context: add examples

Add function level examples to the package.

Partially addresses #16360

Change-Id: I7162aed4e4a969743c19b79c9ffaf9217d2c1c08
Reviewed-on: https://go-review.googlesource.com/26930
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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"
+)
+
+// This example demonstrate the use of a cancelable context preventing a
+// goroutine leak. By the end of the example func's execution, the "count"
+// goroutine is canceled.
+func ExampleWithCancel() {
+	count := func(ctx context.Context, dst chan<- int) {
+		n := 1
+		for {
+			select {
+			case dst <- n:
+				n++
+			case <-ctx.Done():
+				return
+			}
+		}
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	ints := make(chan int)
+	go count(ctx, ints)
+	for n := range ints {
+		fmt.Println(n)
+		if n == 5 {
+			return
+		}
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+}
+
+// This example passes a context with a 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(50 * time.Millisecond)
+	ctx, cancel := context.WithDeadline(context.Background(), d)
+
+	// Even though ctx will be expired, it is good practice to call its
+	// cancelation function in any case. Failure to do so may keep the
+	// context and its parent alive longer than necessary.
+	defer cancel()
+
+	select {
+	case <-time.After(1 * time.Second):
+		fmt.Println("overslept")
+	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(), 50*time.Millisecond)
+	defer cancel()
+
+	select {
+	case <-time.After(1 * time.Second):
+		fmt.Println("overslept")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+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
+}

src/context/withtimeout_test.go

-// Copyright 2014 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"
-)
-
-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(), 50*time.Millisecond)
-
-	select {
-	case <-time.After(1 * time.Second):
-		fmt.Println("overslept")
-	case <-ctx.Done():
-		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
-	}
-
-	// Even though ctx should have expired already, it is good
-	// practice to call its cancelation function in any case.
-	// Failure to do so may keep the context and its parent alive
-	// longer than necessary.
-	cancel()
-
-	// Output:
-	// context deadline exceeded
-}