driver

type State interface {
	// Value returns the current variable value.
	Value() (interface{}, error)
	// UpdateTime returns the update time for the variable.
	UpdateTime() time.Time
}

type Watcher interface {
	// WatchVariable returns the current State of the variable.
	// If the State has not changed, it returns nil.
	//
	// If WatchVariable returns a wait time > 0, the concrete type uses
	// it as a hint to not call WatchVariable again for the wait time.
	//
	// Implementations *may* block, but must return if ctx is Done. If the
	// variable has changed, then implementations *must* eventually return
	// it.
	//
	// A polling implementation should return (State, <poll interval>) for
	// a new State, or (nil, <poll interval>) if State hasn't changed.
	//
	// An implementation that receives notifications from an external source
	// about changes to the underlying variable should:
	// 1. If prev != nil, subscribe to change notifications.
	// 2. Fetch the current State.
	// 3. If prev == nil or if the State has changed, return (State, 0).
	//    A non-zero wait should be returned if State holds an error, to avoid
	//    spinning.
	// 4. Block until it detects a change or ctx is Done, then fetch and return
	//    (State, 0).
	// Note that the subscription in 1 must occur before 2 to avoid race conditions.
	WatchVariable(ctx context.Context, prev State) (state State, wait time.Duration)

	// Close cleans up any resources used by the Watcher object.
	Close() error
}