Code paths for common operations

Code paths for common operations

There is additional information and more examples on how Chromium displays web pages.

Application startup

1. Our WinMain function is in chrome/app/main.cc, and is linked in the chrome project.
2. WinMain launches the Google Update Client, which is the installer/autoupdater. It will find the subdirectory for the current version, and load chrome.dll from there.
3. It calls ChromeMain in the newly loaded library, which is in chrome_main.cc in the chrome_dll project.
4. ChromeMain does initialization for common components, and then forwards to either RendererMain in chrome/renderer/renderer_main.cc if the command line flag indicates that this should be a subprocess, or BrowserMain in chrome/browser/browser_main.cc if not to load a new copy of the application. Since this is startup, we're launching the browser.
5. BrowserMain does common browser initialization. It has different modes for running installed webapps, connecting to the automation system if the browser is being tested, etc.
6. It calls LaunchWithProfile in browser_init.cc which creates a new Browser object in chrome/browser/ui/browser.cc. This object encapsulates one toplevel window in the application. The first tab is appended at this time.

Tab startup & initial navigation

1. Browser::AddTab in chrome/browser/ui/browser.cc is called to append a new tab.
2. It will create a new TabContents object from browser/tab_contents/tab_contents.cc
3. TabContents creates a RenderViewHost (chrome/browser/renderer_host/render_view_host.cc) via the RenderViewHostManager's Init function in chrome/browser/tab_contents/render_view_host_manager.cc). Depending on the SiteInstance, the RenderViewHost either spawns a new renderer process, or re-uses an existing RenderProcessHost. RenderProcessHost is the object in the browser that represents a single renderer subprocess.
4. The NavigationController in chrome/browser/tab_contents/navigation_controller.cc which is owned by the tab contents, is instructed to navigate to the URL for the new tab in NavigationController::LoadURL. "Navigating from the URL bar" from step 3 onward describes what happens from this point.

Navigating from the URL bar

1. When the user types into or accepts an entry in the URL bar, the autocomplete edit box determines the final target URL and passes that to AutocompleteEdit::OpenURL. (This may not be exactly what the user typed - for example, an URL is generated in the case of a search query.)
2. The navigation controller is instructed to navigate to the URL in NavigationController::LoadURL.
3. The NavigationController calls TabContents::Navigate with the NavigationEntry it created to represent this particular page transition. It will create a new RenderViewHost if necessary, which will cause creation of a RenderView in the renderer process. A RenderView won't exist if this is the first navigation, or if the renderer has crashed, so this will also recover from crashes.
4. Navigate forwards to RenderViewHost::NavigateToEntry. The NavigationController stores this navigation entry, but it is marked as "pending" because it doesn't know for sure if the transition will take place (maybe the host can not be resolved).
5. RenderViewHost::NavigateToEntry sends a ViewMsg_Navigate to the new RenderView in the renderer process.
6. When told to navigate, RenderView may navigate, it may fail, or it may navigate somewhere else instead (for example, if the user clicks a link). RenderViewHost waits for a ViewHostMsg_FrameNavigate from the RenderView.
7. When the load is "committed" by WebKit (the server responded and is sending us data), the RenderView sends this message, which is handled in RenderViewHost::OnMsgNavigate.
8. The NavigationEntry is updated with the information on the load. In the case of a link click, the browser has never seen this URL before. If the navigation was browser-initiated, as in the startup case, there may have been redirects that have changed the URL.
9. The NavigationController updates its list of navigations to account for this new information.

Navigations and session history

Each NavigationEntry stores a page ID and a block of history state data. The page ID is used to uniquely identify a page load, so we know which NavigationEntry it corresponds to. It is assigned when the page is committed commit, so a pending NavigationEntry will have a page ID of -1. The history state data is simply a WebCore::HistoryItem serialized to a string. Included on this item are things like the page URL, subframe URLs, and form data.

1. When the browser initiates the request (typing in the URL bar, or clicking back/forward/reload)
   1. A WebRequest is made representing the navigation, along with extra information like a page ID for bookkeeping. New navigations have an ID of -1. Navigations to old entries have the ID assigned to the NavigationEntry when the page was first visited. This extra information will be queried later when the load commits.
   2. The main WebFrame is told to load the new request.
2. When the renderer initiates the request (user clicks a link, javascript changes the location, etc):
   1. WebCore::FrameLoader is told to load the request via one of its bajillion varied load methods.
3. In either case, when the first packet from the server is received, the load is committed (no longer "pending" or "provisional").
4. If this was a new navigation, WebCore will create a new HistoryItem and add it to the BackForwardList, a WebCore class. In this way, we can differentiate which navigations are new, and which are session history navigations.
5. RenderView::DidCommitLoadForFrame handles the commit for the load. Here, the previous page's state is stored in session history, via the ViewHostMsg_UpdateState message. This will tell the browser to update the corresponding NavigationEntry (identified by RenderView's current page ID) with the new history state.
6. RenderView's current page ID is updated to reflect the committed page. For a new navigation, a new unique page ID is generated. For a session history navigation, it will be the page ID originally assigned when it was first visited, which we had stored on the WebRequest when initiating the navigation.
7. A ViewHostMsg_FrameNavigate message is sent to the browser, updating the corresponding NavigationEntry (identified by RenderView's newly updated page ID) with the new URL and other information