Add unavailable_system_classes rule (lyft#34)

This adds the `unavailable_system_classes` rule. This ensures that a given tag (i.e. `<button>`, `<viewController>`, etc.) has one of the specified custom classes.

With the following configuration:

```json
{
  "rules": [
    "unavailable_system_classes"
  ],
  "rules_config": {
    "unavailable_system_classes": {
      "system_classes": {
        "navigationController": ["CoreUI.NavigationController"]
      }
    }
}
```

This has the following redacted output on the Lyft project:

```
Car.storyboard:248: error: E8E-bb-ZGv: `<navigationController>` must use `CoreUI.NavigationController` instead of `LyftUI.NavigationController`. [rule: strict_customization]
Map.storyboard:658: error: TXt-Ri-g2P: `<navigationController>` must use `CoreUI.NavigationController` instead of `LyftUI.NavigationController`. [rule: strict_customization]
Passenger.storyboard:372: error: 9wW-6l-rv8: `<navigationController>` without a custom class is prohibited. Use `CoreUI.NavigationController` instead. [rule: strict_customization]
Driver.storyboard:1336: error: Zpe-RA-erm: `<navigationController>` must use `CoreUI.NavigationController` instead of `LyftUI.NavigationController`. [rule: strict_customization]
```

README.md

@@ -60,6 +60,10 @@ in the .xib or .storyboard file.
 
   Prevent a list of classes from appearing as custom classes. Configure `custom_classes` in a custom rule configuration using `rules_config` (see below). This is a mapping of the full name of the class (`ModuleName.ClassName`) to the suggested replacement.
 
+- `unavailable_system_classes`
+
+  Ensures a system type uses a set of custom clases. Configure `system_classes` in a custom rule configuration using `rules_config` (see below).
+
 ## Usage
 
 For a list of available rules, run `xiblint -h`.

xiblint/rules/unavailable_system_classes.py

@@ -0,0 +1,41 @@
+from xiblint.rules import Rule
+
+
+class UnavailableSystemClasses(Rule):
+    """
+    Ensures given system types are subclassed by a set of classes.
+
+    You must specify a module as part of the class name.
+
+    Example configuration:
+    {
+      "system_classes": {
+          "navigationController": ["ModuleName.CustomNavigationController"],
+          "button": ["ModuleName.CoolButton", "ModuleName.CoolerButton"]
+      }
+    }
+    """
+    def check(self, context):  # type: (Rule, xiblint.xibcontext.XibContext) -> None
+        custom_classes = self.config.get('system_classes', {})
+
+        for tag_name in custom_classes.keys():
+            for element in context.tree.findall('.//{}'.format(tag_name)):
+                tag_name = element.tag
+                options = custom_classes.get(tag_name)
+                custom_class = element.get('customClass')
+                options_string = '`, `'.join(options)
+
+                if not custom_class:
+                    context.error(element, '`<{}>` without a custom class is prohibited. Use `{}` instead.'
+                                  .format(tag_name, options_string))
+                    continue
+
+                full_class_name = self._full_class_name(element)
+                if full_class_name in custom_classes:
+                    continue
+
+                context.error(element, '`<{}>` must use `{}` instead of `{}`.'
+                              .format(tag_name, options_string, full_class_name))
+
+    def _full_class_name(self, element):  # type: (Rule, Element) -> str
+        return "{}.{}".format(element.get('customModule'), element.get('customClass'))