# BrowserMain Initialization

[BrowserMainParts](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=119;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) is a critical class to initialize browser instance on startup.
This class defines the methods executed by BrowserMain() call.
Let's see the each step and see the example of Chrome Impl [ChromeBrowserMainParts](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/chrome_browser_main.h;l=43;drc=d21b40da81ef43caea8c9452514041c1dc60612d)

## Overview
[BrowserMainRunnerImpl::Initialize](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_runner_impl.cc;l=71;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) calls the methods in BrowserMainParts.
It creates [`main_loop_`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_runner_impl.cc;l=108;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) and initialize it.
Then it goes through in the below order.
- EarlyInitialization
- InitializeToolkit
- PreCreateMainMessageLoop
- CreateMainMessageLoop
- PostCreateMainMessageLoop
- CreateStartupTasks

BrowserMainParts class is defined to make these steps clear and let platform-specific implementations align each steps. This design makes it clearer how the initialization goes.

## EarlyInitialization
Pre/PostEarlyInitialization runs at [BrowserMainLoop::EarlyInitialization](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_loop.cc;l=534;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1).

[PreEarlyInitialization](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=128;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) runs soon after the program start. It must contains things to be done ASAP.

For example, [ChromeBrowserMainParts::PreEarlyInitialization](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/chrome_browser_main.cc;l=747;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) first creates BrowserProcessImpl instance, getting chrome feature lists, creating Metrics service, getting local state.

It returns int value as an error code. If the error is non-zero, BrowserMainRunnerImpl immediately calls CreateMessageLoopForEarlyShutdown and skip everything comes next.

On the other hand, [PostEarlyInitialization](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=129;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) runs at the end of [BrowserMainLoop::EarlyInitialization](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_loop.cc;l=534;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1).
Similarly to PreEarlyInitalization, it should be things to be done soon, but can wait or require initialization.
In the [Chrome](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/chrome_browser_main.cc;l=798;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) implementation, it does not do anything special and just delegate it to extra parts.

## InitializeToolkit
[BrowserMainLoop::InitializeToolkit](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_loop.cc;l=1473;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) runs next.
This is for UI related initialization.

This creates aura::Env instance. For non-win platform, that's all.
Then [BrowserMainParts::ToolkipInitialized](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=130;drc=adac219925ef5d9c9a954d189c2e4b8852a4bbed) is called.
It creates [ColorProviderManager](https://source.chromium.org/chromium/chromium/src/+/main:ui/color/color_provider_manager.h;l=32;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1), the class to manages and provides color providers. Here, color means a real color on UI.

## CreateMainMessageLoop
[BrowserMainParts::PreCreateMainMessageLoop](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=131;drc=adac219925ef5d9c9a954d189c2e4b8852a4bbed) is called next.
In general, there is nothing here. Platform-specific codes implement it. For example, Ash creates ChromeSessionManager.

Then, [BrowserMainLoop::CreateMainMessageLoop](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_loop.cc;l=644;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) run.
This creates `main_thread_` initialized by BrowserThreadImpl. The thread type is BrowserThread::UI and the task runner is SingleThreadTaskRunner.

After that, [BrowserMainParts::PostCreateMainMessageLoop](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=132;drc=adac219925ef5d9c9a954d189c2e4b8852a4bbed) runs.
It sets main thread created by the previous step, and creates TraceEventSystemStatsMonitor.

## CreateStartupTasks
[BrowserMainLoop::CreateStartupTasks](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_main_loop.cc;l=848;drc=1d2715064cc686e5eac4e41f34dbc212d47deff1) is the last step in BrowserMainRunnerImpl::Initialize.

This creates `startup_task_runner_` and create threads.
It will trigger [BrowserMainParts::PreCreateThread](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=133;drc=adac219925ef5d9c9a954d189c2e4b8852a4bbed) and [BrowserMainParts::PostCreateThreads](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/browser_main_parts.h;l=134;drc=adac219925ef5d9c9a954d189c2e4b8852a4bbed) before and after the creation.

This CreateThread step is a single-threaded initialization, so immutable Singletons that are initialized once nd read-only from all threads must be created at this point.