//content/public is the API exposed to embedders of the content module.

• isolate developers working on Chrome from inner workings of content
• make the boundary between content and chrome clear to developers and other embedders

In general, we follow the design of the Blink Public API. This makes it easier for people who're already familiar with it, and also keeps things consistent.

• //content/public should contain only interfaces, enums, structs and (rarely) static functions.
  • An exception is //content/public/test. We allow concrete classes that chrome test classes derive from or use in here.
• Tests for files in //content/public should be inside implementation directories, e.g. //content/browser, //content/renderer etc...
• While we don't allow old-style Chrome IPC _messages.h files in //content/public, we do allow .mojom files (see discussion). If a mojom is only used inside content, it should be in //content/common. If it's an interface that is implemented or called by content's embedder, then it belongs in //content/public/common.
• In general, if there is a struct or enum which is only used by an interface, they are put in the same file, but when the struct/enum is used in other places or if it's pretty big, then it should be in its own file.
• All code under //content should be in the "content" namespace.
• Interfaces that content implements usually should be pure abstract, because usually there's only one implementation. These should not be implemented outside of content. (i.e., content will freely assume that it can cast to its implementation(s)).
• Interfaces that embedders implement, especially ones which are used in tests or are observer-style and have many implementations, should have default (empty) implementations.
• Prefer enum classes over enum types. For enum types, the value should start with the name of the type, i.e., PAGE_TRANSITION_LINK in the content::PageTransition enum.
• content implementation code should use other implementations directly and not go through the interface (i.e., code in //content/renderer should use RenderViewImpl instead of content::RenderView).
• It's acceptable to put implementation files that hold constructors/destructors of interfaces/structs which might have member variables. For structs, this covers initializing member variables. For interfaces (i.e. RenderViewObserver) this might cover things like automatic registration/unregistration. Normally we would put this small code in headers, but because of the clang checks against putting code in headers, we're forced to put it in .cc files (we don't want to make a clang exception for the content/public directory since that would lead to confusion).
• When code in chrome implements an interface from content, usually the convention is to prefix the implementation with "Chrome" (i.e. ChromeContentBrowserClient derives from content::ContentBrowserClient).
• Only expose methods in the public API that embedders need. If a method is only used by other code in content, it belongs in foo_impl.h and not foo.h.
• Methods in the API should be there because either content is calling out to its embedder, or the embedder is calling to content. There shouldn't be any methods which are used to call from the embedder to the embedder.
• All classes/structs/enums in the public API must be used by embedders and content. i.e. if the chrome layer uses a struct but content doesn't know about it, it doesn't belong in //content/public but instead some module that's higher level.
• We avoid single-method delegate interfaces, and in those case we use callbacks.
• The const identifier can be added to simple getter APIs implemented by content. Don't add const to interfaces implemented by the embedder, where we can't make assumptions about what the embedder needs to implement it.
• Observer interfaces (i.e. WebContentsObserver, RenderFrameObserver, RenderViewObserver) should only have void methods. This is because otherwise the order that observers are registered would matter, and we don't want that. The only exception is OnMessageReceived(), which is fine since only one observer class handles each particular IPC, so ordering doesn't make a difference.

Large parts of the Chromium codebase depend on //content/public. Some notable directories that depend on it are (parts of) //extensions, //chrome, and //weblayer. Some directories in //components also depend on it, while conversely //content depends on some components.

Directories that do not depend on content include //third_party/blink and //services.

When adding and reviewing DEPS changes that take a dependency on content, some things to consider are:

• Directories outside content can only depend on code inside //content/public and not //content itself or other subdirectories.
• Try to consider whether it makes architectural sense for the directory to depend on content.
• Circular dependencies are not allowed. One reasonable way to check this is git gs <directory> inside //content. There is no complete automated check at this time.
• Figure out if code really needs //content. For example, if it just needs BrowserThread, you could instead inject the main task runner (or they can grab it from the static getter on creation of their object).
• Use //content/public/test if only test code is needed.
• //components subdirectories can depend on //content/public, but be careful of circular dependencies because content depends on some components. Full guidelines for components are in the README.
• Confirm the code is running in the correct process: e.g., //component/foo/browser can only use //content/public/browser. Some modules/components run only in one process and don't have the explicit directory name.
• General DEPS tip: If an ancestor DEPS file already adds a dependency, the descendent DEPS file does not need to add the dependency also, unless something explicitly overrode the dependency. When reviewing a CL that adds a dependency in a descendent directory, confirm that it is required for the build to succeed, and if so determine what overrode the dependency and why. An example of this is content/public/browser/tts_controller.h in /chrome/browser/DEPS.