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Web Apps

What are web apps?

Simply put web apps are sites that the user installs onto their machine mimicking a native app installed on their respective operating system.

User entry points

Sites that meet our install promotion requirements will have an install prompt appear in the omnibox on the right. Users can also install any site they like viaMenu > More tools > Create shortcut....

• Example site:https://developers.google.com/

Users can see all of their web apps onchrome://apps (viewable on non-ChromeOS).

Developer interface

Sites customize how their installed site integrates at the OS level using aweb app manifest. See developer guides for in depth overviews:

• https://web.dev/progressive-web-apps/
• https://web.dev/codelab-make-installable/

Presentation

Seehttps://tinyurl.com/dpwa-architecture-public for presentation slides.

Terms & Phrases

SeeWeb Apps - Concepts.

Debugging

Usechrome://web-app-internals to inspect internal web app state. For Chromium versions prior to M93 usechrome://internals/web-app.

To test the behavior of the web app itself, Chrome DevTools Protocol can be used. SeeInstruction of using PWA via CDP.

Documentation Guidelines

• Markdown documentation (files like this):
  • Contains information that can't be documented in class-level documentation.
  • Answers questions like: What is the goal of a group of classes together? How does a group of classes work together?
  • Explains concepts that are used across different files.
  • Should be unlkely to become out-of-date.
    • Any source links should link to a codesearch ‘search’ page and not the specific line number.
    • Avoid implementation details.
• Class-level documentation (documentation in header files):
  • Answers questions like: Why does this class exist? What is the responsibility of this class? If this class involves a process with stages, what are those stages / steps?
  • Should be updated actively when that given file is changed.
• Documentation inside of methods
  • Should explain the “why” of code if it is not clear.
  • Should be avoided otherwise.

What makes up Chromium's implementation?

The task of turning websites into “apps” in the user's OS environment has many parts to it. Before going into the parts, here is where they live:

See sourcehere.

• TheWebAppProvider core system lives on theProfile object.
• TheWebAppUiManagerImpl also lives on theProfile object (to avoid deps issues).
• TheAppBrowserController (typicallyWebAppBrowserController for our interests) lives on theBrowser object.
• TheWebAppTabHelper lives on theWebContents object.

While most on-disk storage is done in theWebAppSyncBridge, the system also sometimes uses thePrefService. Most of these prefs live on theProfile (profile->GetPrefs()), but some prefs are in the global browser prefs (g_browser_process->local_state()).

Presentation:https://tinyurl.com/dpwa-architecture-public

Older presentation:https://tinyurl.com/bmo-public

Architecture Philosophy

There are a lot of great guidelines within Chromium

• Style guides
• Dos and Don'ts
• etc.

Other than general guidance of minimal complexity and having single-responsibility classes, some goals of our system:

• Tests should operate on thepublic interface as much as possible. Refactors to the internal system should not involve fixing / modifying tests.
• External dependencies should be behind fake-able interfaces, allowing unit & browser tests to swap these out. However, internal parts of our system should not be mocked out or faked - this tightly couples the internal implementation to our tests. If it is impossible to trigger a condition with the public interface, then that condition should be removed (or the public interface improved).
  • Here is a nicepresentation about testing that might clarify our approach.

Public Interface

This public interface should (and will) be improved, however this is the basic state as of 2022/11/09:

• WebAppCommandScheduler. Internally this schedulesWebAppCommands to do safe operations on the system.
  • This already includes a variety of operations like installation, launching, etc.
• Observers like theAppRegistrarObserver orWebAppInstallManagerObserver. However, users of these MUST NOT modify the web app system in the observation call - this can cause race conditions and weird re-entry bugs.
• To query for apps with specific capabilities (like being installable, opening in a window, etc.), use aWebAppFilter. This is the preferred way to find apps that match a given criteria.
• Items exposed from the locks given to commands or callbacks:
  • WebAppRegistrar
  • Writing to the database usingScopedRegistryUpdate and theWebAppSyncBridge.
  • Pref reading & writing
  • WebAppIconManager supports icon fetch for a given web app.
  • etc - see the documentation on the lock for more guidance.

Some parts of the system that are used within commands:

• WebAppUrlLoader &WebAppDataRetriever are used in commands, but this interface could be improved & does not have a formal factory yet.
• WebAppInstallFinalizer is used in commands and could be improved.

External Dependencies

The goal is to have all of these behind an abstraction that has a fake to allow easy unit testing of our system. Some of these dependencies are behind a nice fake-able interface, and some are not (yet).

• Extensions - Some of our code still talks to the extensions system, specifically thePreinstalledWebAppManager.
• content::WebContents: The WebAppProvider system interacts withcontent::WebContents for various tasks like loading URLs (viaWebAppUrlLoader), retrieving web app manifest data and icons (viaWebAppDataRetriever andWebAppIconDownloader respectively), and observing navigations and destruction. TheWebContentsManager serves as a centralized point of dependency for these interactions and acts as a factory for these components, allowing for easier management and faking in tests.
• OS Integration: Each OS integration has fairly custom code on each OS to do the operation. This is difficult to coordinate and test. Currently theOsIntegrationManger manages this, which has a fake version.
• Sync system: There is a tight coupling between our system and the sync system through the WebAppSyncBridge. Faking this is easy and is handled by theFakeWebAppProvider.
• UI: There are parts of the system that are coupled to UI, like showing dialogs, determining information about app windows, etc. These are put behind theWebAppUiManager, and faked by theFakeWebAppUiManager.
• Policy: Our code depends on the policy system setting its policies in appropriate prefs for us to read. Because we just look at prefs, we don't need a “fake” here.

Databases / sources of truth

These store data for our system. Some of it is per-web-app, and some of it is global.

• WebAppRegistrar: This attempts to unify the reading of much of this data, and also holds an in-memory copy of the database data (in WebApp objects).
• WebAppDatabase /WebAppSyncBridge: This stores the web_app.proto object in a database, which is the preferred place to store information about a web app.
• Icons on disk: These are managed by theWebAppIconManager and stored on disk in the user's profile.
• Prefs: ThePrefService is used to store information that is either global, or needs to persist after a web app is uninstalled. Most of these prefs live on theProfile (profile->GetPrefs()), but some prefs are in the global browser prefs (g_browser_process->local_state()). Some users of prefs:
  • AppShimRegistry
  • UserUninstalledPreinstalledWebAppPrefs
• OS Integration: Various OS integration requires storing state on the operating system. Sometimes we are able to read this state back, sometimes not.

None of this information should be accessed without an applicable ‘lock’ on the system.

Managers

These are used to encapsulate common responsibilities or in-memory state that needs to be stored.

WebContentsManager

• Purpose: Manages dependencies oncontent::WebContents for the WebAppProvider system. It acts as a factory forWebAppUrlLoader,WebAppDataRetriever, andWebAppIconDownloader.
• Key Responsibilities:
  • Provides concrete implementations of URL loading and data retrieval abstractions.
  • Enables the use of fakes for these components in unit tests, reducing the need for a full browser environment.
• Location:chrome/browser/web_applications/web_contents/web_contents_manager.h

Commands

Commands are used to encapsulate operations in the system, and use Locks to ensure that your operation has isolation from other operations.

• If you need to change something in the WebAppProvider system, you should probably use a command.
• Commands talk to the system using locks they are granted. The locks should offer access to “managers” that the commands can use.
• Commands expose aToDebugValue() method that is logged on completion and exposed in thechrome://web-app-internals. This can be very helpful for debugging and bug reports.

Note: There are DVLOGs in theWebAppCommandManager that can be helpful.

Locks /WebAppLockManager

Locks allow operations to receive appropriate protections for what they are doing. For example, anAppLock will guarantee that no one is modifying, installing, or uninstalling that AppId while it is granted.

Locks contain assessors that allow the user to access parts of the web app system. This is the safest way to read from the system.

Note: There are DVLOGs in theWebAppLockManager that can be helpful.

OS Integration

Anything that involves talking to the operating system. Usually has to do with adding, modifying, or removing the os entity that we register for the web app.

Deep Dives

• /docs/webapps/installation_pipeline.md
• /docs/webapps/manifest_representations.md
• /docs/webapps/integration-testing-framework.md
• /docs/webapps/os_integration.md
• /docs/webapps/manifest_update_process.md
• /docs/webapps/isolated_web_apps.md
• /docs/webapps/webui_web_app.md

How To Use

See thepublic interface section about which areas are generally “publicly available”.

The system is generally unit-test-compabible through theFakeWebAppProvider, which is created by default in theTestingProfile. Sometimes tests require using theAwaitStartWebAppProviderAndSubsystems function in the setup function of the test to actually start the web app system & wait for it to complete startup. Seetesting.md for more information.

There is a long-term goal of having the system be easily fake-able for customers using it. The best current ‘public interface’ distinction of the system is theWebAppCommandScheduler.

To access or change information about a web app:

• Obtain a lock from theWebAppLockManager, or (preferably) create a command with the relevant lock description.
• When the lock is obtained (or the command is started with the lock), use the lock to access the data you need.
  • Generally, you should use theWebAppRegistrar to get the data you need. This unifies many of our data sources into one place.
• If changing data, change the data depending on the source of truth.
  • For information in the database, use aScopedRegistryUpdate.
  • Otherwise use the relevant manager / helper to modify the data.
  • Some things expect “observers” to be notified. That integration is currently inWebAppSyncBridge, but can be pulled out.

Other guides:

• /docs/webapps/why-is-this-test-failing.md
• /docs/webapps/how-to-create-webapp-integration-tests.md

Debugging

chrome://web-app-internals

This page allows you to see all of the internal information about the WebAppProvider system, including a truncated log of the debug information of the last run commands.

It is often very useful to ask users to attach a copy of this page in bug reports.

The integration tests will print out thecontents of this page if a test fails, which can help debug that failure as well.

DVLOGs

The codebase has a number of useful DVLOGs:

• web_app_command_manager.cc: These will log various state changes of commands and the debug values on completion.
• web_app_lock_manager.cc: This will log lock requests and any ‘held or pending’ lock holders for the requested lock at the time of the request. This does not necessarily mean those locks are blocking (as they may be shared locks) so you will have to look at the request locations to determine the status.

Testing

Please seetesting.md.

Relevant Classes

WebAppProvider

This is a per-profile object housing all the various web app subsystems. This is the “main()” of the web app implementation where everything starts.

WebApp

This is the representation of an installed web app in RAM. Its member fields largely reflect all the ways a site can configure theirweb app manifest plus miscellaneous internal bookkeeping and user settings.

WebAppRegistrar

This is where all theWebApps live in memory, and what many other subsystems query to look up any given web app's fields. Mutations to the registry have to go via ScopedRegistryUpdate orWebAppSyncBridge.

Accessing the registrar should happen through a Lock. If you access it through theWebAppProvider, then know that you are reading uncommitted (and thus unsafe) data.

Why is it full ofGetAppXYZ() getters for every field instead of just returning aWebApp reference? This is primarily done because the value may depend on multiple sources of truth. For example, whether the app should be run on OS login depends on both the user preference (stored in our database) and the administrator's policy (stored separately & given to us in-memory using prefs) Historically this was originally done because WebApps used be stored both in our database and extensions, and this served to unify the two.

WebAppSyncBridge

This is “bridge” between the WebAppProvider system‘s in-memory representation of web apps and the sync system’s database representation (along with sync system functionality like add/remove/modify operations). This integration is a little complex and deserves it's own document, but it basically:Stores all WebApps into a database and updates the database if any fields change. Updates the system when there are changes from the sync system.Installs new apps, uninstalls apps the user uninstalled elsewhere, updates metadata like user display mode preference, etc. Tells the sync system if there are local changes (installs, uninstalls, etc).

There is also a slide in a presentationhere which illustrates how this system works, but it may be out of date.

Note: This only stores per-web-app data, and that data will be deleted if the web app is uninstalled. To store data that persists after uninstall, or applies to a more general scope than a single web app, then thePrefService can be used, either on theProfile object (per-profile data,profile->GetPrefs()) or on the browser process(g_browser_process->local_state()``). Example of needing prefs: Storing if an app was previously installed as a preinstalled app in the past. Information is needed during chrome startup before profiles are loaded. A feature needs to store global data - e.g. “When was the last time we showed the in-product-help banner for any webapp?”

ExternallyManagedAppManager

This is for all installs that are not initiated by the user. This includespreinstalled apps,policy installed apps andsystem web apps.

These all specify a set ofinstall URLs which theExternallyManagedAppManager synchronises the set of currently installed web apps with.

WebAppInstallFinalizer

This is the tail end of the installation process where we write all our web app metadata todisk and deploy OS integrations (likedesktop shortcuts andfile handlers using theOsIntegrationManager.

WebAppUiManager

Sometimes we need to query window state from chrome/browser/ui land even though our BUILD.gn targets disallow this as it would be a circular dependency. Thisabstract class +impl injects the dependency at link time (see [WebAppUiManager::Create()'s][32]declaration and definition locations`).

AppShimRegistry

On Mac OS we sometimes need to reason about the state of installed PWAs in all profiles without loading those profiles into memory. For this purpose,AppShimRegistry stores the needed information in Chrome's “Local State” (global preferences). The information stored here includes:

• All profiles a particular web app is installed in.
• What profiles a particular web app was open in when it was last used.
• What file and protocol handlers are enabled for a web app in each profile it is installed in.

This information is used when launching a web app (to determine what profile or profiles to open the web app in), as well as when updating an App Shim (to make sure all file and protocol handlers for the app are accounted for).