Lobby and matchmaking basics in Unity - Performance & Optimization

Performance: Lobby and matchmaking basics

MEDIUM IMPACT

This concept affects the initial load time and responsiveness of multiplayer game sessions by managing player grouping and connection setup.

Grouping players into a game session quickly and smoothly

Unity

async Task StartMatchmakingAsync() {
  await ConnectAllPlayersAsync(allPlayers);
  UpdateUIOnce();
}

Batch connecting players asynchronously and updating UI once reduces main thread blocking and reflows.

📈 Performance GainSingle UI update and non-blocking matchmaking improves responsiveness and reduces input delay.

Grouping players into a game session quickly and smoothly

Unity

void StartMatchmaking() {
  foreach (var player in allPlayers) {
    ConnectPlayerToSession(player);
    UpdateUI();
  }
}

Connecting players one by one and updating UI each time causes multiple reflows and UI thread blocking.

📉 Performance CostTriggers multiple UI updates and blocks main thread, causing input lag and slow matchmaking.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sequential player connection with UI update per player	High (multiple updates)	Multiple reflows	High paint cost	[X] Bad
Batch player connection with single UI update	Low (single update)	Single reflow	Low paint cost	[OK] Good

Rendering Pipeline

Lobby and matchmaking operations affect the UI rendering pipeline by triggering layout recalculations and paint operations when player lists or statuses update.

→Layout

→Paint

→Composite

⚠️ BottleneckLayout recalculation caused by frequent UI updates during matchmaking

Core Web Vital Affected

INP

This concept affects the initial load time and responsiveness of multiplayer game sessions by managing player grouping and connection setup.

Optimization Tips

1Batch UI updates during matchmaking to reduce layout recalculations.

2Use asynchronous operations to avoid blocking the main thread.

3Minimize frequent DOM or UI changes to improve input responsiveness.

Performance Quiz - 3 Questions

What is a main performance issue when updating the lobby UI after each player joins individually?

A Multiple layout recalculations causing input lag

B Too few DOM nodes created

C No impact on rendering performance

D Faster rendering due to incremental updates

DevTools: Performance

How to check: Record a session during matchmaking, look for long tasks and frequent layout recalculations in the flame chart.

What to look for: Look for reduced main thread blocking and fewer layout recalculations indicating better matchmaking performance.

Practice

1. What is the main purpose of a lobby in Unity multiplayer games?

easy

A. To let players wait and prepare before the game starts

B. To automatically find and connect players

C. To render 3D graphics for the game

D. To save player progress locally

Solution

1. Step 1: Understand the role of a lobby

   A lobby is a waiting area where players gather before the game begins.

2. Step 2: Differentiate from matchmaking

   Matchmaking is the process of finding and connecting players automatically, not the lobby itself.

3. Final Answer:

   To let players wait and prepare before the game starts -> Option A

4. Quick Check:

   Lobby = waiting area before game [OK]

Hint: Lobbies are for waiting; matchmaking is for finding players [OK]

Common Mistakes:

• Confusing lobby with matchmaking
• Thinking lobby handles graphics
• Assuming lobby saves progress

2. Which of the following is the correct way to override a lobby method in Unity C#?

easy

A. public override void OnPlayerJoined() {}

B. override public void OnPlayerJoined() {}

C. public void OnPlayerJoined() override {}

D. void override OnPlayerJoined() {}

Solution

1. Step 1: Recall C# override syntax

   In C#, the correct syntax places 'override' before the return type.

2. Step 2: Check each option

   public override void OnPlayerJoined() {} uses 'public override void MethodName()' which is correct syntax.

3. Final Answer:

   public override void OnPlayerJoined() {} -> Option A

4. Quick Check:

   Override keyword before return type [OK]

Hint: In C#, 'override' comes before the return type [OK]

Common Mistakes:

• Placing override after method name
• Omitting override keyword
• Wrong order of keywords

3. Given this code snippet in a Unity lobby script:

public override void OnPlayerJoined(Player player) {
    Debug.Log($"Player {player.Name} joined.");
    if (Players.Count >= 4) {
        StartGame();
    }
}

What happens when the 4th player joins?

medium

A. The lobby closes without starting the game

B. Nothing happens until manually started

C. The game starts automatically

D. An error occurs because Players.Count is undefined

Solution

1. Step 1: Analyze the OnPlayerJoined method

   When a player joins, it logs their name and checks if Players.Count is 4 or more.

2. Step 2: Understand the condition

   If Players.Count is 4 or more, StartGame() is called, starting the game automatically.

3. Final Answer:

   The game starts automatically -> Option C

4. Quick Check:

   Players.Count >= 4 triggers StartGame() [OK]

Hint: Check conditions that trigger StartGame() call [OK]

Common Mistakes:

• Assuming manual start needed
• Thinking Players.Count is undefined
• Confusing lobby closing with game start

4. Identify the error in this matchmaking code snippet:

public void Matchmake() {
    var match = Matchmaker.CreateMatch();
    match.OnPlayerJoined += OnPlayerJoined;
    match.Start();
}

void OnPlayerJoined(Player player) {
    Debug.Log("Player joined: " + player.Name);
}

medium

A. Start() method does not exist on match object

B. Event subscription syntax is incorrect

C. OnPlayerJoined method must be public to subscribe

D. Matchmaker.CreateMatch() might return null and is not checked

Solution

1. Step 1: Review event subscription

   The syntax 'match.OnPlayerJoined += OnPlayerJoined;' is correct for subscribing to events.

2. Step 2: Check for null safety

   Matchmaker.CreateMatch() might return null if no match is found, but code does not check for null before using match.

3. Final Answer:

   Matchmaker.CreateMatch() might return null and is not checked -> Option D

4. Quick Check:

   Always check for null after CreateMatch() [OK]

Hint: Always check if CreateMatch() returns null before use [OK]

Common Mistakes:

• Assuming event subscription syntax is wrong
• Ignoring null checks
• Assuming Start() method is missing

5. You want to create a lobby that only starts the game when all players are ready. Which approach is best in Unity matchmaking?

hard

A. Call StartGame() immediately when the first player joins

B. Override OnPlayerJoined to check a 'ready' flag on each player before calling StartGame()

C. Use a timer to start the game after 30 seconds regardless of players

D. Disable matchmaking and start game manually from UI

Solution

1. Step 1: Understand the requirement

   The game should start only when all players are ready, not just when they join.

2. Step 2: Choose the best method

   Overriding OnPlayerJoined to check each player's 'ready' status before starting the game ensures all are prepared.

3. Final Answer:

   Override OnPlayerJoined to check a 'ready' flag on each player before calling StartGame() -> Option B

4. Quick Check:

   Check readiness before starting game [OK]

Hint: Check all players' ready flags before starting game [OK]

Common Mistakes:

• Starting game too early
• Ignoring player readiness
• Disabling matchmaking unnecessarily