This directory is meant to house features or subsystems that are used in more than one part of the Chromium codebase.
//ios/chrome) and Chrome on other platforms (//chrome).//ios doesn't depend on //chrome.//chrome and //android_webview.//third_party/blink/common. The distinction comes down to (a) whether Blink is the owner of the code in question or a consumer of it and (b) whether the code in question is shared by Chrome on iOS as well. If the code is conceptually its own cross-process feature with Blink as a consumer, then //components can make sense. If it‘s conceptually Blink code, then //third_party/blink/common likely makes more sense. (In the so-far hypothetical case where it’s conceptually Blink code that is shared by iOS, raise the question on chromium-dev@, where the right folks will see it).Note that the above list is meant to be exhaustive. A component should not be added just to separate it from other code in the same layer that is the only consumer; that can be done with strict DEPS or GN visibility rules.
OWNERS file under //components/{your component} and be responsible for maintaining your addition.//components/OWNER must approve of the location of your code.//content/common, //chrome/utility, etc.).Components cannot depend on the higher layers of the Chromium codebase:
//android_webview//chrome//chromecast//headless//ios/chrome//content/shellComponents can depend on the lower layers of the Chromium codebase:
//base//gpu//mojo//net//printing//uiComponents can depend on each other. This must be made explicit in the DEPS file of the component.
Components can depend on //content/public, //ipc, and //third_party/blink/public. This must be made explicit in the DEPS file of the component. If such a component is used by Chrome for iOS (which does not use content or IPC), the component will have to be in the form of a layered component.
//chrome, //ios/chrome, //content and //ios/web can depend on individual components. The dependency might have to be made explicit in the DEPS file of the higher layer (e.g. in //content/browser/DEPS). Circular dependencies are not allowed: if //content depends on a component, then that component cannot depend on //content/public, directly or indirectly.
As mentioned above, components that depend on //content/public, //ipc, or third_party/blink/public might have to be in the form of a layered component.
Components that have bits of code that need to live in different processes (e.g. some code in the browser process, some in the renderer process, etc.) should separate the code into different subdirectories. Hence for a component named ‘foo’ you might end up with a structure like the following (assuming that foo is not used by iOS and thus does not need to be a layered component):
components/foo - BUILD.gn, DEPS, DIR_METADATA, OWNERS, README.mdcomponents/foo/browser - code that needs the browser processcomponents/foo/common - for e.g. Mojo interfaces and suchcomponents/foo/renderer - code that needs renderer processThese subdirectories should have DEPS files with the relevant restrictions in place, i.e. only components/foo/browser should be allowed to #include from content/public/browser. Note that third_party/blink/public is a renderer process directory except for third_party/blink/public/common which can be used by all processes.
Note that there may also be an android subdir, with a Java source code structure underneath it where the package name is org.chromium.components.foo, and with subdirs after ‘foo’ to illustrate process, e.g. ‘browser’ or ‘renderer’:
components/foo/android/{OWNERS, DEPS}components/foo/android/java/src/org/chromium/components/foo/browser/components/foo/android/javatests/src/org/chromium/components/foo/browser/Code in a component should be placed in a namespace corresponding to the name of the component; e.g. for a component living in //components/foo, code in that component should be in the foo:: namespace.