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2013-06-19T16:28:08.890
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<a href="http://play.golang.org/p/THNb3C-TLq" rel="nofollow">http://play.golang.org/p/THNb3C-TLq</a> <pre><code>package main import ( "fmt" ) type ChangeHandler func(interface{}) type EventedChanger interface { Get(name string) interface{} Set(name string, value interface{}) EventedChanger OnChange(name string, listener ChangeHandler) EventedChanger } type MyChanger struct { data map[string]interface{} listeners map[string][]ChangeHandler } func (m *MyChanger) Get(name string) interface{} { val, ok := m.data[name] if !ok { return nil } return val } func (m *MyChanger) Set(name string, value interface{}) EventedChanger { m.data[name] = value if listeners, ok := m.listeners[name]; ok { for _, l := range listeners { l(value) } } return m } func (m *MyChanger) OnChange(name string, listener ChangeHandler) EventedChanger { m.listeners[name] = append(m.listeners[name], listener) return m } func NewMyChanger() *MyChanger { return &MyChanger{ make(map[string]interface{}), make(map[string][]ChangeHandler), } } func main() { c := NewMyChanger() h := func(value interface{}) { c.Set("fullname", fmt.Sprint(c.Get("firstname"), c.Get("lastname"))) } q := func(value interface{}) { fmt.Println("Full name:", value) } c.OnChange("firstname", h).OnChange("lastname", h).OnChange("fullname", q) c.Set("firstname", "Walter").Set("lastname", "Smith") } </code></pre> Output is: <pre><code>Full name: Walter <nil> Full name: Walter Smith Program exited. </code></pre> You could improve it by making it concurrent and/or execute handlers in parallel for instance. EDIT: <a href="http://play.golang.org/p/msgaBXQwt_" rel="nofollow">http://play.golang.org/p/msgaBXQwt_</a> I have made a more generic version of this to comply with your requirements of being lazy and cached: <pre><code>package main import ( "fmt" ) type Getter func(string) interface{} type Setter func(string, interface{}) type GetSetter interface { Get(string) interface{} Set(string, interface{}) GetSetter RegisterGetter(string, Getter) GetSetter RegisterSetter(string, Setter) GetSetter } type LazyGetSetter struct { data map[string]interface{} getters map[string]Getter setters map[string]Setter } func NewLazyGetSetter() *LazyGetSetter { return &LazyGetSetter{ make(map[string]interface{}), make(map[string]Getter), make(map[string]Setter), } } func (l *LazyGetSetter) Get(name string) interface{} { if getter, ok := l.getters[name]; ok { return getter(name) } if val, ok := l.data[name]; ok { return val } return nil } func (l *LazyGetSetter) Set(name string, value interface{}) *LazyGetSetter { if setter, ok := l.setters[name]; ok { setter(name, value) } else { l.data[name] = value } return l } func (l *LazyGetSetter) RegisterGetter(name string, getter Getter) *LazyGetSetter { l.getters[name] = getter return l } func (l *LazyGetSetter) RegisterSetter(name string, setter Setter) *LazyGetSetter { l.setters[name] = setter return l } type CachedLazyGetSetter struct { *LazyGetSetter cache map[string]interface{} } func NewCachedLazyGetSetter() *CachedLazyGetSetter { return &CachedLazyGetSetter{ NewLazyGetSetter(), make(map[string]interface{}), } } func (c *CachedLazyGetSetter) Cache(name string, value interface{}) *CachedLazyGetSetter { c.cache[name] = value return c } func (c *CachedLazyGetSetter) FetchCache(name string) interface{} { if val, ok := c.cache[name]; ok { return val } return nil } func main() { l := NewCachedLazyGetSetter() l.RegisterGetter("fullname", func(name string) interface{} { if cached := l.FetchCache(name); cached != nil { return cached } f := fmt.Sprintf("%s %s", l.Get("firstname"), l.Get("lastname")) l.Cache(name, f) return f }) l.Set("firstname", "Walter").Set("lastname", "Smith") fmt.Println(l.Get("fullname")) } </code></pre> As to your comment: a map lookup will outperform reflection by orders of magnitude. Cheers!
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