support using BrowserOption in NewContext

While at it, remove the error return from BrowserOption, to make it
consistent with all the other option func types.

We don't have a good test for this feature yet, but at least we check
that it doesn't crash or error via an example.

Fixes #292.

allocate.go

@@ -20,7 +20,7 @@ type Allocator interface {
 	// Allocate creates a new browser. It can be cancelled via the provided
 	// context, at which point all the resources used by the browser (such
 	// as temporary directories) will be freed.
-	Allocate(context.Context) (*Browser, error)
+	Allocate(context.Context, ...BrowserOption) (*Browser, error)
 
 	// TODO: Wait should probably return an error, which can then be
 	// retrieved by the user if just calling cancel().
@@ -73,7 +73,7 @@ type ExecAllocator struct {
 }
 
 // Allocate satisfies the Allocator interface.
-func (p *ExecAllocator) Allocate(ctx context.Context) (*Browser, error) {
+func (p *ExecAllocator) Allocate(ctx context.Context, opts ...BrowserOption) (*Browser, error) {
 	c := FromContext(ctx)
 	if c == nil {
 		return nil, ErrInvalidContext
@@ -152,7 +152,7 @@ func (p *ExecAllocator) Allocate(ctx context.Context) (*Browser, error) {
 	}
 	stderr.Close()
 
-	browser, err := NewBrowser(ctx, wsURL)
+	browser, err := NewBrowser(ctx, wsURL, opts...)
 	if err != nil {
 		return nil, err
 	}

browser.go

@@ -78,9 +78,7 @@ func NewBrowser(ctx context.Context, urlstr string, opts ...BrowserOption) (*Bro
 
 	// apply options
 	for _, o := range opts {
-		if err := o(b); err != nil {
+		o(b)
-			return nil, err
-		}
 	}
 
 	// ensure errf is set
@@ -322,29 +320,22 @@ func (b *Browser) run(ctx context.Context) {
 }
 
 // BrowserOption is a browser option.
-type BrowserOption func(*Browser) error
+type BrowserOption func(*Browser)
 
-// WithLogf is a browser option to specify a func to receive general logging.
+// WithBrowserLogf is a browser option to specify a func to receive general logging.
-func WithLogf(f func(string, ...interface{})) BrowserOption {
+func WithBrowserLogf(f func(string, ...interface{})) BrowserOption {
-	return func(b *Browser) error {
+	return func(b *Browser) { b.logf = f }
-		b.logf = f
-		return nil
-	}
 }
 
-// WithErrorf is a browser option to specify a func to receive error logging.
+// WithBrowserErrorf is a browser option to specify a func to receive error logging.
-func WithErrorf(f func(string, ...interface{})) BrowserOption {
+func WithBrowserErrorf(f func(string, ...interface{})) BrowserOption {
-	return func(b *Browser) error {
+	return func(b *Browser) { b.errf = f }
-		b.errf = f
-		return nil
-	}
 }
 
 // WithConsolef is a browser option to specify a func to receive chrome log events.
 //
 // Note: NOT YET IMPLEMENTED.
 func WithConsolef(f func(string, ...interface{})) BrowserOption {
-	return func(b *Browser) error {
+	return func(b *Browser) {
-		return nil
 	}
 }

chromedp.go

@@ -36,6 +36,11 @@ type Context struct {
 	// have its own unique Target pointing to a separate browser tab (page).
 	Target *Target
 
+	// browserOpts holds the browser options passed to NewContext via
+	// WithBrowserOption, so that they can later be used when allocating a
+	// browser in Run.
+	browserOpts []BrowserOption
+
 	// cancel simply cancels the context that was used to start Browser.
 	// This is useful to stop all activity and avoid deadlocks if we detect
 	// that the browser was closed or happened to crash. Note that this
@@ -56,12 +61,14 @@ type Context struct {
 func NewContext(parent context.Context, opts ...ContextOption) (context.Context, context.CancelFunc) {
 	ctx, cancel := context.WithCancel(parent)
 
-	c := &Context{cancel: cancel}
+	c := &Context{cancel: cancel, first: true}
 	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.
+
+		c.first = c.Browser == nil
 	}
 
 	for _, o := range opts {
@@ -127,12 +134,11 @@ func Run(ctx context.Context, actions ...Action) error {
 		return ErrInvalidContext
 	}
 	if c.Browser == nil {
-		browser, err := c.Allocator.Allocate(ctx)
+		browser, err := c.Allocator.Allocate(ctx, c.browserOpts...)
 		if err != nil {
 			return err
 		}
 		c.Browser = browser
-		c.first = true
 	}
 	if c.Target == nil {
 		if err := c.newSession(ctx); err != nil {
@@ -197,8 +203,31 @@ func (c *Context) newSession(ctx context.Context) error {
 	return nil
 }
 
+// ContextOption is a context option.
 type ContextOption func(*Context)
 
+// WithLogf is a shortcut for WithBrowserOption(WithBrowserLogf(f)).
+func WithLogf(f func(string, ...interface{})) ContextOption {
+	return WithBrowserOption(WithBrowserLogf(f))
+}
+
+// WithErrorf is a shortcut for WithBrowserOption(WithBrowserErrorf(f)).
+func WithErrorf(f func(string, ...interface{})) ContextOption {
+	return WithBrowserOption(WithBrowserErrorf(f))
+}
+
+// WithBrowserOption allows passing a number of browser options to the allocator
+// when allocating a new browser. As such, this context option can only be used
+// when NewContext is allocating a new browser.
+func WithBrowserOption(opts ...BrowserOption) ContextOption {
+	return func(c *Context) {
+		if !c.first {
+			panic("WithBrowserOption can only be used when allocating a new browser")
+		}
+		c.browserOpts = append(c.browserOpts, opts...)
+	}
+}
+
 // Targets lists all the targets in the browser attached to the given context.
 func Targets(ctx context.Context) ([]*target.Info, error) {
 	// Don't rely on Run, as that needs to be able to call Targets, and we
@@ -209,12 +238,11 @@ func Targets(ctx context.Context) ([]*target.Info, error) {
 		return nil, ErrInvalidContext
 	}
 	if c.Browser == nil {
-		browser, err := c.Allocator.Allocate(ctx)
+		browser, err := c.Allocator.Allocate(ctx, c.browserOpts...)
 		if err != nil {
 			return nil, err
 		}
 		c.Browser = browser
-		c.first = true
 	}
 	return target.GetTargets().Do(ctx, c.Browser)
 }

example_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"os"
 	"path/filepath"
 
@@ -48,7 +49,8 @@ func ExampleExecAllocator() {
 	allocCtx, cancel := chromedp.NewExecAllocator(context.Background(), opts...)
 	defer cancel()
 
-	taskCtx, cancel := chromedp.NewContext(allocCtx)
+	// also set up a custom logger
+	taskCtx, cancel := chromedp.NewContext(allocCtx, chromedp.WithLogf(log.Printf))
 	defer cancel()
 
 	// ensure that the browser process is started