Add IPC documentation

docs/Deep Dive/Architecture/WebKit2.md

@@ -21,6 +21,91 @@ and it does not have direct access to the underlying operating system’s [clipb
 microphone, or video camera even though there are Web APIs that grant access to those features.
 Instead, UI process brokers such requests.
 
-FIXME: How is IPC setup
+## How is IPC Setup
 
-FIXME: How to add / modify an IPC message
+In WebKit, Inter-Process Communication (IPC) enables communication between different processes. The IPC setup in WebKit involves several key components:
+
+### Message Definitions
+
+Messages are defined in `*.messages.in` files (e.g., `WebPage.messages.in`). These files specify the structure and types of messages exchanged between processes. Each `*.messages.in` file must map to a header file with the same name (e.g., `WebPage.messages.in` maps to `WebPage.h`). These `*.messages.in` files are processed at build time to generate source code for receiving IPC messages. They support both synchronous and asynchronous messages.
+
+### Message Handlers
+
+Each process has message handlers that process incoming messages and perform the necessary actions. The message definitions in the `*.messages.in` file need to have corresponding implementations in the related class (e.g., `WebPage` implements handlers for the corresponding messages defined in `WebPage.messages.in`). The processing of the `*.messages.in` files during build time generates source code that maps a message definition to its corresponding handler implementation, which will automatically call it for you when that message is received. The message handlers must adhere to the correct function signatures specified in the `*.messages.in` files, otherwise you will get a build error.
+
+### IPC::MessageSender and IPC::MessageReceiver
+
+Classes that send or receive IPC messages typically inherit from `IPC::MessageSender` and `IPC::MessageReceiver`. These base classes automatically handle the serialization and deserialization of messages, as well as the setup of message dispatching.
+
+For a deeper dive, you can found WebKit's IPC code in the `Source/WebKit/Platform/IPC` directory. This directory contains the classes and utilities needed to manage IPC, including message sending and receiving, connection management, and the serialization and deserialization of messages.
+
+## How to Add or Modify an IPC Message
+
+Adding or modifying an IPC message in WebKit involves several steps to ensure proper communication between classes in different processes. Here is a step-by-step guide:
+
+### 1. Define the Message
+
+Locate the appropriate `*.messages.in` file (e.g., `WebPage.messages.in`). If a new message file is needed, it should be placed in the same directory as the class you are implementing it for and it should also be added to the directory's `Sources.txt` file, as mentioned [here](../Build/AddingNewFile.html).
+Now, define the new message in this file. For example, let's say we want to send a message called `LoadURL` from the UI process' `WebPageProxy` class to the WebContent process' `WebPage` class. In `WebPage.messages.in`, you would add an entry like:
+
+```
+LoadURL(String url)
+```
+
+### 2. Implement the Message Handler
+
+In the class that will receive the message (e.g., `WebPage`), implement the handler method. This method will be called when the message is received. For example:
+
+```
+void WebPage::loadURL(const String& url) {
+    // Implementation code to load the URL
+}
+```
+
+### 3. Send the Message
+
+To send the message from another process, use the `send` method of the `IPC::MessageSender` class. For example, to send the `LoadURL` message from the UI process' `WebPageProxy` class to the WebContent's `WebPage` class:
+
+```
+send(Messages::WebPage::LoadURL(url));
+```
+
+Note, `WebPageProxy` inherits from `IPC::MessageSender`, which is what makes this possible.
+
+## Replying to Messages
+
+In addition to sending messages, WebKit's IPC mechanism supports replying to messages, which can be done synchronously or asynchronously.
+
+### 1. Define the Message Reply
+
+In the definition of the message in the `*.messages.in` file, specify that it expects a reply. For example, in `WebPage.messages.in`:
+
+```
+GetTitle() -> (String title)
+```
+
+By default, the reply is handled asynchronously. In order to make it synchronous, add `Synchronous` after the message definition, for example:
+
+```
+GetTitle() -> (String title) Synchronous
+```
+
+### 2. Implement the Message Handler with Reply
+
+Implement the handler method in the receiving class that will process the message and send a reply. For example, in the WebContent process' `WebPage.h`:
+
+```
+void WebPage::getTitle(CompletionHandler<void(String)>&& completionHandler) {
+    completionHandler(m_title);
+}
+```
+
+### 3. Send the Asynchronous Message and Handle Reply
+
+To send the asynchronous message and handle the reply, use the `sendWithAsyncReply` method of the `IPC::MessageSender` class. For example, in the UI process' `WebPageProxy.cpp`:
+
+```
+sendWithAsyncReply(Messages::WebPage::GetTitle(), [this, weakThis = WeakPtr { *this }](String title) {
+    // do something with title...
+});
+```