WaitGroup

它能够一直等到所有的goroutine执行完成，并且阻塞主线程的执行，直到所有的goroutine执行完成。

WaitGroup总共有三个方法

Add:添加或者减少等待goroutine的数量 Done:相当于Add(-1) Wait:执行阻塞，直到所有的WaitGroup数量变成0

实践

实例1跟实例2代码区别：少了一行代码go func() {}() 实例1和实例3代码区别：runtime.GOMAXPROCS的入参不一样

实例1(顺序输出)
package main

import (
				"fmt"
				"sync"
				"runtime"
)

func main() {
				N := 26
				runtime.GOMAXPROCS(1)
				var waitGroup  sync.WaitGroup
				waitGroup.Add(2*N)
				for i := 1; i <= N; i++ {
								go func(i int) {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												fmt.Printf("%c ", 96 + i)
								}(i)
				}


				for i := 1; i <= N; i++ {
								go func(i int)  {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												fmt.Printf("%c ", 64 + i)
								}(i)
				}
				go func() {}()
				waitGroup.Wait()
}

运行结果

a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
实例2（除了最后一个字母，其他字符顺序输出）
package main

import (
				"fmt"
				"sync"
				"runtime"
)

func main() {
				N := 26
				runtime.GOMAXPROCS(1)
				var waitGroup  sync.WaitGroup
				waitGroup.Add(2*N)
				for i := 1; i <= N; i++ {
								go func(i int) {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												fmt.Printf("%c ", 96 + i)
								}(i)
				}


				for i := 1; i <= N; i++ {
								go func(i int)  {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												fmt.Printf("%c ", 64 + i)
								}(i)
				}
				//go func() {}()
				waitGroup.Wait()
}

运行结果 Z a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y

实例3（乱序输出）
package main

import (
				"fmt"
				"sync"
				"runtime"
)

func main() {
				N := 26
				runtime.GOMAXPROCS(2)
				var waitGroup  sync.WaitGroup
				waitGroup.Add(2*N)
				for i := 1; i <= N; i++ {
								go func(i int) {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												fmt.Printf("%c ", 96 + i)
								}(i)
				}


				for i := 1; i <= N; i++ {
								go func(i int)  {
												//defer waitGroup.Add(-1)
												defer waitGroup.Done()
												runtime.Gosched()
												fmt.Printf("%c ", 64 + i)
								}(i)
				}
				go func() {}()
				waitGroup.Wait()
}

运行结果

c a b q d e f g h i j k r l m s n t o u p v w x K y z L M T N U O P V Q W R X S Y C Z D A B H E I F J G