WithCancel
- Go course
- Intermediate
- Lesson 30/80
- 4 min
Learn cancellable context with WithCancel.
WithCancel
context.WithCancel(parent) returns a child context and a cancel function. Calling cancel() cancels the child and every context derived from it; the parent is unaffected.
ctx, cancel := context.WithCancel(context.Background())
defer cancel() // avoid leaking resources
go izle(ctx)
// ...
cancel() // the izle goroutine stopscancel can be called more than once; calls after the first have no effect.
A goroutine listening for cancellation
A long-running goroutine checks for cancellation on every loop iteration:
func izle(ctx context.Context) {
for {
select {
case <-ctx.Done():
fmt.Println("durdu:", ctx.Err())
return
case <-time.After(100 * time.Millisecond):
fmt.Println("kontrol")
}
}
}Cancellation is cooperative: the context doesn't force the goroutine to stop; the goroutine checks the signal and exits on its own.
Practice Cancel it
Create a context with WithCancel. A goroutine waits on <-ctx.Done() and sends ctx.Err() to a channel. main calls cancel() and prints the value from the channel.
Starter code
package main
import (
"context"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"context"
"fmt"
)
func main() {
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
sonuc := make(chan error)
go func() {
<-ctx.Done()
sonuc <- ctx.Err()
}()
cancel()
fmt.Println(<-sonuc)
}Expected output
context canceled
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens if <code>cancel</code> is never called?
- The context cancels itself
- The context and linked resources stay in memory until the parent is cancelled Correct answer
- Nothing
- The program crashes
Explanation: That's why go vet warns "the cancel function is not used"; defer cancel() should be a habit.
2 Release resources
ctx, cancel := context.WithCancel(ctx)
____ cancel()ctx, cancel := context.WithCancel(ctx)
defer cancel()Explanation: defer cancel() frees the resources tied to the context when the function ends.
3 Cancellation chain
| Term | Meaning |
|---|---|
cancel() |
Cancels child contexts |
Parent context |
Unaffected |
ctx.Err() |
context.Canceled |
select + Done |
Listens for cancellation |