soumya92/barista

// Copyright 2018 Google Inc.
//
// 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 multicast provides a method to convert any bar.Module into one that
// can be added to the bar multiple times. When combined with group.Simple, this
// allows multiple combinations of modules without creating extra instances.
package multicast // import "barista.run/modules/meta/multicast"

import (
    "sync"

    "barista.run/bar"
    "barista.run/base/value"
    "barista.run/core"
    "barista.run/sink"
)

type module struct {
    *value.Value
    start func() // called on Stream(), to ensure backing module is started
}

// Stream starts the module, and tries to stream the original module as well.
func (m module) Stream(sink bar.Sink) {
    go m.start()
    for {
        next := m.Next()
        s, _ := m.Get().(bar.Segments)
        sink.Output(s)
        <-next
    }
}

// New creates a multicast module that can be added to the bar any number of
// times, and mirrors the output of the original module at each location.
// IMPORTANT: The original module must not be added to the bar.
func New(original bar.Module) bar.Module {
    output, sink := sink.Value()
    coreModule := core.NewModule(original)
    var once sync.Once
    start := func() {
        once.Do(func() {
            coreModule.Stream(sink)
        })
    }
    return module{output, start}
}