This directory contains the implementation of the Chromium UI. Code in the root of this directory is toolkit- and platform-independent. There are subdirectories with implementations for specific toolkits and OSes. Code in the root of this directory should not be aware of platform-specific implementation details or reach into the platform implementation subdirectories. This directory also should not contain browser-process-scoped items that are not UI-specific, such as parts of the startup logic; these sorts of things belong elsewhere in chrome/browser/.

This directory is often referred to in conversation as “cbui” or “c/b/ui”, pronounced “sea bee you eye”.

Important subdirectories:

• views - the Views implementation of the UI, used on Windows, Mac, Linux, and ChromeOS. This includes things like the browser window itself, tabs, dialogs, etc.
• cocoa - the remaining Cocoa UI, used only on Mac. This directory used to contain a separate full implementation of the UI, parallel to the Views implementation.
• android - part of the Android implementation of the UI. See also //chrome/browser/android.
• webui - the WebUI parts of the browser UI. This includes things like the chrome://settings page and other WebUI pages.

A common pattern is for code in //chrome/browser/ui to define a platform-independent interface which then has implementations in //chrome/browser/ui/views and //chrome/browser/ui/android. This pattern is often followed even for features that don't exist on Android, in which case the Android implementation is often a stub.

This pattern often looks like this:

//chrome/browser/ui/browser_dialogs.h:
void ShowMyDialog(...);

//chrome/browser/ui/views/my_dialog_views.cc:
void ShowMyDialog(...) { ... }

//chrome/browser/ui/android/my_dialog_android.cc:
void ShowMyDialog(...) { ... }

Because “Chromium UI” is such a large surface area, do not add new files directly to this directory; instead, add subdirectories with more specific OWNERS and place new features and files in them. Cleanup of existing scattered files is also welcome.