headless: Add a small C++ example application

headless/app/headless_example.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// A small example application showing the use of the C++ Headless Chrome
+// library. It navigates to a web site given on the command line, waits for it
+// to load and prints out the DOM.
+//
+// Tip: start reading from the main() function below.
+
+#include "base/bind.h"
+#include "base/command_line.h"
+#include "base/memory/weak_ptr.h"
+#include "headless/public/devtools/domains/page.h"
+#include "headless/public/devtools/domains/runtime.h"
+#include "headless/public/headless_browser.h"
+#include "headless/public/headless_devtools_client.h"
+#include "headless/public/headless_devtools_target.h"
+#include "headless/public/headless_web_contents.h"
+#include "ui/gfx/geometry/size.h"
+
+// This class contains the main application logic, i.e., waiting for a page to
+// load and printing its DOM. Note that browser initialization happens outside
+// this class.
+class HeadlessExample : public headless::HeadlessWebContents::Observer,

[alex clarke (OOO till 29th), 2017/02/10 14:46:47]

This should probably extend inspector::ExperimentalObserver so we can add:

 void OnTargetCrashed(const inspector::TargetCrashedParams& params) override {
    LOG(ERROR) << "Abnormal renderer termination.";
     ...
}

[Sami, 2017/02/10 16:25:26]

Hmm, this is supposed to be a really simple example so I'd like to leave stuff
like that out. Added a comment though.

+                        public headless::page::Observer {
+ public:
+  HeadlessExample(headless::HeadlessBrowser* browser,
+                  headless::HeadlessWebContents* web_contents);
+  ~HeadlessExample() override;
+
+  // headless::HeadlessWebContents::Observer implementation:
+  void DevToolsTargetReady() override;
+
+  // headless::page::Observer implementation:
+  void OnLoadEventFired(
+      const headless::page::LoadEventFiredParams& params) override;
+
+  void OnDomFetched(std::unique_ptr<headless::runtime::EvaluateResult> result);
+
+ private:
+  // The headless browser instance. See main().
+  headless::HeadlessBrowser* browser_;

[altimin, 2017/02/10 14:39:43]

UNOWNED?

[Sami, 2017/02/10 16:25:26]

I'm thinking in the age of unique_ptr and friends, "not owned" type comments
aren't that helpful anymore since a naked pointer almost always means lifetime
is managed by someone else. I'll add a comment saying *who* owns these though.

+  // Our tab.
+  headless::HeadlessWebContents* web_contents_;

[altimin, 2017/02/10 14:39:43]

UNOWNED?

[Sami, 2017/02/10 16:25:26]

Done.

+  // The DevTools client used to control the tab.
+  std::unique_ptr<headless::HeadlessDevToolsClient> devtools_client_;
+  // A helper for creating weak pointers to this class.
+  base::WeakPtrFactory<HeadlessExample> weak_factory_;
+};
+
+namespace {
+std::unique_ptr<HeadlessExample> g_example;

[altimin, 2017/02/10 14:39:43]

I'm slightly terrified about the idea that HeadlessExample's destructor may be
called during cleanup of global variables (it will break horribly).  I recommend
using raw pointer here (or base::Singleton).

[Sami, 2017/02/10 16:25:26]

Good point, let's make this a raw ptr.

+}
+
+HeadlessExample::HeadlessExample(headless::HeadlessBrowser* browser,
+                                 headless::HeadlessWebContents* web_contents)
+    : browser_(browser),
+      web_contents_(web_contents),
+      devtools_client_(headless::HeadlessDevToolsClient::Create()),
+      weak_factory_(this) {
+  web_contents_->AddObserver(this);
+}
+
+HeadlessExample::~HeadlessExample() {
+  // Note that we shut down the browser last, because it owns objects such as
+  // the web contents which can no longer be accessed after the browser is gone.
+  web_contents_->GetDevToolsTarget()->DetachClient(devtools_client_.get());

[altimin, 2017/02/10 14:39:43]

We should also add removing observer from Page domain here.

Random thought: we have a number of constructions like if (page_observer)
devtools_client_->GetPage()->RemoveObserver(this). We should consider adding an
RAII-style handle for observers:

page_observer_ = devtools_client_->GetPage()->AddObserver(this);

page_observer_.reset();

[Sami, 2017/02/10 16:25:26]

Oops, done.
 
Not a bad idea. I guess that would be some kind of observer handle. The other
option would be to make AddObserver take a weak pointer.

+  web_contents_->RemoveObserver(this);
+  browser_->Shutdown();
+}
+
+// This method is called when the tab is ready for DevTools inspection.
+void HeadlessExample::DevToolsTargetReady() {
+  // Attach our DevTools client to the tab so that we can send commands to it
+  // and observe events.
+  web_contents_->GetDevToolsTarget()->AttachClient(devtools_client_.get());
+
+  // Start observing events from DevTools's page domain. This lets us get
+  // notified when the page has finished loading. Note that it is possible
+  // the page has already finished loading by now. See
+  // HeadlessShell::DevToolTargetReady for how to handle that case correctly.
+  devtools_client_->GetPage()->AddObserver(this);
+  devtools_client_->GetPage()->Enable();
+}
+
+void HeadlessExample::OnLoadEventFired(
+    const headless::page::LoadEventFiredParams& params) {
+  // The page has now finished loading. Let's grab a snapshot of the DOM by
+  // evaluating the outerHTML property on the body element.
+  devtools_client_->GetRuntime()->Evaluate(
+      "document.body.outerHTML",
+      base::Bind(&HeadlessExample::OnDomFetched, weak_factory_.GetWeakPtr()));
+}
+
+void HeadlessExample::OnDomFetched(
+    std::unique_ptr<headless::runtime::EvaluateResult> result) {
+  std::string dom;

[altimin, 2017/02/10 14:39:43]

Check that we got a valid result here?

[Sami, 2017/02/10 16:25:26]

Done.

+  if (result->GetResult()->GetValue()->GetAsString(&dom))
+    printf("%s\n", dom.c_str());
+
+  // Shut down the browser (see ~HeadlessExample).
+  g_example.reset();
+}
+
+// This function is called by the headless library after the browser has been
+// initialized. It runs on the UI thread.
+void OnHeadlessBrowserStarted(headless::HeadlessBrowser* browser) {
+  // In order to open tabs, we first need a browser context. It corresponds to a
+  // user profile and contains things like the user's cookies, local storage,
+  // cache, etc.
+  headless::HeadlessBrowserContext::Builder context_builder =
+      browser->CreateBrowserContextBuilder();
+
+  // Here we can set options for the browser context. As an example we enable
+  // incognito mode, which makes sure profile data is not written to disk.
+  context_builder.SetIncognitoMode(true);
+
+  // Construct the context and set it as the default. The default browser
+  // context is used by the Target.createTarget() DevTools command when no other
+  // context is given.
+  headless::HeadlessBrowserContext* browser_context = context_builder.Build();
+  browser->SetDefaultBrowserContext(browser_context);
+
+  // Get the URL from the command line.
+  base::CommandLine::StringVector args =
+      base::CommandLine::ForCurrentProcess()->GetArgs();
+  if (args.empty()) {
+    LOG(ERROR) << "No URL to load";
+    browser->Shutdown();
+    return;
+  }
+  GURL url(args[0]);
+
+  // Open a tab (i.e., HeadlessWebContents) in the newly created browser
+  // context.
+  headless::HeadlessWebContents::Builder tab_builder(
+      browser_context->CreateWebContentsBuilder());
+
+  // We can set options for the opened tab here. In this example we are just
+  // setting the initial URL to navigate to.
+  tab_builder.SetInitialURL(url);
+
+  // Create an instance of the example app, which will wait for the page to load
+  // and print its DOM.
+  headless::HeadlessWebContents* web_contents = tab_builder.Build();
+  g_example.reset(new HeadlessExample(browser, web_contents));
+}
+
+int main(int argc, const char** argv) {
+  // This function must be the first thing we call to make sure child processes
+  // such as the renderer are started properly. The headless library starts
+  // child processes by forking and exec'ing the main application.
+  headless::RunChildProcessIfNeeded(argc, argv);
+
+  // Create a headless browser instance. There can be one of these per process
+  // and it can only be initialized once.
+  headless::HeadlessBrowser::Options::Builder builder(argc, argv);
+
+  // Here you can customize browser options. As an example we set the window
+  // size.
+  builder.SetWindowSize(gfx::Size(800, 600));
+
+  // Pass control to the headless library. It will bring up the browser and
+  // invoke the given callback on the browser UI thread. Note: if you need to
+  // pass more parameters to the callback, you can add them to the Bind() call
+  // below.
+  return headless::HeadlessBrowserMain(builder.Build(),
+                                       base::Bind(&OnHeadlessBrowserStarted));
+}