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# Content module
## High-level overview
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.
## Motivation
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](http://blog.chromium.org/2008/10/content-not-chrome.html) :) ).
## content vs chrome
`content` should only contain code that is required to implement the web
platform. Generally, a feature belongs in this category if and only if all of
the following are true:
- Its launch is tracked on the <https://chromestatus.com/> dashboard.
- It has an associated spec.
- It is going through the [feature development
lifecycle](https://www.chromium.org/blink/launching-features).
In contrast, many features that are common to modern web browsers do not satisfy
these criteria and thus, are not implemented in `content`. A non-exhaustive
list:
- Extensions
- NaCl
- SpellCheck
- Autofill
- Sync
- Safe Browsing
- Translate
Instead, these features are implemented in `chrome`, while `content` only
provides generic extension points that allow these features to subscribe to the
events they require. Some features will require adding new extension points: for
more information, see [How to Add New Features (without bloating
RenderView/RenderViewHost/WebContents)](https://www.chromium.org/developers/design-documents/multi-process-architecture/how-to-add-new-features).
Finally, there are a number of browser features that require interaction with
online services supplied by the vendor, e.g. from the above list, Safe Browsing,
Translate, Sync, and Autofill all require various network services to function.
The `chrome` layer is the natural place to encapsulate that vendor-specific
integration behavior. For the rare cases where a web platform feature
implemented in `content` has a dependency on a network service (e.g. the network
location service used by Geolocation), `content` should provide a way for the
embedder to inject an endpoint (e.g. `chrome` might provide the service URL to
use). The `content` module itself must remain generic, with no hardcoded
vendor-specific logic.
## Architectural Diagram
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.
## Content API
The [Content API](public/README.md) 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)](https://www.chromium.org/developers/design-documents/multi-process-architecture/how-to-add-new-features).
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.
`ContentBrowserClient/ContentRendererClient/ContentPluginClient`.
## Status and Roadmap
The current status is `content` doesn't depend on chrome at all (see the meta
[bug](https://bugs.chromium.org/p/chromium/issues/detail?id=76697) and all bugs
it depends on). We now have a basic browser built on top of `content`
("`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_browsertests`.
`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.
## Further documentation
* [Bluetooth](browser/bluetooth/README.md)
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