golang waitgroup 及 sync.Once 示例

下面代码修改自golang示例源码

package main

import (
"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.WaitGroup
var 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.Once
onceBody := 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()
}