【VRChat】Debugging and Testing Basics: Using Debug.Log and ClientSim

Overview

Even when you bring a great idea to life and implement a gimmick, you'll almost always hit the wall of "it doesn't work as expected." The work of finding and fixing program errors (bugs) is called debugging, and the work of confirming that completed gimmicks work correctly in various situations is called testing. These are essential processes for guaranteeing world quality and providing a comfortable experience for visiting players.

Debugging and testing in UdonSharp requires considering VRChat-specific network synchronization and inter-player interactions in addition to standard Unity features, and requires some know-how.

This article explains basic debugging techniques and efficient testing approaches in UdonSharp development.

1. Debug.Log(): The Most Basic Debugging Technique

Debug.Log("message"); is the simplest yet most powerful debugging tool that outputs the specified message to logs.

• Checking Variable Values: Output variable values to logs like Debug.Log($"Current score: {score}"); to verify calculations are correct.
• Confirming Process Execution: Write Debug.Log("Interact event was called"); at the beginning of an Interact event to confirm whether the event is even firing.
• Checking Conditional Branches: Output logs in each block of an if statement to confirm which branch was taken.

Logs are displayed in the Console window during execution in the Unity Editor. When executed in the VRChat client, they are recorded in log files on your PC. The first step of debugging is inserting Debug.Log in the editor and tracing program operation step by step.

public override void Interact()
{
    Debug.Log("Interact started");
    if (isReady)
    {
        Debug.Log("isReady was true. Executing process.");
        // ...processing...
    }
    else
    {
        Debug.Log("isReady was false. Skipping process.");
    }
    Debug.Log("Interact ended");
}

2. ClientSim: Local Multiplayer Testing

Debugging gimmicks involving network synchronization is difficult with solo testing alone. Issues like how things behave when other players join, or whether ownership transfers correctly, only surface in multi-player environments.

However, building in Unity every time and launching multiple VRChat clients for testing is extremely time-consuming. This is where the official VRChat tool ClientSim proves incredibly effective.

• Function: Launches pseudo-multiple players (clients) within the Unity Editor to simulate a multiplayer environment.
• Advantages:
  • Quickly start sync testing without building.
  • Easily confirm behavioral differences between Master and Guest clients.
  • Makes testing network events like OnPlayerJoined and ownership transfers easier.

ClientSim can be easily added to projects through VRChat Creator Companion (VCC). It's no exaggeration to say it's an essential tool for developing synced gimmicks with UdonSharp.

3. Creating Debug UIs

Setting up debug-specific UI in the world is also a very effective technique.

• Status Display Panel: Always display current values of synced variables ([UdonSynced] variables) with Text components. You can see information like who the owner is and what the score is in real-time.
• Debug Buttons: Prepare buttons that forcibly execute specific processing.
  • A button that forcibly calls RequestSerialization().
  • A button that forcibly respawns players.
  • A button that initializes game state.

These UIs are hidden or removed when finally publishing the world, but during development, they greatly improve efficiency in identifying and fixing problems.

// Example debug UI script
public class DebugDisplay : UdonSharpBehaviour
{
    public SyncedGameManager gameManager;
    public Text ownerText;
    public Text scoreText;

    void Update()
    {
        // Display game manager's synced variables every frame
        ownerText.text = $"Owner: {Networking.GetOwner(gameManager.gameObject).displayName}";
        scoreText.text = $"Score: {gameManager.score}";
    }
}

Performance Note: The example above updates UI every frame in Update(), which causes load in production environments. Use only for debugging purposes and remove when publishing, or reduce update frequency (e.g., update every 0.5 seconds).

4. Testing Perspectives

Once a gimmick is complete, test it assuming various situations.

• Unit Testing: First, alone, confirm the gimmick works as intended (local testing).
• Sync Testing: In ClientSim or actual multi-player environments, confirm the following:
  • Late Join: Does state sync correctly when joining the world after other players have operated it (testing sync processing in OnPlayerJoined).
  • Ownership Conflicts: No unexpected behavior when multiple players try to operate the same object simultaneously.
  • Master/Guest Behavior: No behavioral differences depending on whether Master is present, or whether you're Master or Guest.
• Load Testing: Confirm that world frame rate (FPS) doesn't drop drastically with many players or when operating gimmicks intensively in succession.
• Regression Testing: After adding new features or fixing bugs, confirm that other features that previously worked correctly aren't broken.

Summary

• The basics of debugging are using Debug.Log() to trace variable values and processing paths one by one.
• ClientSim is an essential tool for developing synced gimmicks, allowing you to replicate multiplayer environments within the Unity Editor.
• Creating debug UIs in the world that display synced variable states makes understanding problems easier.
• Testing needs to assume multiplayer-specific situations like late joins and ownership conflicts, not just solo operation confirmation.

Debugging and testing may not be glamorous, but they're crucial processes that determine world completion quality. Patiently removing problems one by one is ultimately the shortcut to creating a world that many people will enjoy.