src/context/example_test.go

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package context_test

import (
	"context"
	"fmt"
	"time"
)

const shortDuration = 1 * time.Millisecond // a reasonable duration to block in an example

var neverReady = make(chan struct{}) // never closed

// This example demonstrates the use of a cancelable context to prevent a
// goroutine leak. By the end of the example function, the goroutine started
// by gen will return without leaking.
func ExampleWithCancel() {
	// gen generates integers in a separate goroutine and
	// sends them to the returned channel.
	// The callers of gen need to cancel the context once
	// they are done consuming generated integers not to leak
	// the internal goroutine started by gen.
	gen := func(ctx context.Context) <-chan int {
		dst := make(chan int)
		n := 1
		go func() {
			for {
				select {
				case <-ctx.Done():
					return // returning not to leak the goroutine
				case dst <- n:
					n++
				}
			}
		}()
		return dst
	}

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel() // cancel when we are finished consuming integers

	for n := range gen(ctx) {
		fmt.Println(n)
		if n == 5 {
			break
		}
	}
	// Output:
	// 1
	// 2
	// 3
	// 4
	// 5
}

// This example passes a context with an arbitrary deadline to tell a blocking
// function that it should abandon its work as soon as it gets to it.
func ExampleWithDeadline() {
	d := time.Now().Add(shortDuration)
	ctx, cancel := context.WithDeadline(context.Background(), d)

	// Even though ctx will be expired, it is good practice to call its
	// cancellation function in any case. Failure to do so may keep the
	// context and its parent alive longer than necessary.
	defer cancel()

	select {
	case <-neverReady:
		fmt.Println("ready")
	case <-ctx.Done():
		fmt.Println(ctx.Err())
	}

	// Output:
	// context deadline exceeded
}

// This example passes a context with a timeout to tell a blocking function that
// it should abandon its work after the timeout elapses.
func ExampleWithTimeout() {
	// Pass a context with a timeout to tell a blocking function that it
	// should abandon its work after the timeout elapses.
	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
	defer cancel()

	select {
	case <-neverReady:
		fmt.Println("ready")
	case <-ctx.Done():
		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
	}

	// Output:
	// context deadline exceeded
}

// This example demonstrates how a value can be passed to the context
// and also how to retrieve it if it exists.
func ExampleWithValue() {
	type favContextKey string

	f := func(ctx context.Context, k favContextKey) {
		if v := ctx.Value(k); v != nil {
			fmt.Println("found value:", v)
			return
		}
		fmt.Println("key not found:", k)
	}

	k := favContextKey("language")
	ctx := context.WithValue(context.Background(), k, "Go")

	f(ctx, k)
	f(ctx, favContextKey("color"))

	// Output:
	// found value: Go
	// key not found: color
}