# [Web Apps](README.md) - Operating System Integration
The WebAppProvider system has to provide a lot of integrations with operating system surfaces for web apps. This functionality is usually different per operating system, and is usually invoked through the [`OsIntegrationManager`][2].
The [`OsIntegrationManager`][2]'s main responsibility is support the following operations:
1. Install operating system integration for a given web app.
1. Update operating system integration for a given web app.
1. Uninstall/remove operating system integration for a given web app.
It owns sub-managers who are responsible for each individual operating system integration functionality (e.g. [`web_app_file_handler_manager.h`][1] which owns the file handling feature). That manager will implement the non-os-specific logic, and then call into functions that have os-specific implementations (e.g. `web_app_file_handler_registration.h/_mac.h/_win.h/_linux.h` files).
Below are sections describing how each OS integration works.
## Protocol Handler
The Protocol Handler component is responsible for handling the registration and
unregistration of custom protocols for web apps ([explainer](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/URLProtocolHandler/explainer.md)).
### WebAppProtocolHandlerManager
The entrypoint to the component is `web_app_protocol_handler_manager.cc`, which contains
methods for registering and unregistering custom protocol handlers for web apps,
as well as any other logic related to protocol handling that should be
OS-agnostic - such as URL translation (translating from a protocol URL to a web
app URL) and interactions with the `ProtocolHandlerRegistry`, the browser
component that stores information about handlers internally.
`WebAppProtocolHandlerManager` is also responsible for leveraging the
web_app_protocol_handler_registration API to register and unregister protocols
with the underlying OS, so that custom protocols for a web app can be used
from outside the browser.
### web_app_protocol_handler_registration
`web_app_protocol_handler_registration` exposes a simple API that has
OS-specific files / implementations. This API communicates with other components
responsible for directly managing OS integrations. For example, on Windows this
interacts with the `ShellUtil` class, which manages the interactions with the
Windows registry for us.
Since some of these interactions with the OSes can be a bit costly, we try to do
as much as possible off the main thread. For the Windows implementation, we
post a task and wait for confirmation before proceeding with other steps (such
as updating the browser internal registry).
### ProtocolHandlerRegistry
Protocol handlers, like other Web App features, interact both with the OS and
the browser. On the browser, `protocol_handler_registry.cc` stores and manages
all protocol handler registrations, for both web apps and web sites (registered
via the HTML5 `registerProtocolHandler` API). The `WebAppProtocolHandlerManager` is
responsible for keeping both the OS and the browser in sync, by ensuring OS
changes are reflected in the browser registry accordingly.
### Flow of execution for a protocol registration call (Windows)
The flow of execution is similar for both registrations and unregistrations, so
we only describe registrations below.
1) Other components (typically the `OSIntegrationManager`) call into
WebAppProtocolHandlerManager:
```cpp
protocol_handler_manager_->RegisterOsProtocolHandlers(app_id);
```
2) `WebAppProtocolHandlerManager` forwards that to
`web_app_protocol_handler_registration`, which is OS specific. That API registers
protocols with the OS via `ShellUtil` utilities off the main thread:
```cpp
base::ThreadPool::PostTaskAndReply(
FROM_HERE,
{base::MayBlock(), base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
base::BindOnce(&RegisterProtocolHandlersWithOSInBackground, app_id,
base::UTF8ToUTF16(app_name), profile, profile->GetPath(),
protocol_handlers, app_name_extension),
base::BindOnce(&CheckAndUpdateExternalInstallations, profile->GetPath(),
app_id));
```
3) Once the protocols are registered successfully, a callback is invoked inside
`RegisterProtocolHandlersWithOSInBackground` which then completes the
registration with the `ProtocolHandlerRegistry`.
```cpp
ProtocolHandlerRegistry* registry =
ProtocolHandlerRegistryFactory::GetForBrowserContext(profile);
registry->RegisterAppProtocolHandlers(app_id, protocol_handlers);
```
4) `CheckAndUpdateExternalInstallations` is then called as the reply to the task
in 2) and checks if there is an installation of this app in another profile that
needs to be updated with a profile specific name and executes required update.
[1]: /chrome/browser/web_applications/os_integration/web_app_file_handler_manager.h
[2]: /chrome/browser/web_applications/os_integration/os_integration_manager.h
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