func (chain *ObserverChain) Notify(evt Event) {
	for _, ob := range chain.obs {
		ob.Notify(evt)
	}
}
Notify each observer
package progress
import "fmt"
type Event interface {
	fmt.Stringer
}
type Observer interface {
	Notify(evt Event)
}
type ObserverChain struct {
	obs []Observer
}