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Menus, Hotkeys, & Command Dispatch (Mac)

This document covers how toolbar buttons, the main menu bar, and some context menus are hooked up to the back-end Command infrastructure in Chromium on Mac OS X.


Menus are used in three places: the main menu, the toolbar menu, and the context menu for right clicks. Menus are constructed using the AppKit framework. The two relevant classes are NSMenu, and NSMenuItem. 

Mac OS X has a main menu bar that needs to update based on the front-most window as well as remain functional when there are no windows open (a situation which can't happen on Windows and Linux). 


The class MenuController and its subclasses are used to dynamically generate or modify menus.
The nib file 'MainMenu.xib' generates the static portion of the main menu.

The Bookmarks submenu of the main menu consists of:
1. The static portion of the submenu generated by MainMenu.xib
2. A dynamically updated portion generated by BookmarkMenuBridge.

Enabling/Disabling Menu Items

The cross-platform (XP) infrastructure uses a CommandUpdater object to represent the enabled state of a set of commands. Each Browser object has a CommandUpdater allowing it to maintain unique state that differs from window to window. User interface views, such as menus or buttons, are responsible for querying the appropriate CommandUpdater "model" to obtain their enabled state.

In cocoa, the "enabled" state is determined by sending the validateUserInterfaceItem: selector up the responder chain for each menu item. Despite the fact that MenuController implements  validateUserInterfaceItem:, it may not be a part of the responder chain. The relevant validateUserInterfaceItem: implementation will vary from menu to menu. Most menu items are handled by the BrowserWindowController which asks its corresponding Browser's CommandUpdater for the enabled state of the command id given by the item's tag. This also handles toggling commands and updating the item's title to match current application state.

In the case of buttons, the relevant controller (such as ToolbarController) registers for changes from the CommandUpdater using a CommandObserverBridge. When one of the relevant commands changes its state, the bridge sends enabledStateChangedForCommand:enabled: allowing the controller to update the enabled state of the button.

For more information, see the Apple documentation on User Interface Validation.


NSMenuItem has 3 important characteristics: the tag (XP id of the command), the hotkey/accelerator, and the action/target pair. There are two ways to invoke a menu item - by clicking on it, or by using the hotkey. 

When a user selects a menu item by clicking on it:
  1. BrowserCrApplication receives a sendAction:to:from: callback. It performs logging & exception handling. It forwards to its parent class.
  2. NSApplication invokes the action/target pair.
  3. If the NSMenuItem was created by a MenuController subclass, it may have a custom action/target pair. The target is sent the action, and processes the response. END
  4. By default, the NSMenuItem has action=commandDispatch: target=firstResponder. The tag contains the id of the command. There are 2 possible first responders.
  5. If there exists a relevant browser window, the first responder is BrowserWindowController, which forwards the invocation to BrowserCommandController::ExecuteCommandWithDisposition. 
  6. Otherwise, the first responder is AppController, which processes the event in commandDispatch:.
When a user presses the hotkey associated with a menu item (and there exists a relevant browser window)ChromeEventProcessingWindow performKeyEquivalent: is invoked. It tries to process the hotkey as a extension action, or as a global keyboard shortcut (such as ctr+tab to move to next tab, see for more details). If it fails to process the hotkey, then an event is generated and goes through the same flow from directly clicking the menu item.

When a user presses the hotkey associated with a menu item (and there does not exist a browser window), an event is generated and goes through the same flow from directly clicking the menu item.

There is also a commandDispatchUsingKeyModifiers: for dispatching commands that do different things based on the state of modifier keys (for example, command-clicking the "Home" button opens the home page in a new window).