[GO]关于go的waitgroup

watigroup是用来控制一组goroutine的，用来等待一组goroutine结束

比如关于kafka的消费者代码除了生硬的让程序等待一个小时，也可以这样写

package main

import (
    "sync"
    "github.com/Shopify/sarama"
    "strings"
    "fmt"
)

var wg sync.WaitGroup//用来等到一组goroutine结束

func main() {
    consumer, err := sarama.NewConsumer(strings.Split("localhost:9092", ","), nil)
    if err != nil {
        fmt.Println("failed to start consumer:%s", err)
        return
    }
    partitionList, err := consumer.Partitions("nginx_long")
    if err != nil {
        fmt.Println("failed to get the list of partitions:", err)
    }
    fmt.Println(partitionList)
    for partition := range partitionList{
        pc, err := consumer.ConsumePartition("nginx_log", int32(partition),sarama.OffsetNewest)
        if err != nil {
            fmt.Println("failed to start consumer for partition %d:%s\n", partition, err)
            return
        }
        defer pc.AsyncClose()
        go func(sarama.PartitionConsumer) { //kafka有几个分区就起几个goroutine
            wg.Add() //
            for msg := range pc.Messages(){//返回一个channel
                fmt.Printf("consumer Partition:%d, Offset:%d, key:%s, value:%s", msg.Partition, msg.Offset, string(msg.Key), string(msg.Value))
                fmt.Println()
            }
            wg.Done()
        }(pc)
    }
    wg.Wait()
    //time.Sleep(time.Hour)
    consumer.Close()
}

wg.add()用来加1，wg.done()用来减1，wg.wait()用来确认收到等待信号，程序可以继续执行，对于上面的例子：

1、kafka有几个分区，程序就会启动几个goroutine，在它的匿名函数里，每新起一个goroutine，wg.add()都会被执行

2、每一个for循环结束后，wg.done(）也会被执行，当wg.done()等于0时，wg.wait()收到信号，程序执行consumer.close()