Base class to derive custom Editors from. Use this to create your own custom inspectors and editors for your objects.

Consider a script MyPlayer with variables for armor, damage, and a reference to a gun GameObject:

// MyPlayer.js
// This is not an editor script.
var armor : int = 75;
var damage : int = 25;
var gun : GameObject;
function Update () {
	// Update logic here...
}

Using a custom Editor, the apperance of the script in the Inspector can be changed, for example to look like this:

Custom editor in the Inspector.

You can attach the Editor to a custom component by using the CustomEditor attribute.

There are multiple ways to design custom Editors. If you want the Editor to support multi-object editing, you can use the CanEditMultipleObjects attribute. Instead of modifying script variables directly, it's advantageous to use the SerializedObject and SerializedProperty system to edit them, since this automatically handles multi-object editing, undo, and prefab overrides.

// Custom Editor using SerializedProperties.
// Automatic handling of multi-object editing, undo, and prefab overrides.
#pragma strict
@CustomEditor(MyPlayer)
@CanEditMultipleObjects
class MyPlayerEditor extends Editor {
	var damageProp : SerializedProperty;
	var armorProp : SerializedProperty;
	var gunProp : SerializedProperty;
	function OnEnable () {
		// Setup the SerializedProperties
		damageProp = serializedObject.FindProperty ("damage");
		armorProp = serializedObject.FindProperty ("armor");
		gunProp = serializedObject.FindProperty ("gun");
	}
	function OnInspectorGUI() {
		// Update the serializedProperty - always do this in the beginning of OnInspectorGUI.
		serializedObject.Update ();
		// Show the custom GUI controls
		EditorGUILayout.IntSlider (damageProp, 0, 100, new GUIContent ("Damage"));
		// Only show the damage progress bar if all the objects have the same damage value:
		if (!damageProp.hasMultipleDifferentValues)
			ProgressBar (damageProp.intValue / 100.0, "Damage");
		EditorGUILayout.IntSlider (armorProp, 0, 100, new GUIContent ("Armor"));
		// Only show the armor progress bar if all the objects have the same armor value:
		if (!armorProp.hasMultipleDifferentValues)
			ProgressBar (armorProp.intValue / 100.0, "Armor");
		EditorGUILayout.PropertyField (gunProp, new GUIContent ("Gun Object"));
		// Apply changes to the serializedProperty - always do this in the end of OnInspectorGUI.
		serializedObject.ApplyModifiedProperties ();
	}
	// Custom GUILayout progress bar.
	function ProgressBar (value : float, label : String) {
		// Get a rect for the progress bar using the same margins as a textfield:
		var rect : Rect = GUILayoutUtility.GetRect (18, 18, "TextField");
		EditorGUI.ProgressBar (rect, value, label);
		EditorGUILayout.Space ();
	}
}

Alternatively, if automatic handling of multi-object editing, undo, and prefab overrides is not needed, the script variables can be modified directly by the editor without using the SerializedObject and SerializedProperty system, as in the example below.

// Custom Editor the "old" way by modifying the script variables directly.
// No handling of multi-object editing, undo, and prefab overrides!
@CustomEditor (MyPlayer)
class MyPlayerEditor extends Editor {
	function OnInspectorGUI () {
		target.damage = EditorGUILayout.IntSlider ("Damage", target.damage, 0, 100);
		ProgressBar (target.damage / 100.0, "Damage");
		target.armor = EditorGUILayout.IntSlider ("Armor", target.armor, 0, 100);
		ProgressBar (target.armor / 100.0, "Armor");
		var allowSceneObjects : boolean = !EditorUtility.IsPersistent (target);
		target.gun = EditorGUILayout.ObjectField ("Gun Object", target.gun, GameObject, allowSceneObjects);
	}
	// Custom GUILayout progress bar.
	function ProgressBar (value : float, label : String) {
		// Get a rect for the progress bar using the same margins as a textfield:
		var rect : Rect = GUILayoutUtility.GetRect (18, 18, "TextField");
		EditorGUI.ProgressBar (rect, value, label);
		EditorGUILayout.Space ();
	}
}