Go组件学习——cron定时器

package main

import (
	"fmt"
	"github.com/robfig/cron"
	"time"
)

func main() {
	c := cron.New()
	c.AddFunc("*/3 * * * * *", func() {
		fmt.Println("every 3 seconds executing")
	})

	go c.Start()
	defer c.Stop()


	select {
	case <-time.After(time.Second * 10):
		return
	}
}

// AddFunc adds a func to the Cron to be run on the given schedule.
func (c *Cron) AddFunc(spec string, cmd func()) error {
	return c.AddJob(spec, FuncJob(cmd))
}

// AddJob adds a Job to the Cron to be run on the given schedule.
func (c *Cron) AddJob(spec string, cmd Job) error {
	schedule, err := Parse(spec)
	if err != nil {
		return err
	}
	c.Schedule(schedule, cmd)
	return nil
}

// Parse returns a new crontab schedule representing the given spec.
// It returns a descriptive error if the spec is not valid.
// It accepts crontab specs and features configured by NewParser.
func (p Parser) Parse(spec string) (Schedule, error) {
	if len(spec) == 0 {
		return nil, fmt.Errorf("Empty spec string")
	}
	if spec[0] == '@' && p.options&Descriptor > 0 {
		return parseDescriptor(spec)
	}

	// Figure out how many fields we need
	max := 0
	for _, place := range places {
		if p.options&place > 0 {
			max++
		}
	}
	min := max - p.optionals

	// Split fields on whitespace
	fields := strings.Fields(spec)	// 使用空白符拆分cron表达式

	// Validate number of fields
	if count := len(fields); count < min || count > max {
		if min == max {
			return nil, fmt.Errorf("Expected exactly %d fields, found %d: %s", min, count, spec)
		}
		return nil, fmt.Errorf("Expected %d to %d fields, found %d: %s", min, max, count, spec)
	}

	// Fill in missing fields
	fields = expandFields(fields, p.options)

	var err error
	field := func(field string, r bounds) uint64 {	// 抽象出filed函数，方便下面调用
		if err != nil {
			return 0
		}
		var bits uint64
		bits, err = getField(field, r)
		return bits
	}

	var (
		second     = field(fields[0], seconds)
		minute     = field(fields[1], minutes)
		hour       = field(fields[2], hours)
		dayofmonth = field(fields[3], dom)
		month      = field(fields[4], months)
		dayofweek  = field(fields[5], dow)
	)
	if err != nil {
		return nil, err
	}

	return &SpecSchedule{
		Second: second,
		Minute: minute,
		Hour:   hour,
		Dom:    dayofmonth,
		Month:  month,
		Dow:    dayofweek,
	}, nil
}

// The Schedule describes a job's duty cycle.
type Schedule interface {
	// Return the next activation time, later than the given time.
	// Next is invoked initially, and then each time the job is run.
	Next(time.Time) time.Time
}

// Start the cron scheduler in its own go-routine, or no-op if already started.
func (c *Cron) Start() {
	if c.running {
		return
	}
	c.running = true
	go c.run()
}

// Run the scheduler. this is private just due to the need to synchronize
// access to the 'running' state variable.
func (c *Cron) run() {
	// Figure out the next activation times for each entry.
	now := c.now()
	for _, entry := range c.entries {
		entry.Next = entry.Schedule.Next(now)
	}

	for {
		// Determine the next entry to run.
		sort.Sort(byTime(c.entries))

		var timer *time.Timer
		if len(c.entries) == 0 || c.entries[0].Next.IsZero() {	// 如果没有要执行的任务或者第一个任务的待执行时间为空，则睡眠
			// If there are no entries yet, just sleep - it still handles new entries
			// and stop requests.
			timer = time.NewTimer(100000 * time.Hour)
		} else {
			timer = time.NewTimer(c.entries[0].Next.Sub(now))	// 否则新建一个距离现在到下一个要触发执行的Timer
		}

		for {
			select {
			case now = <-timer.C:	// 触发时间到，执行任务
				now = now.In(c.location)
				// Run every entry whose next time was less than now
				for _, e := range c.entries {
					if e.Next.After(now) || e.Next.IsZero() {
						break
					}
					go c.runWithRecovery(e.Job)
					e.Prev = e.Next
					e.Next = e.Schedule.Next(now)
				}

			case newEntry := <-c.add:	// 添加任务
				timer.Stop()
				now = c.now()
				newEntry.Next = newEntry.Schedule.Next(now)
				c.entries = append(c.entries, newEntry)

			case <-c.snapshot:	// 调用c.Entries()返回一个现有任务列表的snapshot
				c.snapshot <- c.entrySnapshot()
				continue

			case <-c.stop:	// 任务结束，退出
				timer.Stop()
				return
			}

			break
		}
	}
}

package main

import (
	"fmt"
	"time"
)

func main() {
	timer1 := time.NewTimer(2 * time.Second)

	<-timer1.C
	fmt.Println("Timer 1 expired")

	timer2 := time.NewTimer(time.Second)
	go func() {
		<-timer2.C
		fmt.Println("Timer 2 expired")
	}()

	stop2 := timer2.Stop()
	if stop2 {
		fmt.Println("Timer 2 stopped")
	}
}

Timer 1 expired
Timer 2 stopped

case newEntry := <-c.add:	// 添加任务
	timer.Stop()
	now = c.now()
	newEntry.Next = newEntry.Schedule.Next(now)
	c.entries = append(c.entries, newEntry)

case now = <-timer.C:	// 触发时间到，执行任务
	now = now.In(c.location)
    // Run every entry whose next time was less than now
    for _, e := range c.entries {
    	if e.Next.After(now) || e.Next.IsZero() {
    		break
    	}
    go c.runWithRecovery(e.Job)
    e.Prev = e.Next
    e.Next = e.Schedule.Next(now)
    }

func (c *Cron) runWithRecovery(j Job) {
	defer func() {
		if r := recover(); r != nil {
			const size = 64 << 10
			buf := make([]byte, size)
			buf = buf[:runtime.Stack(buf, false)]
			c.logf("cron: panic running job: %v\n%s", r, buf)
		}
	}()
	j.Run()
}

func (f FuncJob) Run() { f() }

func () {
    fmt.Println("test")
}()