Handle back button behavior.

Previously, the back button immediately navigated backwards in the
Assistant container's UI state.

Now, the back button will navigate backwards in the WebContents history
stack. Only when there is no more history do we return control to the
Assistant container for handling.

See bug for demo.

Bug: b:112355878
Change-Id: I943407c85497f09a5360f21353301b6bc6823315
Reviewed-on: https://chromium-review.googlesource.com/1167646
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582736}

ash/assistant/assistant_controller.cc

@@ -144,6 +144,12 @@ void AssistantController::ReleaseWebContents(
   web_contents_manager_->ReleaseAllWebContents(id_tokens);
 }
 
+void AssistantController::NavigateWebContentsBack(
+    const base::UnguessableToken& id_token,
+    mojom::WebContentsManager::NavigateWebContentsBackCallback callback) {
+  web_contents_manager_->NavigateWebContentsBack(id_token, std::move(callback));
+}
+
 void AssistantController::DownloadImage(
     const GURL& url,
     mojom::AssistantImageDownloader::DownloadCallback callback) {

ash/assistant/assistant_controller.h

@@ -73,6 +73,13 @@ class ASH_EXPORT AssistantController
   // |id_token_list|.
   void ReleaseWebContents(const std::vector<base::UnguessableToken>& id_tokens);
 
+  // Navigates the WebContents uniquely identified by |id_token| back relative
+  // to the current history entry. The supplied |callback| will run specifying
+  // true if navigation occurred, false otherwise.
+  void NavigateWebContentsBack(
+      const base::UnguessableToken& id_token,
+      mojom::WebContentsManager::NavigateWebContentsBackCallback callback);
+
   // Downloads the image found at the specified |url|. On completion, the
   // supplied |callback| will be run with the downloaded image. If the download
   // attempt is unsuccessful, a NULL image is returned.

ash/assistant/ui/assistant_web_view.cc

@@ -7,7 +7,6 @@
 #include "ash/assistant/assistant_controller.h"
 #include "ash/assistant/assistant_ui_controller.h"
 #include "ash/assistant/ui/assistant_ui_constants.h"
-#include "ash/assistant/ui/caption_bar.h"
 #include "ash/assistant/util/deep_link_util.h"
 #include "ash/public/cpp/app_list/answer_card_contents_registry.h"
 #include "ash/public/interfaces/web_contents_manager.mojom.h"
@@ -54,11 +53,36 @@ void AssistantWebView::InitLayout() {
 
   // Caption bar.
   caption_bar_ = new CaptionBar();
-  caption_bar_->set_delegate(assistant_controller_->ui_controller());
+  caption_bar_->set_delegate(this);
   caption_bar_->SetButtonVisible(CaptionButtonId::kMinimize, false);
   AddChildView(caption_bar_);
 }
 
+bool AssistantWebView::OnCaptionButtonPressed(CaptionButtonId id) {
+  CaptionBarDelegate* delegate = assistant_controller_->ui_controller();
+
+  // We need special handling of the back button. When possible, the back button
+  // should navigate backwards in the managed WebContents' history stack.
+  if (id == CaptionButtonId::kBack && web_contents_id_token_.has_value()) {
+    assistant_controller_->NavigateWebContentsBack(
+        web_contents_id_token_.value(),
+        base::BindOnce(
+            [](CaptionBarDelegate* delegate, bool navigated) {
+              // If the WebContents' did not navigate it is because we are
+              // already at the first entry in the history stack and we cannot
+              // navigate back any further. In this case, we give back control
+              // to our primary caption button delegate.
+              if (!navigated)
+                delegate->OnCaptionButtonPressed(CaptionButtonId::kBack);
+            },
+            base::Unretained(delegate)));
+    return true;
+  }
+
+  // For all other buttons we defer to our primary caption button delegate.
+  return delegate->OnCaptionButtonPressed(id);
+}
+
 void AssistantWebView::OnDeepLinkReceived(
     assistant::util::DeepLinkType type,
     const std::map<std::string, std::string>& params) {

ash/assistant/ui/assistant_web_view.h

@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "ash/assistant/assistant_controller_observer.h"
+#include "ash/assistant/ui/caption_bar.h"
 #include "base/macros.h"
 #include "base/optional.h"
 #include "ui/views/view.h"
@@ -19,14 +20,14 @@ class UnguessableToken;
 namespace ash {
 
 class AssistantController;
-class CaptionBar;
 
 // AssistantWebView is a child of AssistantBubbleView which allows Assistant UI
 // to render remotely hosted content within its bubble. It provides a CaptionBar
 // for window level controls and a WebView/RemoteViewHost for embedding web
 // contents.
 class AssistantWebView : public views::View,
-                         public AssistantControllerObserver {
+                         public AssistantControllerObserver,
+                         public CaptionBarDelegate {
  public:
   explicit AssistantWebView(AssistantController* assistant_controller);
   ~AssistantWebView() override;
@@ -36,6 +37,9 @@ class AssistantWebView : public views::View,
   int GetHeightForWidth(int width) const override;
   void ChildPreferredSizeChanged(views::View* child) override;
 
+  // CaptionBarDelegate:
+  bool OnCaptionButtonPressed(CaptionButtonId id) override;
+
   // AssistantControllerObserver:
   void OnDeepLinkReceived(
       assistant::util::DeepLinkType type,

ash/public/interfaces/web_contents_manager.mojom

@@ -32,6 +32,12 @@ interface WebContentsManager {
   // Releases any resources associated with any WebContents uniquely identified
   // by one of the specified |id_tokens|.
   ReleaseAllWebContents(array<mojo_base.mojom.UnguessableToken> id_tokens);
+
+  // Navigates the WebContents uniquely identified by |id_token| back relative
+  // to the current history entry. The callback returns true if the WebContents
+  // were navigated, false otherwise.
+  NavigateWebContentsBack(mojo_base.mojom.UnguessableToken id_token)
+    => (bool navigated);
 };
 
 // Defines parameters for a managed WebContents.

chrome/browser/ui/ash/assistant/web_contents_manager.cc

@@ -83,6 +83,18 @@ class ManagedWebContents : public content::WebContentsDelegate,
       std::move(callback_).Run(embed_token_);
   }
 
+  void NavigateBack(
+      ash::mojom::WebContentsManager::NavigateWebContentsBackCallback
+          callback) {
+    content::NavigationController& controller = web_contents_->GetController();
+    if (controller.CanGoBack()) {
+      controller.GoBack();
+      std::move(callback).Run(true);
+    } else {
+      std::move(callback).Run(false);
+    }
+  }
+
  private:
   void InitWebContents(Profile* profile,
                        ash::mojom::ManagedWebContentsParamsPtr params) {
@@ -191,3 +203,9 @@ void WebContentsManager::ReleaseAllWebContents(
   for (const base::UnguessableToken& id_token : id_tokens)
     managed_web_contents_map_.erase(id_token);
 }
+
+void WebContentsManager::NavigateWebContentsBack(
+    const base::UnguessableToken& id_token,
+    ash::mojom::WebContentsManager::NavigateWebContentsBackCallback callback) {
+  managed_web_contents_map_[id_token]->NavigateBack(std::move(callback));
+}

chrome/browser/ui/ash/assistant/web_contents_manager.h

@@ -39,6 +39,10 @@ class WebContentsManager : public ash::mojom::WebContentsManager {
   void ReleaseWebContents(const base::UnguessableToken& id_token) override;
   void ReleaseAllWebContents(
       const std::vector<base::UnguessableToken>& id_tokens) override;
+  void NavigateWebContentsBack(
+      const base::UnguessableToken& id_token,
+      ash::mojom::WebContentsManager::NavigateWebContentsBackCallback callback)
+      override;
 
  private:
   mojo::Binding<ash::mojom::WebContentsManager> binding_;