Remote procedure calls in Unity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Remote Procedure Call (RPC) in Unity?

An RPC in Unity is a way to call a function on another computer or player over the network, making multiplayer games work smoothly.

beginner

How do you mark a method as an RPC in Unity's networking system?

You mark a method with the [PunRPC] attribute when using Photon, or use [ClientRpc] in Unity's Netcode for GameObjects to indicate it can be called remotely.

intermediate

What is the difference between a ClientRpc and a Command in Unity networking?

A Command is called by a client to run code on the server, while a ClientRpc is called by the server to run code on clients.

intermediate

Why should RPCs be used carefully in multiplayer games?

Because RPCs send data over the network, using too many or large RPCs can cause lag or slow down the game experience.

beginner

Give an example of a simple RPC method in Unity using Photon.

Example:

[PunRPC]
void Jump() {
  // Make the player jump
}

What does an RPC do in Unity multiplayer games?

A Calls a function on another player or server over the network

B Saves game data locally

C Draws graphics on the screen

D Handles user input only

RPCs let you run code on other players' computers or the server, enabling multiplayer interaction.

Which attribute is used to mark a method as an RPC in Photon Unity Networking?

A [ClientRpc]

B [PunRPC]

C [Command]

D [Remote]

Photon uses [PunRPC] to mark methods that can be called remotely.

In Unity's Netcode for GameObjects, what does a ClientRpc do?

A Runs code on clients from the server

B Runs code on the server from clients

C Saves player preferences

D Handles physics calculations

ClientRpc methods are called by the server to execute code on all or specific clients.

Why should you limit the number of RPC calls in your game?

A RPCs slow down graphics rendering

B RPCs use too much memory locally

C Too many RPCs can cause network lag

D RPCs are not supported on mobile devices

Excessive RPC calls increase network traffic and can cause lag.

Which of these is NOT a typical use of RPCs in Unity multiplayer?

A Synchronizing player actions

B Triggering animations on other players

C Updating game state across players

D Loading local game assets

Loading local assets is done locally, not via RPCs.

Explain how Remote Procedure Calls help multiplayer games in Unity work.

Think about how players see each other's actions in real time.

Describe the difference between ClientRpc and Command in Unity networking.

Who calls whom in each case?

Practice

1. What attribute do you use in Unity to make a method run on the server when called from a client?

easy

A. [ServerRpc]

B. [ClientRpc]

C. [RunOnServer]

D. [RemoteCall]

Solution

1. Step 1: Understand the direction of the call

   Client-to-server calls require a special attribute to mark the method that runs on the server.

2. Step 2: Identify the correct attribute in Unity

   Unity uses [ServerRpc] to mark methods that clients call to run on the server.

3. Final Answer:

   [ServerRpc] -> Option A

4. Quick Check:

   Client-to-server calls use [ServerRpc] [OK]

Hint: Client calls server methods with [ServerRpc] attribute [OK]

Common Mistakes:

• Confusing [ClientRpc] as client-to-server
• Using non-existent attributes like [RunOnServer]
• Thinking [RemoteCall] is a valid Unity attribute

2. Which of the following is the correct syntax to declare a server RPC method in Unity?

easy

A. public void MyMethod() { }

B. [ServerRpc] public void MyMethod() { }

C. [ClientRpc] public void MyMethod() { }

D. void [ServerRpc] MyMethod() { }

Solution

1. Step 1: Recall the attribute placement

   Attributes in C# go before the method declaration.

2. Step 2: Check the correct attribute and syntax

   The correct syntax is [ServerRpc] public void MyMethod() { }. void [ServerRpc] MyMethod() { } places the attribute incorrectly.

3. Final Answer:

   [ServerRpc] public void MyMethod() { } -> Option B

4. Quick Check:

   Attributes precede method declarations [OK]

Hint: Attributes go before method signature in C# [OK]

Common Mistakes:

• Placing attribute after method name
• Using [ClientRpc] for server RPC
• Omitting the attribute entirely

3. Consider this Unity code snippet:

[ServerRpc]
public void SendMessageToClients(string msg) {
    Debug.Log("Server received: " + msg);
    BroadcastMessageClientRpc(msg);
}

[ClientRpc]
void BroadcastMessageClientRpc(string message) {
    Debug.Log("Client received: " + message);
}

What will be printed on the client when SendMessageToClients("Hello") is called from a client?

medium

A. Server received: Hello
Client received: Hello

B. Server received: Hello

C. Client received: Hello

D. Nothing is printed on client

Solution

1. Step 1: Understand the call flow

   The client calls SendMessageToClients which runs on the server due to [ServerRpc]. The server logs "Server received: Hello".

2. Step 2: Server calls client method

   The server then calls BroadcastMessageClientRpc on all clients, which logs "Client received: Hello" on each client.

3. Final Answer:

   Client received: Hello -> Option C

4. Quick Check:

   ClientRpc runs on clients, so clients see "Client received: Hello" [OK]

Hint: Client sees ClientRpc logs, server sees ServerRpc logs [OK]

Common Mistakes:

• Thinking server logs appear on client
• Assuming both logs print on client
• Confusing which method runs where

4. What is wrong with this Unity RPC method?

[ServerRpc]
public void UpdateScoreClientRpc(int score) {
    Debug.Log("Score updated: " + score);
}

medium

A. [ServerRpc] methods must be private

B. Missing return type for RPC method

C. RPC methods cannot have parameters

D. Method name suggests client RPC but uses [ServerRpc] attribute

Solution

1. Step 1: Analyze method name and attribute

   The method name contains "ClientRpc" but the attribute is [ServerRpc], which is confusing and likely a mistake.

2. Step 2: Check RPC method rules

   RPC methods can have parameters and must have a return type (void is valid). They can be public. The mismatch between name and attribute is the main issue.

3. Final Answer:

   Method name suggests client RPC but uses [ServerRpc] attribute -> Option D

4. Quick Check:

   Method name and attribute must match RPC direction [OK]

Hint: Match method name with RPC attribute to avoid confusion [OK]

Common Mistakes:

• Ignoring method name and attribute mismatch
• Thinking parameters are not allowed
• Assuming RPC methods must be private

5. You want to send a message from the server to all clients except the one who sent the original request. Which approach using Unity RPCs is correct?

hard

A. Use [ClientRpc] with a special NetworkConnection parameter to exclude sender

B. Use [ServerRpc] to send message to clients except sender

C. Use [ClientRpc] and check inside the client to ignore messages from itself

D. Use [ClientRpc] with a parameter to exclude the sender client inside the method

Solution

1. Step 1: Understand RPC direction

   Server sends messages to clients using [ClientRpc]. To exclude a client, the server must know which client to exclude.

2. Step 2: Use NetworkConnection parameter

   Unity allows [ClientRpc] methods to take a NetworkConnection parameter to exclude that client from receiving the RPC.

3. Step 3: Confirm other options

   Use [ClientRpc] with a parameter to exclude the sender client inside the method is incorrect because exclusion logic cannot be done inside the client method. Use [ServerRpc] to send message to clients except sender is wrong direction. Use [ClientRpc] and check inside the client to ignore messages from itself relies on client ignoring messages, which is inefficient.

4. Final Answer:

   Use [ClientRpc] with a special NetworkConnection parameter to exclude sender -> Option A

5. Quick Check:

   Exclude sender with NetworkConnection parameter in ClientRpc [OK]

Hint: Exclude sender using NetworkConnection param in ClientRpc [OK]

Common Mistakes:

• Trying to exclude sender inside client code
• Using ServerRpc to send to clients
• Not using NetworkConnection parameter for exclusion