英文:
How to access to the global variable from an anonymous function in the Go?
问题
在匿名函数中如何更改test变量的值？谢谢！
英文:
I have a code:
var test string
func main() {
http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
test = "index"
})
fmt.Println(test)
if error := http.ListenAndServe(":9001", nil); error != nil {
log.Fatal("Error!", error)
}
}
How to change the value of the test variable in the anonymous function? I will be thankful!
答案1
得分: 5
HTTP处理程序确实更改了全局变量；但是你没有保护对全局变量的访问，因此存在竞态条件。也就是说，在你的“fmt.Println(test)”运行之前，"http.HandleFunc"已经运行了。
我假设这只是一个玩具示例：如果你想要改变你的代码，等待值被改变/等待HTTP请求结束后再终止，你可以这样做：
var test string
var doneCh = make(chan bool, 1)
func main() {
http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
test = "index"
select {
case doneCh <- true:
default:
}
})
go func() {
<-doneCh
fmt.Println(test)
}()
if error := http.ListenAndServe(":9001", nil); error != nil {
log.Fatal("Error!", error)
}
}
这里使用一个通道来保护“test”的状态。在真实的代码中，更可能使用sync.Once或sync.Mutex。另外，我应该提到（希望你已经意识到）当可能时，应该尽量避免改变全局状态。
英文:
The HTTP handler does change your global variable; however you don't protect access to the global variable and thus have a race condition. That is to say fmt.Println(test) runs before your http.HandleFunc.
I'm assuming this is a toy example: If you want to change your code to wait for the value to be changed / wait for an HTTP hit, before terminating, then you could do this:
var test string
var doneCh = make(chan bool, 1)
func main() {
http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
test = "index"
select {
case doneCh <- true:
default:
}
})
go func() {
<-doneCh
fmt.Println(test)
}()
if error := http.ListenAndServe(":9001", nil); error != nil {
log.Fatal("Error!", error)
}
}
This uses a channel to guard the state of test. In real code it would be more likely to use sync.Once or sync.Mutex. Also, I should mention (and I hope you already realize) mutating global state is always something to be avoided when possible.
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