责任链（Chain of Responsibility）模式是一种行为设计模式，允许你将请求沿着处理者链进行发送。收到请求后，每个处理者均可对请求进行处理，或将其传递给链上的下个处理者。
以医院为例。在一个医院中会有如下职责成员：
- 挂号处
- 医生
- 收银处
- 药房
当病人来医院看病时，他会先去挂号处挂号，然后找医生看病，看完病后去收银处缴费，最后去药房拿药。在这个过程中，病人需要经过一个由四个职责部门组成的链条。病人在一个职责部门的事务结束后才能进入后续的职责部门，在这个过程中我们可以清晰地看到一条职责链。
现在可以说说责任链模式的适用场景了：
- 当一个请求需要经过多个环节的处理时；
- 因为有多个对象可以处理某个请求，但我们并不想让客户端选择处理请求的对象，同时我们还想将客户端和处理器解耦，此时我们可以选择职责链模式：客户端只需要和职责链中的第一个处理器接触就可以了。
代码示例：
package main
import (
"fmt"
)
// 病人
type patient struct {
name string
registrationDone bool
doctorCheckUpDone bool
medicineDone bool
paymentDone bool
}
// 医院部门接口
type department interface {
execute(*patient)
setNext(department)
}
// 挂号处
type reception struct {
next department
}
func (r *reception) execute(p *patient) {
if p.registrationDone {
fmt.Println("Patient registration already done")
r.next.execute(p)
return
}
fmt.Println("Reception registering patient")
p.registrationDone = true
if r.next != nil {
r.next.execute(p)
}
}
func (r *reception) setNext(next department) {
r.next = next
}
// 医生
type doctor struct {
next department
}
func (d *doctor) execute(p *patient) {
if p.doctorCheckUpDone {
fmt.Println("Doctor checkup already done")
d.next.execute(p)
return
}
fmt.Println("Doctor checking patient")
p.doctorCheckUpDone = true
if d.next != nil {
d.next.execute(p)
}
}
func (d *doctor) setNext(next department) {
d.next = next
}
// 收银处
type cashier struct {
next department
}
func (c *cashier) execute(p *patient) {
if p.paymentDone {
fmt.Println("Payment Done")
}
fmt.Println("Cashier getting money from patient patient")
p.paymentDone = true
if c.next != nil {
c.next.execute(p)
}
}
func (c *cashier) setNext(next department) {
c.next = next
}
// 药房
type medical struct {
next department
}
func (m *medical) execute(p *patient) {
if p.medicineDone {
fmt.Println("Medicine already given to patient")
m.next.execute(p)
return
}
fmt.Println("Medical giving medicine to patient")
p.medicineDone = true
if m.next != nil {
m.next.execute(p)
}
}
func (m *medical) setNext(next department) {
m.next = next
}
func main() {
reception := &reception{}
doctor := &doctor{}
cashier := &cashier{}
medical := &medical{}
//Set next for reception department
reception.setNext(doctor)
//Set next for doctor department
doctor.setNext(cashier)
//Set next for cashier department
cashier.setNext(medical)
//Patient visiting
patient := &patient{name: "abc"}
fmt.Printf("patient:%+v", patient)
reception.execute(patient)
fmt.Printf("patient:%+v", patient)
}
运行代码，输出结果：
patient:&{name:abc registrationDone:false doctorCheckUpDone:false medicineDone:false paymentDone:false}Reception registering patient
Doctor checking patient
Cashier getting money from patient patient
Medical giving medicine to patient
patient:&{name:abc registrationDone:true doctorCheckUpDone:true medicineDone:true paymentDone:true}
