        FirstField: "first value",
        SecondField: 2,
        ThirdField: "third value",
    }

    // has == true
    has, _ := reflections.HasField(s, "FirstField")

    // has == false
    has, _ := reflections.HasField(s, "FourthField")

Fields

Fields returns the list of a structure field names, so you can access or modify them later on. You can whether provide Fields a structure or a pointer to structure as first argument.

    s := MyStruct {
        FirstField: "first value",
        SecondField: 2,
        ThirdField: "third value",
    }

    var fields []string

    // Fields will list every structure exportable fields.
    // Here, it's content would be equal to:
    // []string{"FirstField", "SecondField", "ThirdField"}
    fields, _ = reflections.Fields(s)

Items

Items returns the structure's field name to values map. You can whether provide Items a structure or a pointer to structure as first argument.

    s := MyStruct {
        FirstField: "first value",
        SecondField: 2,
        ThirdField: "third value",
    }

    var structItems map[string]interface{}

    // Items will return a field name to
    // field value map
    structItems, _ = reflections.Items(s)

Tags

Tags returns the structure's fields tag with the provided key. You can whether provide Tags a structure or a pointer to structure as first argument.

    s := MyStruct {
        FirstField: "first value",      `matched:"first tag"`
        SecondField: 2,                 `matched:"second tag"`
        ThirdField: "third value",      `unmatched:"third tag"`
    }

    var structTags map[string]string

    // Tags will return a field name to tag content
    // map. Nota that only field with the tag name
    // you've provided which will be matched.
    // Here structTags will contain:
    // {
    //     "FirstField": "first tag",
    //     "SecondField": "second tag",
    // }
    structTags, _ = reflections.Tags(s, "matched")

Set a structure field value

SetField update's a structure's field value with the one provided. Note that unexported fields cannot be set, and that field type and value type have to match.

    s := MyStruct {
        FirstField: "first value",
        SecondField: 2,
        ThirdField: "third value",
    }

    // In order to be able to set the structure's values,
    // a pointer to it has to be passed to it.
    _ := reflections.SetField(&s, "FirstField", "new value")

    // If you try to set a field's value using the wrong type,
    // an error will be returned
    err := reflection.SetField(&s, "FirstField", 123)  // err != nil

Important notes

• unexported fields cannot be accessed or set using reflections library: the golang reflect library intentionaly prohibits unexported fields values access or modifications.

Contribute

• Check for open issues or open a fresh issue to start a discussion around a feature idea or a bug.
• Fork the repository_ on GitHub to start making your changes to the master branch (or branch off of it).
• Write tests which shows that the bug was fixed or that the feature works as expected.
• Send a pull request and bug the maintainer until it gets merged and published. :) Make sure to add yourself to AUTHORS_.

the repository AUTHORS