Bug OnPostprocessPrefab() doesn't serialize changes

The AssetPostprocessor is designed to let users hook into the import pipeline and control the importers output - the files created in the Library folder. The Prefab post processor is doing the same thing - its input is a Prefab from the Assets folder (source Prefab) and its output is a Prefab in the Library folder (imported Prefab). When opening a Prefab in Prefab View the source Prefab is used, not the imported Prefab which means that any change made in OnPostprocessPrefab is not visible is Prefab view.

For example OnPostprocessPrefab can be used to attach scriptable objects to imported Prefabs or to do complicated/time consuming computations that can be done when importing a Prefab instead of doing them at runtime.
Following is a simple example of a Prefab Post Processor:

NPCProperties.cs:

Code (CSharp):

1. using UnityEngine;
2.  
3. public class NPCProperties : MonoBehaviour
4. {
5.     public enum Type
6.     {
7.         TheGoodOne,
8.         TheBadOne
9.     }
10.  
11.     public Type type;
12.     public NPCPropertySheet propertySheet;
13.     public Vector2[] path;
14. }

NPCPropertySheet.cs:

Code (CSharp):

1. using UnityEngine;
2.  
3. public class NPCPropertySheet : ScriptableObject
4. {
5.     public int hp;
6.     public int damage;
7. }

BadGuyPropertySheet.cs:

Code (CSharp):

1. public class BadGuyPropertySheet : NPCPropertySheet
2. {
3.     public BadGuyPropertySheet()
4.     {
5.         hp = 100;
6.         damage = 10;
7.     }
8. }

GoodGuyPropertySheet.cs:

Code (CSharp):

1. public class GoodGuyPropertySheet : NPCPropertySheet
2. {
3.     public GoodGuyPropertySheet()
4.     {
5.         hp = 100;
6.         damage = 0;
7.     }
8. }

PrefabPostProcessor.cs:

Code (CSharp):

1. using UnityEngine;
2. using UnityEditor;
3.  
4. public class PrefabPostProcessor : AssetPostprocessor
5. {
6.     private static Vector2[] ComputeTheGoodGuyPath()
7.     {
8.         var points = new Vector2[10];
9.         /// Complicated and inefficient algorithm that you don't want to implement in Awake/Start
10.         return points;
11.     }
12.  
13.     private static Vector2[] ComputeTheBadGuyPath()
14.     {
15.         var points = new Vector2[10];
16.         /// Complicated and inefficient algorithm that you don't want to implement in Awake/Start
17.         return points;
18.     }
19.  
20.     private void OnPostprocessPrefab(GameObject root)
21.     {
22.         var npcProperties = root.GetComponentInChildren<NPCProperties>();
23.         if(npcProperties != null)
24.         {
25.             switch(npcProperties.type)
26.             {
27.                 case NPCProperties.Type.TheGoodOne:
28.                     npcProperties.path = ComputeTheGoodGuyPath();
29.                     npcProperties.propertySheet = ScriptableObject.CreateInstance<GoodGuyPropertySheet>();
30.                     context.AddObjectToAsset("GoodGuyPropertySheet", npcProperties.propertySheet);
31.                     break;
32.  
33.                 case NPCProperties.Type.TheBadOne:
34.                     npcProperties.path = ComputeTheBadGuyPath();
35.                     npcProperties.propertySheet = ScriptableObject.CreateInstance<BadGuyPropertySheet>();
36.                     context.AddObjectToAsset("BadGuyPropertySheet", npcProperties.propertySheet);
37.                     break;
38.             }
39.         }
40.     }
41. }

If the desired behavior is to alter the source Prefab one can use Editor scripts like this:

EditPrefabAsset.cs:

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.UI;
3. using UnityEditor;
4.  
5. public class EditPrefabAsset : MonoBehaviour
6. {
7.     static string PrefabPath = "Assets/MyPrefab.prefab";
8.  
9.     [MenuItem("Prefabs/Change Prefab Text")]
10.     static void ChangePrefabText()
11.     {
12.         GameObject prefabRoot = AssetDatabase.LoadMainAssetAtPath(PrefabPath) as GameObject;
13.         prefabRoot.GetComponentInChildren<Text>().text = "Hello World!";
14.         PrefabUtility.SavePrefabAsset(prefabRoot);
15.     }
16. }