Take a dive deep into the new prefab features with developers Steen Lund. What's the inside versus outside of a prefab? How does it relate to Prefab mode, overrides, unpacking, and prefab variants? How has the new prefab back-end changed? This session will give you a better understanding of the Prefab system, the pros and cons of various implementation decisions, and some Aha moments working with Prefab nesting.

1. GenerativeArt–MadewithUnity
1
Technical Deep
Dive into the New
Prefab System
Steen Lund
Tech Lead, Unity Technologies
@steenlund

2. Agenda
2
● Prefab Assets, Instances and Content
● Prefab Importing
● Prefab Mode
● Unpacking
● Scripting and Tooling

3. Takeaway
3
● Knowledge of how the backend works
● Changes to Unity’s handling of Prefabs assets
● Knowledge of Prefab tooling in the Editor

4. Project Goals
4

5. 5
Nested Prefabs
TableScene
Table
LegLeg
Table
VaseVase
Vase
Vase

6. 6
Prefab Variants
CatScene
Cat
(instance
of Animal)
Cat Animal
Animal

7. Editing in isolation
7

8. Give some, take some
8
• No more disconnected Prefab instances
• No more editing directly in the Project Browser

9. Prefab Assets,
Instances and Content
9

10. 10
GameObject
in scene

11. 11
Make GameObject
into Prefab instance

12. 12
Drag the GameObject
to Project Browser

13. 13

14. What is a Prefab
14
Instance?

15. 15
PrefabInstance
Handle
Instance
Leg
Table
More than you see with the
naked eye

16. 16
corresponding object
Instance Asset

17. 17
AssetInstance
Table
LegLeg
Table

18. 18
Prefabs in Scene files
Main Camera
Directional Light
Scene File
PrefabInstance
Handle
Loaded Scene
PrefabInstance
Handle

19. What is a Prefab Asset?
19
Prefab File
Table
Leg

20. 20
Consider a Prefab Asset being a scene
Prefab File
Table
Leg
PrefabInstance
Handle

21. Prefab Importing
21

22. 22
Prefab File in Assets Folder
Prefab Importer
Table
Leg
Prefab File in Library Folder
Prefab Asset Handle
Table
Leg

23. 23
Prefab File in Assets Folder Prefab File in Library Folder
Prefab Importer
Table
Leg
PrefabInstance
Handle
Prefab Asset Handle
Table
Leg
Vase
PrefabInstance
Handle

24. 24
Prefab Variant File in Assets Folder Prefab File in Library Folder
Prefab Importer
PrefabInstance
Handle
Prefab Asset Handle
Vase
PrefabInstance
Handle

25. Corresponding Objects
With Nesting
25

26. Corresponding Objects with Nesting
26
Table
LegLeg
Table
VaseVase Vase
TableScene Vase

27. Corresponding Objects with Variants
27
CatCat Animal
CatScene Animal

28. 28
Dependency chain
Prefab Importer Prefab Importer
Inner Middle Outer

29. 29
Build size implications
● No change for Scene, have always been baked out at build time
● Resources and AssetBundles are affected
AA A
B C

30. 30
PrefabAsset
Handle
PrefabInstance
Handle
Asset in Library FolderPrefab Instance
Table
LegLeg
Table
VaseVase
PrefabInstance
Handle

31. Prefab Mode
31

32. 32
Edit Prefab
in Prefab Mode

33. Prefab Mode
Editing Prefab Asset
content
not Prefab instance
GameObject icon
not Prefab icon
Saving
not applying
33

34. 34
Prefab
Asset
Prefab
instance
Prefab contents
in Prefab Mode
automatic
update
apply
open from instance
open from asset
save

35. Unpacking
35

36. 36
Prefabs can no longer be disconnected

37. 37

38. 38
Make Prefab instance
into GameObject
Unpack Prefab Contents
(one layer)

39. 39
Make Prefab instance
into GameObject
Unpack Prefab Contents
(one layer)
Prefab Variant
becomes its base

40. 40

41. Scripting and Tooling
41

42. Editor Stages
42
• Main Stage
• Prefab Stage
• ….

43. Editor Stages
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• [ExecuteInEditMode]
• Will leave Prefab Mode
• [ExecuteAlways]

44. Editor Stages: [ExecuteAlways]
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void Update()
{
// Rotate only in playmode
// We don't want this in edit mode nor in prefab mode
// as it would update the prefab asset
if (Application.IsPlaying(this))
{
Transform t = GetComponent<Transform>();
t.Rotate(rotationSpeed * Time.deltaTime, 0, 0);
}
++updateCount;
Debug.Log(string.Format("Updates {0}", updateCount));
}

45. Case:
45
I want to dynamically create an editing
environment for my prefab

46. 46
I want to dynamically create an editing
environment for my prefab
using UnityEditor.Experimental.SceneManagement;
[InitializeOnLoad]
class CustomPrefabEnvironment
{
static CustomPrefabEnvironment()
{
PrefabStage.prefabStageOpened += OnPrefabStageOpened;
}
static void OnPrefabStageOpened(PrefabStage prefabStage)
{
…

47. Case:
47
Is my GameObject a Prefab (and what kind)?

48. Is my GameObject a Prefab (and what kind)?
48
PrefabInstanceStatus GetPrefabInstanceStatus(Object componentOrGameObject)
PrefabAssetType GetPrefabAssetType(Object componentOrGameObject)
bool IsAnyPrefabInstanceRoot(GameObject gameObject)
bool IsOutermostPrefabInstanceRoot(GameObject gameObject)
bool IsPartOfAnyPrefab([Object componentOrGameObject)
bool IsPartOfImmutablePrefab(Object componentOrGameObject)
bool IsPartOfPrefabThatCanBeAppliedTo(Object componentOrGameObject)
bool IsPartOfPrefabAsset(Object componentOrGameObject)
bool IsPartOfPrefabInstance(Object componentOrGameObject)
bool IsPartOfNonAssetPrefabInstance(Object componentOrGameObject)
bool IsPartOfRegularPrefab(Object componentOrGameObject)
bool IsPartOfModelPrefab(Object componentOrGameObject)
bool IsPartOfVariantPrefab(Object componentOrGameObject)
bool IsDisconnectedFromPrefabAsset(Object componentOrGameObject)
bool IsPrefabAssetMissing(Object instanceComponentOrGameObject)

49. Is my GameObject a Prefab (and what kind)?
49
if (PrefabUtility.IsPartOfPrefabInstance(go))
{
var type = PrefabUtility.GetPrefabAssetType(go);
Debug.Log(string.Format("GameObject is part of a Prefab Instance of type {0}" ,type));
}
else
{
Debug.Log("Selected GameObject is not part of a Prefab Instance");
}

50. Case:
50
How do I know if 2 GameObjects are part of the
same Prefab Instance?

51. 51
How do I know if 2 GameObjects are part of the same Prefab Instance?
var firstHandle = PrefabUtility.GetPrefabInstanceHandle(Selection.gameObjects[0]);
var secondHandle = PrefabUtility.GetPrefabInstanceHandle(Selection.gameObjects[1]);
if (firstHandle == secondHandle)
Debug.Log("GameObject belongs to same instance");
else
Debug.Log("GameObject are not from the same instance");

52. Case:
52
I want to create a Prefab from script

53. I want to create a Prefab from script
53
var cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
// If you simply want the cube to be written as an asset but not turn into a prefab instance
// use this method
PrefabUtility.SaveAsPrefabAsset(cube, path);
// This method will create a Prefab asset and make the GameObject an instance of the
// new prefab asset
PrefabUtility.SaveAsPrefabAssetAndConnect(cube, path, InteractionMode.AutomatedAction);

54. Case:
54
I want to create a Prefab Variant from script

55. I want to create a Prefab Variant from script
55
if (isAsset)
{
path = AssetDatabase.GetAssetPath(go);
go = (GameObject)PrefabUtility.InstantiatePrefab(go);
}
else if (isInstance)
{
go = PrefabUtility.GetOutermostPrefabInstanceRoot(go);
path = AssetDatabase.GetAssetPath(PrefabUtility.GetCorrespondingObjectFromSource(go));
}
var prefabName = System.IO.Path.GetFileNameWithoutExtension(path);
path = System.IO.Path.GetDirectoryName(path);
PrefabUtility.SaveAsPrefabAsset(go, path + "/" + prefabName + " Variant.prefab");

56. Case:
56
I want to nest Prefabs from script

57. 57
I want to nest Prefabs from script
var instanceA = (GameObject)PrefabUtility.InstantiatePrefab(goA);
var instanceB = (GameObject)PrefabUtility.InstantiatePrefab(goB);
instanceB.GetComponent<Transform>().parent = instanceA.GetComponent<Transform>();
PrefabUtility.ApplyPrefabInstance(instanceA, InteractionMode.AutomatedAction);

58. 58
I want to nest Prefabs from script
var instanceB = (GameObject)PrefabUtility.InstantiatePrefab(goB);
var root = PrefabUtility.LoadPrefabContents(path);
instanceB.GetComponent<Transform>().parent = root.GetComponent<Transform>();
PrefabUtility.SaveAsPrefabAsset(root, path);
PrefabUtility.UnloadPrefabContents(root);

59. Case:
59
I want to change a flag on multiple Prefabs assets

60. 60
I want to change a flag on multiple Prefabs assets
// Load the content of the Prefab asset so we can modify it
var assetRoot = PrefabUtility.LoadPrefabContents(path);
GameObjectUtility.SetStaticEditorFlags(assetRoot, StaticEditorFlags.LightmapStatic);
// Write the updated GameObject to the Prefab asset
PrefabUtility.SaveAsPrefabAsset(assetRoot, path);
// Clean up is important or we will leak scenes and
// eventually run out of temporary scenes.
PrefabUtility.UnloadPrefabContents(assetRoot);

61. Case:
61
I want to add assets to my prefab file
AssetDatebase.AddObjectToAsset(newObject, asset);

62. Case:
62
Advanced: I modified a Library folder object,
now I want to update the source asset

63. 63
Advanced: I modified a Library folder object,
now I want to update the source asset
string path = "Assets/Prefabs/A.prefab";
var goA = (GameObject)AssetDatabase.LoadMainAssetAtPath(path);
GameObjectUtility.SetStaticEditorFlags(goA, ~GameObjectUtility.GetStaticEditorFlags(goA));
PrefabUtility.SavePrefabAsset(goA);

64. 64
Example code:
https://github.com/Unity-Technologies/UniteLA2018Examples
Scripting reference
https://docs.unity3d.com/2018.3/Documentation/ScriptReference/PrefabUtility.html
Prefabs landing page: https://unity3d.com/prefabs
Nikoline Høgh:
Oct. 24th 10 AM
Prefab Workflow Improvements – An Introduction
Location: Diamond 5

65. 65
Questions?