Add enforce_system_properties rule (lyft#55)

* Add enforce_system_properties rule

README.md

@@ -76,6 +76,10 @@ in the .xib or .storyboard file.
 
   Ensures a system type uses a set of custom clases. Configure `system_classes` in a custom rule configuration using `rules_config` (see below).
 
+- `enforce_system_properties`
+
+  Ensures a property in a system type is set to one of the allowed properties. Configure `system_properties` in a custom rule configuration using `rules_config` (see below). Use `null` as an option to allow default value.
+
 ## Usage
 
 For a list of available rules, run `xiblint -h`.

xiblint/__init__.py

@@ -1 +1 @@
-__version__ = '0.9.12'
+__version__ = '0.9.13'

xiblint/rules/enforce_system_properties.py

@@ -0,0 +1,37 @@
+from xiblint.rules import Rule
+from xiblint.xibcontext import XibContext
+
+
+class EnforceSystemProperties(Rule):
+    """
+    Ensures unavailable system properties are not used.
+
+    Example configuration:
+    {
+      "system_properties": {
+        "label": {
+          "adjustsFontForContentSizeCategory": ["YES"]
+        },
+        "button": {
+          "reversesTitleShadowWhenHighlighted": [null, "NO"]
+        }
+      }
+    }
+    """
+    def check(self, context):  # type: (XibContext) -> None
+        system_properties = self.config.get('system_properties', {})
+
+        for tag_name in system_properties.keys():
+            for element in context.tree.findall('.//{}'.format(tag_name)):
+                tag_name = element.tag
+                enforced_properties = system_properties.get(tag_name)
+                for property_name in enforced_properties.keys():
+                    property_allowed_values = enforced_properties.get(property_name)
+                    property_value = element.get(property_name)
+
+                    if property_value in property_allowed_values:
+                        continue
+
+                    options_string = '`, `'.join(map(str, property_allowed_values))
+                    context.error(element, '`<{}>` property `{}` must use `{}` instead of `{}`.'
+                                  .format(tag_name, property_name, options_string, property_value))