Subscription as iterator to reduce goroutine usage

[NateScarlet]

Currently, we need 2+ goroutine for each subscription (1 read by gqlgen, 1 write by resolver, more for data transform).

If we use iterator func(yield func(T, error) bool) as return value ,
then resolver can push item and do data transform directly in yield.

So required goroutine for each subscription can be reduced to 1.

This also allows return error during subscription without using transport.AddSubscriptionError

current subscription resolver

require 1 goroutine

func (r *subscriptionResolver) CurrentTime(ctx context.Context) (<-chan *model.Time, error) {
	ch := make(chan *model.Time)
	go func() {
		defer close(ch)
		for {
			time.Sleep(1 * time.Second)
			fmt.Println("Tick")
			currentTime := time.Now()
			t := &model.Time{
				UnixTime:  int(currentTime.Unix()),
				TimeStamp: currentTime.Format(time.RFC3339),
			}
			select {
			case <-ctx.Done():
			case ch <- t:
			}
		}
	}()
	return ch, nil
}

proposed subscription resolver

no goroutine needed

func (r *subscriptionResolver) CurrentTime(ctx context.Context) func(func(*model.Time, error) bool) {
	return func(yield func(*model.Time, error) bool) {
		var ticker = time.NewTicker(1 * time.Second)
		defer ticker.Stop()
		for {
			select {
			case <-ctx.Done():
				return
			case <-ticker.C:
				fmt.Println("Tick")
			}
			currentTime := time.Now()
			t := &model.Time{
				UnixTime:  int(currentTime.Unix()),
				TimeStamp: currentTime.Format(time.RFC3339),
			}
			var err error // can returns error during subscription
			if !yield(t, err) {
				return
			}
		}
	}
}