I am using an infinite for loop with a label. Outside the scope of the for loop, I have a scheduled function running as a go routine. I want to break the for loop from the scheduled function when a certain condition is met. How can I accomplish the same ? This is what I am trying, which obviously won't work due to scope issue.
package main
import (
"fmt"
"time"
"sync"
)
func main() {
count := 0
var wg sync.WaitGroup
wg.Add(1)
t := time.NewTicker(time.Second*1)
go func (){
for {
fmt.Println("I will print every second", count)
count++
if count > 5 {
break myLoop;
wg.Done()
}
<-t.C
}
}()
i := 1
myLoop:
for {
fmt.Println("iteration", i)
i++
}
wg.Wait()
fmt.Println("I will execute at the end")
}
Make a signalling channel.
quit := make(chan struct{}{})
Close it when you want to break a loop.
go func (){
for {
fmt.Println("I will print every second", count)
count++
if count > 5 {
close(quit)
wg.Done()
return
}
<-t.C
}
}()
Read on closed channel returns immediately with zero value (but we dont need it in this case). Otherwise reading from it blocks and select pass execution to "default" case.
myLoop:
for {
select {
case <- quit:
break myLoop
default:
fmt.Println("iteration", i)
i++
}
}
Darigaaz's answer works for a single goroutine, but closing a closed channel panics (and you also don't need a waitgroup in that instance). If you have multiple goroutines, and want the loop to exit after all of them have finished, use a waitgroup with a closer routine:
https://play.golang.org/p/RhmUzWhneT
package main
import (
"fmt"
"sync"
"time"
)
func main() {
var wg sync.WaitGroup
quitCh := make(chan struct{})
for i := 1; i <= 5; i++ {
wg.Add(1)
go func(i int) {
count := 1
t := time.NewTicker(time.Millisecond)
for count <= 5 {
fmt.Printf("Goroutine %v iteration %v\n", i, count)
count++
<-t.C
}
wg.Done()
}(i)
}
// This is the closer routine.
go func() {
wg.Wait()
close(quitCh)
}()
t := time.NewTicker(500 * time.Microsecond)
loop:
for i := 1; ; i++ { // this is still infinite
select {
case <-quitCh:
break loop // has to be named, because "break" applies to the select otherwise
case <-t.C:
fmt.Println("Main iteration", i)
}
}
fmt.Println("End!")
}
As an alternative to the named loop style, you can instead use a fallthrough break in that select:
for i := 1; ; i++ { // this is still infinite
select {
case <-quitCh:
// fallthrough
case <-t.C:
fmt.Println("Main iteration", i)
continue
}
break // only reached if the quitCh case happens
}
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