1、cond结构介绍

// Cond implements a condition variable, a rendezvous point
// for goroutines waiting for or announcing the occurrence
// of an event.
// Cond结构实现一个条件变量和约定指针，为了等待协程或者通知一个事件的发生。
// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
// which must be held when changing the condition and
// when calling the Wait method.
// 每一个cond都有一个与之关联Locker L的锁（通常是读锁或者读写锁）
// 当条件改变条件和调用Wait方法时，这个锁是锁住的。
// A Cond must not be copied after first use.
// 这个Cond变量，在初始化之后，不能再被复制。
type Cond struct {
    noCopy noCopy
    // L is held while observing or changing the condition
    // 当状态一直是改变或者是观察的状态时，L将被锁住
       L Locker

    notify  notifyList  // 事件通知列表
    checker copyChecker   // 检查变量是否被copy
}

2、cond方法介绍

// NewCond returns a new Cond with Locker l.
// 实例化一个带有Locker的Cond变量
func NewCond(l Locker) *Cond {
    return &Cond{L: l}
}

// Wait atomically unlocks c.L and suspends execution
// of the calling goroutine. After later resuming execution,
// Wait locks c.L before returning. Unlike in other systems,
// Wait cannot return unless awoken by Broadcast or Signal.
// 等待释放锁和暂缓调用这个协程，在恢复执行之后，wait将一直锁住，
//这点不像其他操作系统，如果不调用Broadcast或者Signal, Wait将不返回
// Because c.L is not locked when Wait first resumes, the caller
// typically cannot assume that the condition is true when
// Wait returns. Instead, the caller should Wait in a loop:
// 因为Wait第一次恢复时，c.L是不被锁住的；当Wait返回时，调用者通常不认为这是真的
// 相反，调用者，应该在循环中等待。
//    c.L.Lock()
//    for !condition() {
//        c.Wait()
//    }
//    ... make use of condition ...
//    c.L.Unlock()
//
func (c *Cond) Wait() {
    c.checker.check()
    t := runtime_notifyListAdd(&c.notify)
    c.L.Unlock()
    runtime_notifyListWait(&c.notify, t)
    c.L.Lock()
}

// Signal wakes one goroutine waiting on c, if there is any.
// Signal 唤醒一个作用在c上等待的协程，无论是哪一个。
// It is allowed but not required for the caller to hold c.L
// during the call.
// 在调用期间，c.L 是不一定要持有这个c.L
func (c *Cond) Signal() {
    c.checker.check()
    runtime_notifyListNotifyOne(&c.notify)
}

// Broadcast wakes all goroutines waiting on c.
// Broadcast 唤醒所有作用于c上而等待的协程。
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Broadcast() {
    c.checker.check()
    runtime_notifyListNotifyAll(&c.notify)
}

3、代码示例

package main

import (
    "sync"
    "fmt"
    "time"
)

var locker = new(sync.Mutex)
var cond = sync.NewCond(locker)
func main()  {
    for i:=0; i < 5; i++ {
        go func(x int) {
            cond.L.Lock() // wait 前，必须要先加锁
            defer cond.L.Unlock()
            cond.Wait()
            fmt.Println(x)
            time.Sleep(time.Second * 1)
        }(i)
    }
    time.Sleep(time.Second*1)
    fmt.Println("Signal....")
    cond.Signal() // 下发一个通知给已经获取锁的goroutine
    time.Sleep(time.Second * 3)
    cond.Signal() // 3 秒之后，下发一个通知给已经获取锁的goroutine
    time.Sleep(time.Second *3)
    cond.Broadcast() // 3 秒
    fmt.Println("Broadcast...")
    time.Sleep(time.Second * 60)
}

5、应用场景

• 限流器

参考：类库dropbox中，rate_limiter.go