// A WaitGroup waits for a collection of goroutines to finish.
// The main goroutine calls Add to set the number of
// goroutines to wait for. Then each of the goroutines
// runs and calls Done when finished. At the same time,
// Wait can be used to block until all goroutines have finished.
//
// A WaitGroup must not be copied after first use.
type WaitGroup struct {
   noCopy noCopy

   // 64-bit value: high 32 bits are counter, low 32 bits are waiter count.
   // 64-bit atomic operations require 64-bit alignment, but 32-bit
   // compilers do not ensure it. So we allocate 12 bytes and then use
   // the aligned 8 bytes in them as state.
   state1 [12]byte
   sema   uint32
}

示例代碼：

package main

import (
    "sync"
    "fmt"
)

func doWorker(id int, ch chan int, wg *sync.WaitGroup) {
    for n := range ch {
        fmt.Printf("Worker %d received %c\n", id, n)
        wg.Done()    // 減少一個計數
    }
}

type worker struct {
    in chan int
    wg *sync.WaitGroup
}

func createWorker(id 
 
int, wg *sync.WaitGroup) worker {
    w := worker{
        in: make(chan int),
        wg: wg,
    }

    go doWorker(id, w.in, wg)
    return w
}

func chanDemo() {
    var wg sync.WaitGroup

    var workers [10]worker

    for i:=0; i<10; i++ {
        workers[i] = createWorker(i, &wg)
    }


    
 
for i, worker := range workers {
        wg.Add(1)      // 添加一個計數
        worker.in <- ‘a‘ + i
    }
    wg.Wait()    // 阻塞，等待所有任務完成

}
func main() {
    chanDemo()
}

// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct {
   state int32
   sema  uint32
}

示例代碼：

package main

import (
    "sync"
    "fmt"
)

var x = 0

func increment(wg *sync.WaitGroup, mu *sync.Mutex) {
    mu.Lock()
    x++
    mu.Unlock()
    wg.Done()
}

func main() {
    var wg sync.WaitGroup
    var mu sync.Mutex

    for i := 0; i < 1000; i++ {
        wg.Add(1)
        go increment(&wg, &mu)
    }
    wg.Wait()
    fmt.Println("final value of x", x)
}

Go sync.WaitGroup 等待Goroutine執行完成