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Golang reflector

First of all, don't use reflection if you don't have to.

But if you really have to... This library offers a simplified Golang reflection abstraction.

Getting and setting fields

Let's suppose we have structs like:

type Address struct {
    Street string `tag:"be" tag2:"1,2,3"`
    Number int    `tag:"bi"`
}

type Person struct {
    Name string `tag:"bu"`
    Address
}

func (p Person) Hi(name string) string {
    return fmt.Sprintf("Hi %s my name is %s", name, p.Name)
}

Initialize the reflector's object wrapper:

import "github.com/tkrajina/go-reflector/reflector"

p := Person{}
obj := reflector.New(p)

Check if a field is valid:

obj.Field("Name").IsValid()

Get field value:

val, err := obj.Field("Name").Get()

Set field value:

p := Person{}
obj := reflector.New(&p)
err := obj.Field("Name").Set("Something")

Don't forget to use a pointer in New(), otherwise setters won't work. Field "settability" can be checked by using field.IsSettable().

Tags

Get a tag:

jsonTag := obj.Field("Name").Tag("json")

Get tag values array (exploded with "," as a delimiter):

jsonTag := obj.Field("Name").TagExpanded("json")

Or get a map with all field tags:

fieldTagsMap := obj.Field("Name").Tags()

Listing fields

There are three ways to list fields:

  • List all fields: This will include anonymous structs and fields declared in anonymous structs (Name, Address, Street, Number).
  • List flattened fields: Includes fields declared in anonymous structs without anonymous structs (Name, Street, Number).
  • List nonflattened fields: Includes anonymous structs without their fields (Name, Address). This is the way fields are actually declared in the code.

Depending on which listing you want, you can use:

fields := obj.FieldsAll()
fields := obj.FieldsFlattened()
fields := obj.Fields()

You can only get the list of anonymous fields with obj.FieldsAnonymous().

Be aware that because of anonymous structs, some field names can be returned twice! In most cases this is not a desired situation, but you can use reflector to detect such situations in your code:

doubleDeclaredFields := obj.FindDoubleFields()
if len(doubleDeclaredFields) > 0 {
    fmt.Println("Detected multiple fields with same name:", doubleDeclaredFields)
}

The field listing will contain both exported and unexported fields. Unexported fields are not gettable/settable, but their tags are readable.

Calling methods

obj := reflector.New(&Person{})
resp, err := obj.Method("Hi").Call("John", "Smith")

The err is not nil only if something was wrong with the method (for example invalid method name, or wrong argument number/types), not with the actual method call. If the call finished, err will be nil. If the method call returned an error, you can check it with:

if resp.IsError() {
    fmt.Println("Got an error:", resp.Error.Error())
} else {
    fmt.Println("Method call response:", resp.Result)
}

Listing methods

for _, method := range obj.Methods() {
    fmt.Println("Method", method.Name(), "with input types", method.InTypes(), "and output types", method.OutTypes())
}

Getting length, getting and setting slice/array/string/map elements

Map:

m := map[string]interface{}{"aaa", 17}
o := reflector.New(m)
fmt.Println("Length", o.Len())
val, found := o.GetByKey("aaa")
o.SetByKey("bbb", "new value")
fmt.Println("keys:", o.Keys())

Slice, string:

l := []int{1, 2, 3}
o := reflector.New(o)
fmt.Println("Length", o.Len())
val, found := o.GetByIndex(0)
o.SetByIndex(0, 19)

Performance

When reflecting the same type multiple times, reflector will cache as much reflection metadata as possible only once and use that in future.

If you make any changes to the library, run make test-performance to check performance improvement/deterioration before/after your change.

$ make test-performance
N=1000000 go test -v ./... -run=TestPerformance
=== RUN   TestPerformance
WITH REFLECTION
    n= 1000000
    started: 2016-05-25 08:35:15.5258
    ended: 2016-05-25 08:35:19.5258
    duration: 4.269112s
--- PASS: TestPerformance (4.27s)
=== RUN   TestPerformancePlain
WITHOUT REFLECTION
    n= 1000000
    started: 2016-05-25 08:35:19.5258
    ended: 2016-05-25 08:35:19.5258
    duration: 0.005237s
--- PASS: TestPerformancePlain (0.01s)
PASS
ok      github.com/tkrajina/go-reflector/reflector      4.285s

Keep those numbers in mind before deciding to use reflection :)

License

Reflector is licensed under the Apache License, Version 2.0