Partager l’état de l’application en direct

Imagine multiple people working on the same document or playing the same game. They need to see the same information and changes in real-time. This common, synchronized information is called shared application state.

It's the data that reflects the current status of an application, accessible and modifiable by all connected participants.

Sharing state live is crucial for creating truly collaborative and interactive experiences. Think about:

• Collaborative Editing: Multiple users typing in a document simultaneously.
• Multi-user Drawing: Everyone sees lines appear as they are drawn.
• Online Games: Synchronizing player positions, scores, and game events.
• Shared Whiteboards: Real-time updates to drawings and notes.

It makes applications feel responsive and connected.

In WebRTC, RTCDataChannel is your go-to tool for sharing application state. Unlike media streams (audio/video), data channels are designed for sending arbitrary data, from text messages to binary files.

They provide a fast, direct, and secure peer-to-peer connection, making them ideal for small, frequent state updates without latency.

Before sending, you need to decide how to structure your application state. A common and flexible way is using plain JavaScript objects or JSON (JavaScript Object Notation).

This allows you to easily store different types of data like text, numbers, and nested objects, ready for serialization.

Try running this example of a simple state object:

When a user makes a change, you update your local state. Then, you need to send this change to other peers. Data Channels expect string or binary data.

We use JSON.stringify() to convert our JavaScript object into a JSON string before sending it via dataChannel.send() (conceptually).

Here's how to prepare an update:

When another peer sends an update, your dataChannel.onmessage event handler will receive it. The received data will be a string (or ArrayBuffer).

You then use JSON.parse() to convert the JSON string back into a JavaScript object. Finally, you apply these changes to your local application state, often using Object.assign() for merging.

See how to process a received update:

When sharing state, you can either send the entire application state every time a change occurs, or just send the "diff" (delta update), which is only the part of the state that changed.

For large states or frequent small changes, sending only diffs is more efficient as it uses less bandwidth. However, sending the full state can be simpler to implement and more robust against missed messages in some scenarios.

What happens if two users try to change the same part of the state at the exact same time? This is a state conflict.

Simple solutions include "last write wins" (the last received update overrides previous ones). More advanced systems use techniques like CRDTs (Conflict-free Replicated Data Types), which are data structures designed to merge changes from different sources automatically without conflicts.

Let's imagine a simple shared counter application. Each peer has a button to increment the counter.

1. Initial State: Both peers start with counter: 0.
2. User Action: Peer A clicks "Increment". Local counter becomes 1.
3. Send Update: Peer A sends {"counter": 1} via Data Channel.
4. Receive Update: Peer B receives {"counter": 1}, parses it, and updates its local counter to 1.
5. Synchronization: Both peers now show counter: 1.

This simple flow demonstrates the core of live state synchronization.

In this lesson, you learned about the importance of sharing application state live for collaborative features. We covered how RTCDataChannel is ideal for this, and the process of representing, sending, and receiving state updates using JSON serialization.

You also explored challenges like state conflicts and strategies for choosing between full state and delta updates. These techniques are fundamental for building responsive, multi-user applications.