The “content” module is located in
src/content, and is the core code needed to render a page using a multi-process sandboxed browser. It includes all the web platform features (i.e. HTML5) and GPU acceleration. It does not include Chrome features, e.g. extensions/autofill/spelling etc.
As the Chromium code has grown, features inevitably hooked into the wrong places, causing layering violations and dependencies that shouldn‘t exist. It’s been hard for developers to figure out what the “best” way is because the APIs (when they existed) and features were together in the same directory. To avoid this happening, and to add a clear separation between the core pieces of the code that render a page using a multi-process browser, consensus was reached to move the core Chrome code into
src/content (content not chrome :) ).
As discussed above,
content should only have the core code needed to render a page. Chrome features use APIs that are provided by
content to filter IPCs and get notified of events that they require. How to Add New Features (without bloating RenderView/RenderViewHost/WebContents) describes how to do this.
As an example, here‘s a (non-exhaustive) list of features that are Chrome only, and so are not in content. This means that
content code shouldn’t have to know anything about them, only providing generic APIs that they can be built upon.
As the list above shows, even browser features that are common to modern browsers are not in
content. The dividing line is that
src/content only has code that is required to implement the web platform. Features that aren't covered by web specs should live in
src/chrome. If a feature is being implemented and the team foresees that it would be a spec, it should still go in
src/chrome. Once it has a spec, then it can move to
Where code interacts with online network services that must be supplied by the vendor, the favored approach is to fully implement that feature outside of the
content module. E.g. from the list above Safe Browsing, Translate, Sync and Autofill require various network services to function, and the
chrome layer is the natural place to encapsulate that behavior. For those few cases where we need to make network requests using code in the content module in order to implement generic HTML5 features (e.g. the network location service for Geolocation), the embedder must fully define the the endpoint to connect to, typically it might do this by injecting the service URL. We do not want any such policy coded into the
content module at all, again to keep it generic.
TODO: Draw a modern diagram.
See an older diagram at: https://www.chromium.org/developers/content-module.
The diagram illustrates the layering of the different modules. A module can include code directly from lower modules. However, a module can not include code from a module that is higher than it. This is enforced through DEPS rules. Modules can implement embedder APIs so that modules lower than them can call them. Examples of these APIs are the WebKit API and the Content API.
The Content API is how code in content can indirectly call Chrome. Where possible, Chrome features try to hook in by filtering IPCs and listening to events per How to Add New Features (without bloating RenderView/RenderViewHost/WebContents). When there isn't enough context (i.e. callback from WebKit) or when the callback is a one-off, we have a
ContentClient interface that the embedder (Chrome) implements.
ContentClient is available in all processes. Some processes also have their own callback API as well, i.e.
The current status is
content doesn't depend on chrome at all (see the meta bug and all bugs it depends on). We now have a basic browser built on top of
content_shell”) that renders pages using
content on all platforms. This allow developers working on the web platform and core code to only have to build/test content, instead of all of chrome.
We have a separate target for
content's unit tests in
content_unittests, and integration tests in
content is build at a separate dll to speed up the build.
We‘ve created an API around
content, similar to our WebKit API. This isolates embedders from content’s inner workings, and makes it clear to people working on content which methods are used by embedders.