1.反射的简介
Golang提供了一种机制，在编译时不知道类型的情况下，可更新变量、运行时查看值、调用方法以及直接对他们的布局进行操作的机制，称为反射。
2.为什么使用反射？
打个比方，有时候我们需要一个函数可以处理各种类型的值。在不知道类型的情况下，你可能会这么写：
// 伪代码
switch value := value.(type) {
case string:
// …一些操作
case int:
// …一些操作
case cbsStruct: // 自定义的结构体
// …一些操作
// …
}
有没发现什么问题？
这边存在一个问题：类型很多，这个函数会写的非常长，而且还可能存在自定的类型，也就是说这个判断日后可能还要一直改，因为无法知道未知值到底属于什么类型。
无法透视一个未知类型的时候，以上代码其实不是很合理，这时候就需要有反射来帮忙你处理，反射使用TypeOf和ValueOf函数从接口中获取目标对象的信息，轻松完成目的。
3.反射具体能做什么？
1.获取变量内部信息
reflect提供了两种类型来进行访问接口变量的内容：
reflect.ValueOf() 获取输入参数接口中的数据的值，如果为空则返回0 <- 注意是0
reflect.TypeOf() 动态获取输入参数接口中的值的类型，如果为空则返回nil <- 注意是nil
package main
import (
“fmt”
“reflect”
)
func main() {
var name string = “咖啡色的羊驼”
// TypeOf会返回目标数据的类型，比如int/float/struct/指针等
reflectType := reflect.TypeOf(name)
// valueOf返回目标数据的的值，比如上文的”咖啡色的羊驼”
reflectValue := reflect.ValueOf(name)
fmt.Println(“type: “, reflectType)
fmt.Println(“value: “, reflectValue)
}
更深一层：在以上操作发生的时候，反射将“接口类型的变量”转为了“反射的接口类型的变量”，比如上文实际上返回的是reflect.Value和reflect.Type的接口对象。（可以根据ide跟踪一下相关函数返回类型便知）
2.struct的反射
package main
import (
“fmt”
“reflect”
)
type Student struct {
Id int
Name string
}
func (s Student) Hello(){
fmt.Println(“我是一个学生”)
}
func main() {
s := Student{Id: 1, Name: “咖啡色的羊驼”}
// 获取目标对象
t := reflect.TypeOf(s)
// .Name()可以获取取这个类型的名称
fmt.Println(“这个类型的名称是:”, t.Name())
// 获取目标对象的值类型
v := reflect.ValueOf(s)
// .NumField()来获取其包含的字段的总数
for i := 0; i < t.NumField(); i++ {
// 从0开始获取Student所包含的key
key := t.Field(i)
// 通过interface方法来获取key所对应的值
value := v.Field(i).Interface()
fmt.Printf(“第%d个字段是：%s:%v = %v n”, i+1, key.Name, key.Type, value)
}
// 通过.NumMethod()来获取Student里头的方法
for i:=0;i<t.NumMethod(); i++ {
m := t.Method(i)
fmt.Printf(“第%d个方法是：%s:%vn”, i+1, m.Name, m.Type)
}
}
3.匿名或嵌入字段的反射
package main
import (
“reflect”
“fmt”
)
type Student struct {
Id int
Name string
}
type People struct {
Student // 匿名字段
}
func main() {
p := People{Student{Id: 1, Name: “咖啡色的羊驼”}}
t := reflect.TypeOf(p)
// 这里需要加一个#号，可以把struct的详情都给打印出来
// 会发现有Anonymous:true，说明是匿名字段
fmt.Printf(“%#vn”, t.Field(0))
// 取出这个学生的名字的详情打印出来
fmt.Printf(“%#vn”, t.FieldByIndex([]int{0, 1}))
// 获取匿名字段的值的详情
v := reflect.ValueOf(p)
fmt.Printf(“%#vn”, v.Field(0))
}
4.判断传入的类型是否是我们想要的类型
package main
import (
“reflect”
“fmt”
)
type Student struct {
Id int
Name string
}
func main() {
s := Student{Id: 1, Name: “咖啡色的羊驼”}
t := reflect.TypeOf(s)
// 通过.Kind()来判断对比的值是否是struct类型
if k := t.Kind(); k == reflect.Struct {
fmt.Println(“bingo”)
}
num := 1;
numType := reflect.TypeOf(num)
if k := numType.Kind(); k == reflect.Int {
fmt.Println(“bingo”)
}
}
5.通过反射修改内容
package main
import (
“reflect”
“fmt”
)
type Student struct {
Id int
Name string
}
func main() {
s := &Student{Id: 1, Name: “咖啡色的羊驼”}
v := reflect.ValueOf(s)
// 修改值必须是指针类型否则不可行
if v.Kind() != reflect.Ptr {
fmt.Println(“不是指针类型，没法进行修改操作”)
return
}
// 获取指针所指向的元素
v = v.Elem()
// 获取目标key的Value的封装
name := v.FieldByName(“Name”)
if name.Kind() == reflect.String {
name.SetString(“小学生”)
}
fmt.Printf(“%#v n”, *s)
// 如果是整型的话
test := 888
testV := reflect.ValueOf(&test)
testV.Elem().SetInt(666)
fmt.Println(test)
}
6.通过反射调用方法
package main
import (
“fmt”
“reflect”
)
type Student struct {
Id int
Name string
}
func (s Student) EchoName(name string){
fmt.Println(“我的名字是：”, name)
}
func main() {
s := Student{Id: 1, Name: “咖啡色的羊驼”}
v := reflect.ValueOf(s)
// 获取方法控制权
// 官方解释：返回v的名为name的方法的已绑定（到v的持有值的）状态的函数形式的Value封装
mv := v.MethodByName(“EchoName”)
// 拼凑参数
args := []reflect.Value{reflect.ValueOf(“咖啡色的羊驼”)}
// 调用函数
mv.Call(args)
}
##4.反射的一些小点
1.使用反射时需要先确定要操作的值是否是期望的类型，是否是可以进行“赋值”操作的，否则reflect包将会毫不留情的产生一个panic。
2.反射主要与Golang的interface类型相关，只有interface类型才有反射一说。如果有兴趣可以看一下TypeOf和ValueOf，会发现其实传入参数的时候已经被转为接口类型了。
// 以下为截取的源代码
func TypeOf(i interface{}) Type {
eface := *(*emptyInterface)(unsafe.Pointer(&i))
return toType(eface.typ)
}
func ValueOf(i interface{}) Value {
if i == nil {
return Value{}
}
escapes(i)
return unpackEface(i)
}
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