Add RequireInterface attribute

RequireInterface.meta

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RequireInterface/Editor.meta

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RequireInterface/Editor/RequireInterfaceDrawer.cs

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+using UnityEngine;
+using UnityEditor;
+/// <summary>
+/// Drawer for the RequireInterface attribute.
+/// </summary>
+[CustomPropertyDrawer(typeof(RequireInterfaceAttribute))]
+public class RequireInterfaceDrawer : PropertyDrawer
+{
+    /// <summary>
+    /// Overrides GUI drawing for the attribute.
+    /// </summary>
+    /// <param name="position">Position.</param>
+    /// <param name="property">Property.</param>
+    /// <param name="label">Label.</param>
+    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
+    {
+        // Check if this is reference type property.
+        if (property.propertyType == SerializedPropertyType.ObjectReference)
+        {
+            // Get attribute parameters.
+            var requiredAttribute = this.attribute as RequireInterfaceAttribute;
+            // Begin drawing property field.
+            EditorGUI.BeginProperty(position, label, property);
+            // Draw property field.
+            property.objectReferenceValue = EditorGUI.ObjectField(position, label, property.objectReferenceValue, requiredAttribute.requiredType, true);
+            // Finish drawing property field.
+            EditorGUI.EndProperty();
+        }
+        else
+        {
+            // If field is not reference, show error message.
+            // Save previous color and change GUI to red.
+            var previousColor = GUI.color;
+            GUI.color = Color.red;
+            // Display label with error message.
+            EditorGUI.LabelField(position, label, new GUIContent("Property is not a reference type"));
+            // Revert color change.
+            GUI.color = previousColor;
+        }
+    }
+}

RequireInterface/Editor/RequireInterfaceDrawer.cs.meta

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RequireInterface/RequireInterfaceAttribute.cs.meta

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