How to import Objective-C types as optionals or non-optionals into Swift

When working on a project that consists of a mixed code base, we might need to import and use Objective-C code in Swift files. Since every object in Objective-C can become nil, Swift imports every type as an implicitly unwrapped optional without any nullability annotations.

To be more expressive in Swift by using either optional or non-optional types, we have the possibility to extend the Objective-C code with nullability declarations and macros.

Let's look at how to do that.

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Nullability declarations

We can annotate Objective-C code to indicate whether an instance can have a nil value. Swift uses those annotations to import declarations either as optional or non-optional:

• types with a nonnullable annotation are imported as non-optionals
• types with a nullable annotation are imported as optionals

- (nullable Tea *)search:(nonnull NSString *)name;

The example above is imported to Swift as follows:

search(name: String) -> Tea?

Without a nullability annotation, Swift would import a type as an implicitly unwrapped optional, which would look as follows:

search(name: String!) -> Tea!

Nullability macros

Additionally, we can use the macros NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END to annotate regions in Objective-C as nonnullable.

This can simplify the process of annotating Objective-C code if we'd like to import most of the types as non-optionals into Swift.

NS_ASSUME_NONNULL_BEGIN

@interface SomeClass : NSObject

- (nullable String *)subtitleForItem:(SomeItem *)item;
@property NSString *title;
@property NSArray<SomeItem *> *items;
@end

NS_ASSUME_NONNULL_END

In the example above, all types will be imported as non-optionals into Swift, except those we annotated with a nullable declaration.

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