For-Select-Done

我们应该防止程序中发生任何泄露。所以我们应该对于留在程序中的go例程发送信号，让它知道它可以退出。
最常见的就是将for-select循环与通道结合起来，向go程序发送一个关闭信号。我们称它为“完成”通道。

func printIntegers(done <-chan struct{}, intStream <-chan int) {
	for{
		 select {
		 case i := <-intStream:
		 fmt.Println(i)
		 case <-done:
		 return
		 }
	 }
}

没有看懂，先记在这里……

扇入模式

func fanIn(ctx context.Context, fetchers ...<-chan interface{}) <-chan interface{} {
 combinedFetcher := make(chan interface{})
 // 1
 var wg sync.WaitGroup
 wg.Add(len(fetchers))
 
 // 2
 for _, f := range fetchers {
 f := f
 go func() {
 // 3
 
 defer wg.Done()
 for{
 select{
 case res := <-f:
 combinedFetcher <- res
 case <-ctx.Done():
 return
 }
 }
 }()
 }
 
 // 4
 // Channel cleanup
 go func() {
 wg.Wait()
 close(combinedFetcher)
 } ()
 return combinedFetcher
}

从流中获取前 n 个值

func takeFirstN(ctx context.Context, dataSource <-chan interface{}, n int) <-chan interface{} {
 // 1
 takeChannel := make(chan interface{})
 
 // 2
 go func() {
 defer close(takeChannel)
 // 3 
 for i := 0; i< n; i++ {
 select {
 case val, ok := <-dataSource:
 if !ok{
 return
 }
 takeChannel <- val
 case <-ctx.Done():
 return
 }
 }
 }()
 return takeChannel
}

订阅模式

type Subscription interface {
 Updates() <-chan Item
}
//On the other hand, we are going to use another interface as an abstraction to fetch the data we 
//need:
//另一方面，我们将使用另一个接口作为抽象来获取我们需要的数据。
type Fetcher interface {
 Fetch() (Item, error)
}
//For each of these we are going to have a concrete type implementing them.
//对于其中的每一个，我们都将有一个具体的类型来实现它们。
//For the subscription:
//对于订阅来说。
func NewSubscription(ctx context.Context, fetcher Fetcher, freq int) Subscription {
 s := &sub{
 fetcher: fetcher,
 updates: make(chan Item),
 }
// Running the Task Supposed to fetch our data
 go s.serve(ctx, freq)
 return s
}
type sub struct {
 fetcher Fetcher
 updates chan Item
}
func (s *sub) Updates() <-chan Item {
 return s.updates
}
//We are going to go into more details about what happens inside the serve method.
我们将更详细地介绍服务方法内部发生的事情。
//For the fetcher:
//对于取物者来说。
func NewFetcher(uri string) Fetcher {
 f := &fetcher{
 uri: uri,
 }
 return f
}
type fetcher struct {
 uri string
}
//Inside the serve method

//The serve method consists of a for-select-done type of loop:
//服务方法由一个 for-select-done 类型的循环组成。
func (s *sub) serve(ctx context.Context, checkFrequency int) {
 clock := time.NewTicker(time.Duration(checkFrequency) * time.Second)
 type fetchResult struct {
 fetched Item
 err error
 }
 fetchDone := make(chan fetchResult, 1)
 
 for {
 select {
 // Clock that triggers the fetch
 case <-clock.C:
 go func() {
fetched, err := s.fetcher.Fetch()
fetchDone <- fetchResult{fetched, err}
 }()
 // Case where the fetch result is
 // Ready to be consumed
 case result := <-fetchDone:
 fetched := result.fetched
 if result.err != nil {
 log.Println("Fetch error: %v \n Waiting the next iteration", result.err.Error())
break
 }
 s.updates <-fetched
 // Case where we need to close the server
 case <-ctx.Done():
 return
 }
 }
}

有一个修正，暂时先不提。

地图模式

func Map(done <-chan struct{}, inputStream <-chan int, operator func(int)int) <-chan int {
 // 1
 mappedStream := make(chan int)
 go func() {
 defer close(mappedStream)
 // 2
 for {
 select {
 case <-done:
 return
 // 3
 case i, ok := <-inputStream:
 if !ok { return }
 //4
 select {
 case <-done:
 return
 case mappedStream <- operator(i):
 }
 }
 }
 }()
 return mappedStream

func main() {
 done := make(chan struct{})
 defer close(done)
 
 // Generates a channel sending integers
 // From 0 to 9
 range10 := rangeChannel(done, 10)
 
 multiplierBy10 := func(x int) int {
 return x * 10
 }
 for num := range Map(done, range10, multiplierBy10) {
 fmt.Println(num)
 }
}

过滤模式

func Filter(done <-chan struct{}, inputStream <-chan int, operator func(int)bool) <-chan int {
 filteredStream := make(chan int)
 go func() {
 defer close(filteredStream)
 
 for {
 select {
 case <-done:
 return
 case i, ok := <-inputStream:
 if !ok {
 return
 }
 
 if !operator(i) { break }
 select {
 case <-done:
 return
 case filteredStream <- i:
 }
 }
 }
 }()
 return filteredStream
}

func main() {
 done := make(chan struct{})
 defer close(done)
 
 // Generates a channel sending integers
 // From 0 to 9
 range10 := rangeChannel(done, 10)
 
 isEven := func(x int) bool {
 return x % 2 == 0
 }
 for num := range Filter(done, range10, isEven) {
 fmt.Println(num)
 }
}

这五个模式是构建更大、更复杂的 Golang 应用程序的基石。这些解决方案可以解决你在处
理 GO 中的并发问题时可能遇到的问题。此外，你还可以在此基础上修改、扩展和创建新的
模式。