使用反射(reflect)对结构体赋值
上一篇 中，我们看了GIN是如何绑定参数并且校验的，本着知道如何使用也要知道底层原理的
探索精神，这一篇中，我们自己来使用 reflect 实现一个轻量版的参数绑定。
不过在此之前，我们需要先了解和熟悉 reflect 库。
学习 reflect
对于代码中的一个变量来说，它有两个信息：1. 类型；2. 值。类型是指，这个变量具体是什么类型，比如是否是 string, int,
bool, *ptr；对于值，就是具体的赋值，比如 x := 1，x 的类型是 int, 值是 1。这就引入了 reflect 库中最重要的
两个函数：
reflect.TypeOf。类型是func TypeOf(i interface{}) Type。reflect.ValueOf。类型是func ValueOf(i interface{}) Value。
我们来看看两者分别是什么：
// TypeOf returns the reflection Type that represents the dynamic type of i.
// If i is a nil interface value, TypeOf returns nil.
func TypeOf(i interface{}) Type {
eface := *(*emptyInterface)(unsafe.Pointer(&i))
return toType(eface.typ)
}
// Type is the representation of a Go type.
//
// Not all methods apply to all kinds of types. Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of type before
// calling kind-specific methods. Calling a method
// inappropriate to the kind of type causes a run-time panic.
//
// Type values are comparable, such as with the == operator,
// so they can be used as map keys.
// Two Type values are equal if they represent identical types.
type Type interface {
// Methods applicable to all types.
// Align returns the alignment in bytes of a value of
// this type when allocated in memory.
Align() int
// FieldAlign returns the alignment in bytes of a value of
// this type when used as a field in a struct.
FieldAlign() int
// Method returns the i'th method in the type's method set.
// It panics if i is not in the range [0, NumMethod()).
//
// For a non-interface type T or *T, the returned Method's Type and Func
// fields describe a function whose first argument is the receiver,
// and only exported methods are accessible.
//
// For an interface type, the returned Method's Type field gives the
// method signature, without a receiver, and the Func field is nil.
//
// Methods are sorted in lexicographic order.
Method(int) Method
// MethodByName returns the method with that name in the type's
// method set and a boolean indicating if the method was found.
//
// For a non-interface type T or *T, the returned Method's Type and Func
// fields describe a function whose first argument is the receiver.
//
// For an interface type, the returned Method's Type field gives the
// method signature, without a receiver, and the Func field is nil.
MethodByName(string) (Method, bool)
// NumMethod returns the number of methods accessible using Method.
//
// Note that NumMethod counts unexported methods only for interface types.
NumMethod() int
// Name returns the type's name within its package for a defined type.
// For other (non-defined) types it returns the empty string.
Name() string
// PkgPath returns a defined type's package path, that is, the import path
// that uniquely identifies the package, such as "encoding/base64".
// If the type was predeclared (string, error) or not defined (*T, struct{},
// []int, or A where A is an alias for a non-defined type), the package path
// will be the empty string.
PkgPath() string
// Size returns the number of bytes needed to store
// a value of the given type; it is analogous to unsafe.Sizeof.
Size() uintptr
// String returns a string representation of the type.
// The string representation may use shortened package names
// (e.g., base64 instead of "encoding/base64") and is not
// guaranteed to be unique among types. To test for type identity,
// compare the Types directly.
String() string
// Kind returns the specific kind of this type.
Kind() Kind
// Implements reports whether the type implements the interface type u.
Implements(u Type) bool
// AssignableTo reports whether a value of the type is assignable to type u.
AssignableTo(u Type) bool
// ConvertibleTo reports whether a value of the type is convertible to type u.
// Even if ConvertibleTo returns true, the conversion may still panic.
// For example, a slice of type []T is convertible to *[N]T,
// but the conversion will panic if its length is less than N.
ConvertibleTo(u Type) bool
// Comparable reports whether values of this type are comparable.
// Even if Comparable returns true, the comparison may still panic.
// For example, values of interface type are comparable,
// but the comparison will panic if their dynamic type is not comparable.
Comparable() bool
// Methods applicable only to some types, depending on Kind.
// The methods allowed for each kind are:
//
// Int*, Uint*, Float*, Complex*: Bits
// Array: Elem, Len
// Chan: ChanDir, Elem
// Func: In, NumIn, Out, NumOut, IsVariadic.
// Map: Key, Elem
// Ptr: Elem
// Slice: Elem
// Struct: Field, FieldByIndex, FieldByName, FieldByNameFunc, NumField
// Bits returns the size of the type in bits.
// It panics if the type's Kind is not one of the
// sized or unsized Int, Uint, Float, or Complex kinds.
Bits() int
// ChanDir returns a channel type's direction.
// It panics if the type's Kind is not Chan.
ChanDir() ChanDir
// IsVariadic reports whether a function type's final input parameter
// is a "..." parameter. If so, t.In(t.NumIn() - 1) returns the parameter's
// implicit actual type []T.
//
// For concreteness, if t represents func(x int, y ... float64), then
//
// t.NumIn() == 2
// t.In(0) is the reflect.Type for "int"
// t.In(1) is the reflect.Type for "[]float64"
// t.IsVariadic() == true
//
// IsVariadic panics if the type's Kind is not Func.
IsVariadic() bool
// Elem returns a type's element type.
// It panics if the type's Kind is not Array, Chan, Map, Ptr, or Slice.
Elem() Type
// Field returns a struct type's i'th field.
// It panics if the type's Kind is not Struct.
// It panics if i is not in the range [0, NumField()).
Field(i int) StructField
// FieldByIndex returns the nested field corresponding
// to the index sequence. It is equivalent to calling Field
// successively for each index i.
// It panics if the type's Kind is not Struct.
FieldByIndex(index []int) StructField
// FieldByName returns the struct field with the given name
// and a boolean indicating if the field was found.
FieldByName(name string) (StructField, bool)
// FieldByNameFunc returns the struct field with a name
// that satisfies the match function and a boolean indicating if
// the field was found.
//
// FieldByNameFunc considers the fields in the struct itself
// and then the fields in any embedded structs, in breadth first order,
// stopping at the shallowest nesting depth containing one or more
// fields satisfying the match function. If multiple fields at that depth
// satisfy the match function, they cancel each other
// and FieldByNameFunc returns no match.
// This behavior mirrors Go's handling of name lookup in
// structs containing embedded fields.
FieldByNameFunc(match func(string) bool) (StructField, bool)
// In returns the type of a function type's i'th input parameter.
// It panics if the type's Kind is not Func.
// It panics if i is not in the range [0, NumIn()).
In(i int) Type
// Key returns a map type's key type.
// It panics if the type's Kind is not Map.
Key() Type
// Len returns an array type's length.
// It panics if the type's Kind is not Array.
Len() int
// NumField returns a struct type's field count.
// It panics if the type's Kind is not Struct.
NumField() int
// NumIn returns a function type's input parameter count.
// It panics if the type's Kind is not Func.
NumIn() int
// NumOut returns a function type's output parameter count.
// It panics if the type's Kind is not Func.
NumOut() int
// Out returns the type of a function type's i'th output parameter.
// It panics if the type's Kind is not Func.
// It panics if i is not in the range [0, NumOut()).
Out(i int) Type
common() *rtype
uncommon() *uncommonType
}
Type 代表Go语言中的类型，它是一个接口，但是有一大堆方法。我们需要仔细读一下注释，注释中有说明，其中很多方法都是
特定类型才能使用的，否则会panic。
接下来我们来看 Value:
// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i. ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
if i == nil {
return Value{}
}
// TODO: Maybe allow contents of a Value to live on the stack.
// For now we make the contents always escape to the heap. It
// makes life easier in a few places (see chanrecv/mapassign
// comment below).
escapes(i)
return unpackEface(i)
}
// Value is the reflection interface to a Go value.
//
// Not all methods apply to all kinds of values. Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of value before
// calling kind-specific methods. Calling a method
// inappropriate to the kind of type causes a run time panic.
//
// The zero Value represents no value.
// Its IsValid method returns false, its Kind method returns Invalid,
// its String method returns "<invalid Value>", and all other methods panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
//
// A Value can be used concurrently by multiple goroutines provided that
// the underlying Go value can be used concurrently for the equivalent
// direct operations.
//
// To compare two Values, compare the results of the Interface method.
// Using == on two Values does not compare the underlying values
// they represent.
type Value struct {
// typ holds the type of the value represented by a Value.
typ *rtype
// Pointer-valued data or, if flagIndir is set, pointer to data.
// Valid when either flagIndir is set or typ.pointers() is true.
ptr unsafe.Pointer
// flag holds metadata about the value.
// The lowest bits are flag bits:
// - flagStickyRO: obtained via unexported not embedded field, so read-only
// - flagEmbedRO: obtained via unexported embedded field, so read-only
// - flagIndir: val holds a pointer to the data
// - flagAddr: v.CanAddr is true (implies flagIndir)
// - flagMethod: v is a method value.
// The next five bits give the Kind of the value.
// This repeats typ.Kind() except for method values.
// The remaining 23+ bits give a method number for method values.
// If flag.kind() != Func, code can assume that flagMethod is unset.
// If ifaceIndir(typ), code can assume that flagIndir is set.
flag
// A method value represents a curried method invocation
// like r.Read for some receiver r. The typ+val+flag bits describe
// the receiver r, but the flag's Kind bits say Func (methods are
// functions), and the top bits of the flag give the method number
// in r's type's method table.
}
除了 Type 和 Value，我们还需要知道一个东西，那就是 Kind。我们可以通过一个简单的例子来看看他们的区别：
package main
import (
"fmt"
"reflect"
)
type MyStruct struct {
i int
}
func main() {
m := MyStruct{1}
t := reflect.TypeOf(m)
v := reflect.ValueOf(m)
fmt.Printf("t: %s, kind: %s, v: %s, kind: %s\n", t, t.Kind(), v, v.Kind())
}
运行一下：
$ go run main.go
t: main.MyStruct, kind: struct, v: {%!s(int=1)}, kind: struct
可以看到，Type 保存的是变量的类型，而 kind 是变量最终在Go里存在时的原生类型。并且从 Type 和 Value 都能拿到这个信息。
Kind 的种类有：
// A Kind represents the specific kind of type that a Type represents.
// The zero Kind is not a valid kind.
type Kind uint
const (
Invalid Kind = iota
Bool
Int
Int8
Int16
Int32
Int64
Uint
Uint8
Uint16
Uint32
Uint64
Uintptr
Float32
Float64
Complex64
Complex128
Array
Chan
Func
Interface
Map
Ptr
Slice
String
Struct
UnsafePointer
)
自己实现参数绑定
有了上述知识，我们就可以自己实现一个参数绑定甚至是JSON序列化和反序列化的库了。首先我们确定好函数的签名：
func mapping(dst interface{}, m map[string][]string)
入参是一个结构体的指针，因为Go里所有的函数调用传参都是 pass by value，也就是值拷贝，如果想要修改一个变量，必须把指针
传进去，而第二个参数的类型则是 map[string][]string，这是因为我们在上一篇文章中已经了解到了，url.Values，headers
的底层表示，都是这个类型，所以我们直接使用这个类型。
接下来就是大体的逻辑：
- 首先我们要检验参数的类型
- 然后我们拿到结构体的类型信息，依次迭代结构体的每一个成员并且根据类型尝试解析
m里的值，最后赋值
逻辑不难，最主要是要搞清楚 reflect 提供的能力，我们直接看代码，代码中有注释：
package main
import (
"fmt"
"log"
"reflect"
"strconv"
)
func mapping(dst interface{}, m map[string][]string) {
typ := reflect.TypeOf(dst)
// 首先判断传入参数的类型
if !(typ.Kind() == reflect.Ptr && typ.Elem().Kind() == reflect.Struct) {
log.Printf("Should pass ptr to destination struct object. Usage: mapping(&someStruct, m)")
return
}
// 拿到指针所指向的元素的类型
typ = typ.Elem()
// 拿到指针所指向的元素的值
value := reflect.ValueOf(dst).Elem()
// 遍历每一个字段
for i := 0; i < typ.NumField(); i++ {
field := typ.Field(i)
// 忽略非导出字段
if !field.IsExported() {
log.Printf("field %s is not exported, ignore", field.Name)
continue
}
// 判断是否设置了这个tag
formTag := field.Tag.Get("form")
if formTag == "" {
log.Printf("tag `form` not exist in field, ignore")
continue
}
// 查看是否有取值
vs := m[formTag]
if len(vs) == 0 {
log.Printf("vs by formTag %s not found, ignore", formTag)
continue
}
v := vs[len(vs)-1]
// 根据类型来设置值
switch fieldType := field.Type.Kind(); fieldType {
case reflect.Int, reflect.Int16, reflect.Int32, reflect.Int64:
typedV, _ := strconv.ParseInt(v, 10, 64)
value.Field(i).SetInt(typedV)
case reflect.String:
value.Field(i).SetString(v)
case reflect.Uint, reflect.Uint16, reflect.Uint32, reflect.Uint64:
typedV, _ := strconv.ParseUint(v, 10, 64)
value.Field(i).SetUint(typedV)
case reflect.Bool:
value.Field(i).SetBool(v == "true")
default:
log.Printf("field type %s not support yet", fieldType)
}
}
}
func main() {
m := map[string][]string{
"name": {"jhony"},
"age": {"1"},
"money": {"10010010"},
}
type Person struct {
Name string `form:"name"`
Age uint `form:"age"`
Money int64 `form:"money"`
unexport string `form:"unexport"`
NotFound bool `form:"not_found"`
NoTag int8
}
i := 1
mapping(i, m)
mapping(&i, m)
p := Person{}
mapping(&p, m)
fmt.Printf("%v\n", p)
}
运行一下：
$ go run main.go
2022/01/09 17:21:55 Should pass ptr to destination struct object. Usage: mapping(&someStruct, m)
2022/01/09 17:21:55 Should pass ptr to destination struct object. Usage: mapping(&someStruct, m)
2022/01/09 17:21:55 field unexport is not exported, ignore
2022/01/09 17:21:55 vs by formTag not_found not found, ignore
2022/01/09 17:21:55 tag `form` not exist in field, ignore
{jhony 1 10010010 false 0}
搞定！
总结
上一篇文章中，我们看到GIN大概是如何绑定参数的，这一篇文章中，我们自己来实现一套轻量版的逻辑，通过这样实战一番，对
reflect 就会更加熟悉。
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