OnInspectorGui : how to get selected field ?

I've had a hard time understanding what exactly you want, so I'll try to give broad answer. I'll provide an example setup of YourEditor:

     [CustomEditor(typeof(YourType))]
     public class YourEditor : Editor
     {
         private YourType _yourCtrl = null;
         void OnEnable()
         {
             _yourCtrl = (YourType)target;
         }
         public override void OnInspectorGUI()
         {
             //blablabla
             _yourCtrl.yourObjectInYourScript = EditorGUILayout.ObjectField(_yourCtrl.yourObjectInYourScript, typeof(SomeType), false) as SomeType;
             //blablabla
         }
     }

As far as I understood you either want to set it to None (as example for ObjectField) or to remove it from displaying.

For the first one you'll need to either set your object to null:

 _yourCtrl.yourObjectInYourScript = null;//anywhere in Editor or the script itself

or to explicitly set it null:

 EditorGUILayout.ObjectField(null, typeof(SomeType), false) as SomeType;

If you just don't want to show that field then add some boolean either in script or youreditor (for example let it be showMyField) and change it to false/true on click on condition or on whatever else you want, and then:

 if (showMyField) EditorGUILayout.ObjectField(null, typeof(SomeType), false) as SomeType;

I'm not sure exactly what you meant, but the title is "How to get selected field?" which I'm going to take to mean "How to get the field that is currently selected in the editor?"

You could use EditorGUIUtility.hotControl which returns the ControlID of the last field the left mouse was clicked on.

You could essentially save the value of hotControl as it updates and compare that to the ID's of the controls you are wanting to check.

 int currentControlID = 0;
 int myFieldControlID = /*Whatever the ID of the field is*/;
 
 if(EditorGUIUtility.hotControl != 0)
     currentControlID = EditorGUIUtility.hotControl;
 
 if(currentControlID == myFieldControlID)
     // Create your popup here.
 
 

As far as getting the ControlID of the fields you want, I would just suggest making a LabelField that displays EditorGUIUtility.hotControl and what what the value is whenever you click on the field you want.

Here you go. I just encountered this problem. Thanks to zeman97 I was able to get started! using UnityEditor; using UnityEngine; using System.Collections;

 [CustomEditor (typeof(BoxColliderHelper))]
 [CanEditMultipleObjects]
 public class BoxColliderHelperEditor : Editor
 {
   int val1FieldControlValue, val2FieldControlValue, val3FieldControlValue, selectedField;
   float val1 = 0f, val2 = 0f, val3 = 0f;
 
   public override void OnInspectorGUI ()
   {
     val1FieldControlValue = EditorGUIUtility.GetControlID (new GUIContent ("value01"), FocusType.Passive);
     val1 = EditorGUILayout.FloatField (new GUIContent ("value01"), val1);
     val2FieldControlValue = EditorGUIUtility.GetControlID (new GUIContent ("value02"), FocusType.Passive);
     val2 = EditorGUILayout.FloatField ("value02", val2);
     val3FieldControlValue = EditorGUIUtility.GetControlID (new GUIContent ("value03"), FocusType.Passive);
     val3 = EditorGUILayout.FloatField ("value03", val3);
     //get current selected control
     if (EditorGUIUtility.hotControl != 0)
       selectedField = EditorGUIUtility.hotControl - 1;

     //display values
     EditorGUILayout.LabelField (selectedField.ToString ());
     EditorGUILayout.LabelField ("01's label " + val1FieldControlValue.ToString ());
     EditorGUILayout.LabelField ("02's label " + val2FieldControlValue.ToString ());
     EditorGUILayout.LabelField ("03's label " + val3FieldControlValue.ToString ());
   }
 }