Custom blueprint import ability added

[astralmaster]

Description

Adds the ability to import custom blueprints created in UE4 to Carla's blueprint library, spawnable via Python API. Ported from Carla ue5-dev branch. In order to add your blueprint, create BlueprintParameters.json file in /CarlaUE4/Content/Carla/Config/ with following format:

{
  "Blueprints": [
    {
      "Name": "SomeBlueprintName",
      "Path": "/Game/Path/To/SomeBlueprintName"
    },
	{
      "Name": "SomeOtherBlueprintName",
      "Path": "/Game/Path/To/SomeOtherBlueprintName"
    }
  ]
}

If, for example, in your UE4 Content Browser you have a blueprint with path "Content -> Blueprints -> SomeBlueprintName", then you would modify the Path in BlueprintParameters.json as "Path": "/Game/Blueprints/SomeBlueprintName".

Then, from Python API:

world = client.get_world()
blueprint_library = world.get_blueprint_library()
blueprint_name = 'SomeBlueprintName'
bp = blueprint_library.find(blueprint_name)

Fixes #9018

Where has this been tested?

• Platform(s): Windows 10 x64
• Python version(s): 3.7
• Unreal Engine version(s): UE 4.26.2

Possible Drawbacks

The code uses FindFunction() to determine whether PostProcessBlueprint has been used/implemented in UE4, should probably check if there is an easier, more robust way. This was needed to correctly determine whether the return value of PostProcessBlueprint should be taken into account when setting the SpawnResult.Status in BlueprintActorFactory.cpp's SpawnActor(). In the original BlueprintActorFactory.cpp from UE5, the code checks if PostProcessBlueprint returns true/false and sets the SpawnResult.Status accordingly, but if PostProcessBlueprint is not used in UE4, then the default return value is false and that wrongly (I assume) sets the SpawnResult.Status to EActorSpawnResultStatus::UnknownError.

---

This change is 

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update our CHANGELOG.md based on your changes.

[Blyron]

Could you add a small .md on how have you done it to help people in the community?

[astralmaster]

Could you add a small .md on how have you done it to help people in the community?

As part of the CHANGELOG.md or separate .md? Where should that go?

[Blyron]

Could you add a small .md on how have you done it to help people in the community?

As part of the CHANGELOG.md or separate .md? Where should that go?

You should add the title of the feature in the CHANGELOG.md and a tutorial of how to do it in
Docs/
in a file named something like tuto_add_blueprints_to_factory or something like that

[astralmaster]

That should do it!

[Blyron]

Yeah!
It makes it. We will review asap

[Blyron]

The PR has compilinng issues;
Parsing headers for CarlaUE4Editor
Running UnrealHeaderTool "/__w/carla/carla/Unreal/CarlaUE4/CarlaUE4.uproject" "/__w/carla/carla/Unreal/CarlaUE4/Intermediate/Build/Linux/B4D820EA/CarlaUE4Editor/Development/CarlaUE4Editor.uhtmanifest" -LogCmds="loginit warning, logexit warning, logdatabase error" -Unattended -WarningsAsErrors -abslog="/unreal-engine/Engine/Programs/UnrealBuildTool/Log_UHT.txt"
/__w/carla/carla/Unreal/CarlaUE4/Plugins/Carla/Source/Carla/Actor/ActorBlueprintFunctionLibrary.h(175) : Error: Unrecognized type 'FBlueprintParameters' - type must be a UCLASS, USTRUCT or UENUM
make[1]: *** [Makefile:1080: CarlaUE4Editor] Error 6
make[1]: Leaving directory '/__w/carla/carla/Unreal/CarlaUE4'
make: *** [Util/BuildTools/Linux.mk:88: CarlaUE4Editor] Error 2