func main() {
    // 輸出奇數(shù)
    printOdd := func() {
        for i := 1; i <= 10; i += 2 {
            fmt.Println(i)
            time.Sleep(100 * time.Millisecond)
        }
    }

    // 輸出偶數(shù)
    printEven := func() {
        for i := 2; i <= 10; i += 2 {
            fmt.Println(i)
            time.Sleep(100 * time.Millisecond)
        }
    }

    go printOdd()
    go printEven()

    // 阻塞等待
    time.Sleep(time.Second)
}

func main() {
    wg := sync.WaitGroup{}
    // 輸出奇數(shù)
    printOdd := func() {
        defer wg.Done()
        for i := 1; i <= 10; i += 2 {
            fmt.Printf("%d ", i)
            time.Sleep(100 * time.Millisecond)
        }
    }
    // 輸出偶數(shù)
    printEven := func() {
        defer wg.Done()
        for i := 2; i <= 10; i += 2 {
            fmt.Printf("%d ", i)
            time.Sleep(100 * time.Millisecond)
        }
    }
    wg.Add(2)
    go printOdd()
    go printEven()
    // 阻塞等待
    fmt.Println("waiting...")
    wg.Wait()
    fmt.Println("\nfinish...")
}

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 only guarantee that 64-bit fields are 32-bit aligned.
   // For this reason on 32 bit architectures we need to check in state()
   // if state1 is aligned or not, and dynamically "swap" the field order if
   // needed.
   state1 uint64
   state2 uint32
}

func (wg *WaitGroup) Add(delta int) {
   ...
   state := atomic.AddUint64(statep, uint64(delta)<<32)
   ...
}
func (wg *WaitGroup) Done() {
   wg.Add(-1)
}
func (wg *WaitGroup) Wait() {
   ...
   for {
      state := atomic.LoadUint64(statep)
      v := int32(state >> 32)
      w := uint32(state)
      if v == 0 {
         // Counter is 0, no need to wait.
         if race.Enabled {
            race.Enable()
            race.Acquire(unsafe.Pointer(wg))
         }
         return
      }
      // Increment waiters count.
      if atomic.CompareAndSwapUint64(statep, state, state+1) {
         if race.Enabled && w == 0 {
            // Wait must be synchronized with the first Add.
            // Need to model this is as a write to race with the read in Add.
            // As a consequence, can do the write only for the first waiter,
            // otherwise concurrent Waits will race with each other.
            race.Write(unsafe.Pointer(semap))
         }
         runtime_Semacquire(semap)
         if *statep != 0 {
            panic("sync: WaitGroup is reused before previous Wait has returned")
         }
         if race.Enabled {
            race.Enable()
            race.Acquire(unsafe.Pointer(wg))
         }
         return
      }
   }
}