Filtering

Nullarihyon 1.2 introduced filtering, which helps programmers to have a working set of programs. Filter is a set of class names, and warnings irrelevant to given classes are ignored.

Getting started with filtering

You can try filtering with nullarihyon check command.

$ nullarihyon check --filter NSString test.m

For nullarihyon xcode, add nullfilter file in the directory which contains .xcodeproj you are working with. The content of nullfilter is like the following.

NSString
YourViewController
# AnotherViewController
/ViewController/

Each line contains a class name, or a regular expression for class name enclosed by /. You can have comments with # symbol.

Semantics

When you setup filtering with ViewController, following warnings will be printed.

1. Warnings found in implementation of ViewController
2. Warnings found about message call expression to method of ViewController
3. Warnings about nullability introduced from method of ViewController

1 is trivial. 2 and 3 should look a bit ambiguous. Let's explain them with example.

Method of a class

Method of ViewController means a method which is declared in @interface declaration of ViewController. Assume we have a @interface declaration as the following:

@interface ViewController : UIViewController

// ViewController specific method
@property (nonatomic, nonnull) NSString *name;

// UIViewController method but with explicit declaration
- (void)presentViewController:(nonnull UIViewController *)viewController animated:(BOOL)animated completion:(void(^nullable){})completion;

@end

-[ViewController setName:] is a method of ViewController. -[ViewController prepareForSegue:sender:] is also a method of ViewController. However, other methods inherited from UIViewController including -[UIViewController addChildViewController:] is not.

ViewController *viewController;
NSString * _Nullable string;
UIViewController * _Nullable anotherViewController;

// Next two lines will have warning
viewController.name = string;
[viewController presentViewController:anotherViewController animated:YES completion:nil];

// This line will not have warning, though addChildViewController expects _Nonnull argument
[viewController addChildViewController:anotherViewController];

If you override a method of super class but do not expose it to @interface, the method is not of the class.

Nullability introduced by a class

Let's explain the motivation.

- (void)example:(nullable ViewController viewController) {
  // Dont want warning for this
  ViewController * _Nonnull vc = viewController;

  NSString * _Nullable name = [viewController nameOrNil];
  // Want warning for this
  BOOL x = [@"example" isEqualToString:name];  // isEqualToString expects _Nonnull argument
}

The nullability of viewController is not related to definition of ViewController. It is by the definition of the method. We don't want warning for that.

However, nullability of name is by the definition of ViewController (assume nameOrNil method returns _Nullable). And we want warning reported around isEqualToString:.

Warning about an expression will be reported if

• Expression is result of method call of ViewController, or
• Expression is a variable and a variable has assignment with result of method call of ViewController