# 描述

sync.Once是 Golang package 中使方法只執行一次的物件實現，作用與init函式類似。但也有所不同。

• init函式是在檔案包首次被載入的時候執行，且只執行一次
• sync.Onc是在程式碼執行中需要的時候執行，且只執行一次

當一個函式不希望程式在一開始的時候就被執行的時候，我們可以使用sync.Once。

例如：

package main

import (
	"fmt"
	"sync"
)

func main() {
	var once sync.Once
	onceBody := func() {
		fmt.Println("Only once")
	}
	done := make(chan bool)
	for i := 0; i < 10; i++ {
		go func() {
			once.Do(onceBody)
			done <- true
		}()
	}
	for i := 0; i < 10; i++ {
		<-done
	}
}

# Output:
Only once
 

sync.Once使用變數done來記錄函式的執行狀態，使用sync.Mutex和sync.atomic來保證執行緒安全的讀取done。

# 原始碼

package sync

import (
	"sync/atomic"
)

// Once is an object that will perform exactly one action.
type Once struct {
	m    Mutex
	done uint32
}

// Do calls the function f if and only if Do is being called for the
// first time for this instance of Once. In other words, given
// 	var once Once
// if once.Do(f) is called multiple times, only the first call will invoke f,
// even if f has a different value in each invocation. A new instance of
// Once is required for each function to execute.
//
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
// 	config.once.Do(func() { config.init(filename) })
//
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
//
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
//
func (o *Once) Do(f func()) {
	if atomic.LoadUint32(&o.done) == 1 {
		return
	}
	// Slow-path.
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 {
		defer atomic.StoreUint32(&o.done, 1)
		f()
	}
}