filter.go

/*
Copyright 2014 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package watch

// FilterFunc should take an event, possibly modify it in some way, and return
// the modified event. If the event should be ignored, then return keep=false.
type FilterFunc func(in Event) (out Event, keep bool)

// Filter passes all events through f before allowing them to pass on.
// Putting a filter on a watch, as an unavoidable side-effect due to the way
// go channels work, effectively causes the watch's event channel to have its
// queue length increased by one.
//
// WARNING: filter has a fatal flaw, in that it can't properly update the
// Type field (Add/Modified/Deleted) to reflect items beginning to pass the
// filter when they previously didn't.
//
func Filter(w Interface, f FilterFunc) Interface {
	fw := &filteredWatch{
		incoming: w,
		result:   make(chan Event),
		f:        f,
	}
	go fw.loop()
	return fw
}

type filteredWatch struct {
	incoming Interface
	result   chan Event
	f        FilterFunc
}

// ResultChan returns a channel which will receive filtered events.
func (fw *filteredWatch) ResultChan() <-chan Event {
	return fw.result
}

// Stop stops the upstream watch, which will eventually stop this watch.
func (fw *filteredWatch) Stop() {
	fw.incoming.Stop()
}

// loop waits for new values, filters them, and resends them.
func (fw *filteredWatch) loop() {
	defer close(fw.result)
	for {
		event, ok := <-fw.incoming.ResultChan()
		if !ok {
			break
		}
		filtered, keep := fw.f(event)
		if keep {
			fw.result <- filtered
		}
	}
}