Unreal Live Coding

Unreal Live Coding

Trigger and manage Live Coding (runtime C++ recompilation) through UCP's call command. The Compile function uses deferred response — UCP.py will automatically wait for compilation to finish without blocking the UE editor or requiring polling.

Prerequisite: The unreal-client-protocol skill must be available, the UE editor must be running with UCP enabled, and Live Coding must be enabled in Editor Preferences (Windows only).

Custom Function Library

ULiveCodingOperationLibrary

CDO Path: /Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary

Deferred Response

Compile returns FUCPDeferredResponse — a special UCP return type that enables non-blocking async execution:

1. UCP.py sends the Compile request
2. UE starts compilation and immediately frees the game thread (no blocking)
3. UCP.py keeps the TCP connection open, waiting for the response (no polling needed)
4. When compilation finishes, UE pushes the result back to UCP.py
5. UCP.py receives the result and returns it to the caller

Important: Set UE_TIMEOUT environment variable to a larger value (e.g. 120) before calling Compile, as compilation may take longer than the default 30-second timeout:

$env:UE_TIMEOUT = "120"

Examples

Trigger compilation (waits for completion):

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"Compile"}

Check if currently compiling:

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"IsCompiling"}

Check if Live Coding is enabled:

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"IsLiveCodingEnabled"}

Enable Live Coding:

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"EnableLiveCoding","params":{"bEnable":true}}

Common Patterns

Edit C++ and recompile

After modifying .cpp files, trigger Live Coding to apply changes without restarting the editor:

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"Compile"}

The response will contain {"status": "success"} when compilation succeeds.

Check availability before compiling

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"IsLiveCodingEnabled"}

If false, enable it first:

{"object":"/Script/UnrealClientProtocolEditor.Default__LiveCodingOperationLibrary","function":"EnableLiveCoding","params":{"bEnable":true}}

Live Coding Limitations

• Windows only — Live Coding is not available on other platforms
• Header file changes not supported — changes to UCLASS, USTRUCT, UENUM, UFUNCTION declarations, or any changes in header files require a full editor restart
• Data layout changes — modifying struct/class member variables can cause crashes; only function body changes are safe
• New classes — adding entirely new C++ classes may require a full rebuild
• .cpp body changes only — the safest use case is modifying function implementations in .cpp files