add support for opening multiple tabs

On a single browser, that is. And port the example from _example,
proving that it works.

_example/tabs.go

@@ -1,31 +0,0 @@
-package main
-
-import (
-	"context"
-	"log"
-
-	"github.com/chromedp/chromedp"
-)
-
-func main() {
-	// first tab
-	ctx1, cancel := chromedp.NewContext(context.Background())
-	defer cancel()
-
-	// create new tab
-	ctx2, _ := chromedp.NewContext(ctx1)
-
-	// runs in first tab
-	if err := chromedp.Run(ctx1, myTask()); err != nil {
-		log.Fatal(err)
-	}
-
-	// runs in second tab
-	if err := chromedp.Run(ctx2, myTask()); err != nil {
-		log.Fatal(err)
-	}
-}
-
-func myTask() chromedp.Tasks {
-	return []chromedp.Action{}
-}

context.go

@@ -19,7 +19,7 @@ type Context struct {
 
 	Browser *Browser
 
-	sessionID target.SessionID
+	SessionID target.SessionID
 }
 
 // NewContext creates a browser context using the parent context.
@@ -29,6 +29,9 @@ func NewContext(parent context.Context, opts ...ContextOption) (context.Context,
 	c := &Context{}
 	if pc := FromContext(parent); pc != nil {
 		c.Allocator = pc.Allocator
+		c.Browser = pc.Browser
+		// don't inherit SessionID, so that NewContext can be used to
+		// create a new tab on the same browser.
 	}
 
 	for _, o := range opts {
@@ -69,12 +72,12 @@ func Run(ctx context.Context, action Action) error {
 		}
 		c.Browser = browser
 	}
-	if c.sessionID == "" {
+	if c.SessionID == "" {
 		if err := c.newSession(ctx); err != nil {
 			return err
 		}
 	}
-	return action.Do(ctx, c.Browser.executorForTarget(ctx, c.sessionID))
+	return action.Do(ctx, c.Browser.executorForTarget(ctx, c.SessionID))
 }
 
 func (c *Context) newSession(ctx context.Context) error {
@@ -107,7 +110,7 @@ func (c *Context) newSession(ctx context.Context) error {
 		}
 	}
 
-	c.sessionID = sessionID
+	c.SessionID = sessionID
 	return nil
 }
 

example_test.go

@@ -76,3 +76,36 @@ func ExampleExecAllocatorOption() {
 	// Output:
 	// DevToolsActivePort has 2 lines
 }
+
+func ExampleManyTabs() {
+	// new browser, first tab
+	ctx1, cancel := chromedp.NewContext(context.Background())
+	defer cancel()
+
+	// ensure the first tab is created
+	if err := chromedp.Run(ctx1, chromedp.Tasks{}); err != nil {
+		panic(err)
+	}
+
+	// same browser, second tab
+	ctx2, _ := chromedp.NewContext(ctx1)
+
+	// ensure the second tab is created
+	if err := chromedp.Run(ctx2, chromedp.Tasks{}); err != nil {
+		panic(err)
+	}
+
+	c1 := chromedp.FromContext(ctx1)
+	c2 := chromedp.FromContext(ctx2)
+
+	fmt.Printf("Same browser: %t\n", c1.Browser == c2.Browser)
+	fmt.Printf("Same tab: %t\n", c1.SessionID == c2.SessionID)
+
+	// wait for the resources to be cleaned up
+	cancel()
+	c1.Allocator.Wait()
+
+	// Output:
+	// Same browser: true
+	// Same tab: false
+}