Learn how Cocoa error parameters are converted to Swift throwing methods.
Overview
In Cocoa, methods that produce errors take an NSError
pointer parameter as their last parameter, which populates its argument with an NSError
object if an error occurs. Swift automatically translates Objective-C methods that produce errors into methods that throw an error according to Swift’s native error handling functionality.
Understand How Error Parameters Are Imported
Swift examines Objective-C method declarations and translates them into Swift throwing methods, with shorter names when possible.
For example, consider the remove
method from File
. In Objective-C, it's declared like this:
- (BOOL)removeItemAtURL:(NSURL *)URL
error:(NSError **)error;
In Swift, it’s imported like this:
func removeItem(at: URL) throws
Notice that the remove
method is imported by Swift with a Void
return type, no error parameter, and a throws
declaration.
If the last non-block parameter of an Objective-C method is of type NSError **
, Swift replaces it with the throws keyword, to indicate that the method can throw an error. If the Objective-C method’s error parameter is also its first parameter, Swift attempts to simplify the method name further, by removing the With
or And
suffix, if present, from the first part of the selector. If another method is declared with the resulting selector, the method name is not changed.
If an error producing Objective-C method returns a BOOL
value to indicate the success or failure of a method call, Swift changes the return type of the function to Void
. Similarly, if an error producing Objective-C method returns a nil
value to indicate the failure of a method call, Swift changes the return type of the function to a nonoptional type.
Otherwise, if no convention can be inferred, the method is left intact.
Note
You use the NS
macro on Objective-C method declarations that produce an NSError
to prevent it from being imported by Swift as a method that throws.