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propertyobserver.go
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propertyobserver.go
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package main
import (
"container/list"
"fmt"
)
type Observable struct {
subs *list.List
}
func (o *Observable) Subscribe(x Observer) {
o.subs.PushBack(x)
}
func (o *Observable) Unsubscribe(x Observer) {
for z := o.subs.Front(); z != nil; z = z.Next() {
if z.Value.(Observer) == x {
o.subs.Remove(z)
}
}
}
func (o *Observable) Fire(data interface{}) {
for z := o.subs.Front(); z != nil; z = z.Next() {
z.Value.(Observer).Notify(data)
}
}
// ---------------
// Observer that knows
// how to send notification
// ---------------
type Observer interface {
Notify(data interface{})
}
// ---------------
// PropertyChange can
// communicate with struct's properties
// access it's old value and new value.
// The setter and getter are below
// But we may have problems with
// property dependencies
// (see propertydependencies.go)
// ---------------
type PropertyChange struct {
Name string //property string
NewValue interface{} //new value for the property
}
type Person struct {
Observable
age int //private property we want to get notified if changed
}
func NewPerson(age int) *Person {
return &Person{
Observable: Observable{new(list.List)},
age: age,
}
}
//Age is a getter for age member
func (p *Person) Age() int { return p.age }
//SetAge is a setter for age member
func (p *Person) SetAge(newAge int) {
if newAge == p.age {
return
}
p.age = newAge
//notify about changing in age
p.Fire(PropertyChange{"Age", p.age})
}
// ---------------
// Real-life Example:
// TrafficManagement notification
// ---------------
type TrafficManagement struct {
o Observable
}
func (t *TrafficManagement) Notify(data interface{}) {
if pc, ok := data.(PropertyChange); ok {
if pc.NewValue.(int) >= 16 {
fmt.Println("Congrats, you can drive now!")
// we no longer care
t.o.Unsubscribe(t)
}
}
}
func main() {
p := NewPerson(15)
t := &TrafficManagement{p.Observable}
p.Subscribe(t)
for i := 16; i <= 20; i++ {
fmt.Println("Setting age to", i)
p.SetAge(i)
}
}