golang waitgroup 及 sync.Once 樣本

這是一個建立於 的文章，其中的資訊可能已經有所發展或是發生改變。

下面代碼修改自golang樣本源碼

package mainimport ("fmt""sync""net/http""io/ioutil""log")func GetDemo(addr string) {res, err := http.Get(addr)if err != nil {log.Fatal(err)}robots, err := ioutil.ReadAll(res.Body)res.Body.Close()if err != nil {log.Fatal(err)}fmt.Printf("%s", robots)}// This example fetches several URLs concurrently,// using a WaitGroup to block until all the fetches are complete.func ExampleWaitGroup() {var wg sync.WaitGroupvar urls = []string{"http://www.golang.org/","http://www.google.com/","http://www.baidu.com/",}for _, url := range urls {// Increment the WaitGroup counter.wg.Add(1)// Launch a goroutine to fetch the URL.go func(url string) {// Decrement the counter when the goroutine completes.defer wg.Done()// Fetch the URL.GetDemo(url)}(url)}// Wait for all HTTP fetches to complete.wg.Wait()fmt.Println("--------------------group wait over ---------------------------")}func ExampleOnce() {var once sync.OnceonceBody := func() {fmt.Println("Only once")}done := make(chan bool)for i := 0; i < 10; i++ {go func() {once.Do(onceBody)done <- true}()}for i := 0; i < 10; i++ {<-done}fmt.Println("once over -------------------------------")// Output:// Only once}func main() {ExampleOnce()ExampleWaitGroup()}