Go并发编程之sync.Once使用实例详解

package main
 
import (
	"fmt"
	"sync"
)
 
func main() {
	var (
		once sync.Once
		wg   sync.WaitGroup
	)
 
	for i := 0; i < 10; i++ {
		wg.Add(1)
		// 这里要注意讲i显示的当参数传入内部的匿名函数
		go func(i int) {
			defer wg.Done()
			// fmt.Println("once", i)
			once.Do(func() {
				fmt.Println("once", i)
			})
		}(i)
	}
 
	wg.Wait()
	fmt.Printf("over")
}

type Once struct {
    done uint32
    m    Mutex
}

func (o *Once) Do(f func()) {
	// Note: Here is an incorrect implementation of Do:
	//
	//	if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
	//		f()
	//	}
	//
	// Do guarantees that when it returns, f has finished.
	// This implementation would not implement that guarantee:
	// given two simultaneous calls, the winner of the cas would
	// call f, and the second would return immediately, without
	// waiting for the first's call to f to complete.
	// This is why the slow path falls back to a mutex, and why
	// the atomic.StoreUint32 must be delayed until after f returns.
 
	if atomic.LoadUint32(&o.done) == 0 {
		// Outlined slow-path to allow inlining of the fast-path.
		o.doSlow(f)
	}
}

if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
	f()
}

if atomic.LoadUint32(&o.done) == 0 {
    // Outlined slow-path to allow inlining of the fast-path.
    o.doSlow(f)
}

func (o *Once) doSlow(f func()) {
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 {
		defer atomic.StoreUint32(&o.done, 1)
		f()
	}
}

type singleton struct {}
 
var (
    instance    *singleton
    initialized uint32
    mu          sync.Mutex
)
 
func Instance() *singleton {
    if atomic.LoadUint32(&initialized) == 1 {
        return instance
    }
 
    mu.Lock()
    defer mu.Unlock()
 
    if instance == nil {
        defer atomic.StoreUint32(&initialized, 1)
        instance = &singleton{}
    }
    return instance
}

type singleton struct {}
 
var (
    instance *singleton
    once     sync.Once
)
 
func Instance() *singleton {
    once.Do(func() {
        instance = &singleton{}
    })
    return instance
}

var icons map[string]image.Image
 
func loadIcons() {
    icons = map[string]image.Image{
        "left":  loadIcon("left.png"),
        "up":    loadIcon("up.png"),
        "right": loadIcon("right.png"),
        "down":  loadIcon("down.png"),
    }
}
 
// Icon 被多个goroutine调用时不是并发安全的
// 因为map类型本就不是类型安全数据结构
func Icon(name string) image.Image {
    if icons == nil {
        loadIcons()
    }
    return icons[name]
}

var icons map[string]image.Image
 
var loadIconsOnce sync.Once
 
func loadIcons() {
    icons = map[string]image.Image{
        "left":  loadIcon("left.png"),
        "up":    loadIcon("up.png"),
        "right": loadIcon("right.png"),
        "down":  loadIcon("down.png"),
    }
}
 
// Icon 是并发安全的,并且保证了在代码运行的时候才会加载配置
func Icon(name string) image.Image {
    loadIconsOnce.Do(loadIcons)
    return icons[name]
}