Only Objective-C methods are translated to throwing Swift methods, which do return a BOOL (not lower-cased bool), or a nullable-object.
(Tested with Xcode 11.7, and Swift 5 language.)
The reason is that Cocoa methods always use a return value NO or nil
to indicate the failure of a method, and not just set an error object.
This is documented in
Using and Creating Error Objects:
Important: Success or failure is indicated by the return value of the method.
Although Cocoa methods that indirectly return error objects in the Cocoa error
domain are guaranteed to return such objects if the method indicates failure
by directly returning nil or NO, you should always check that the return
value is nil or NO before attempting to do anything with the NSError object.
For example, the Objective-C interface
@interface OClass : NSObject
NS_ASSUME_NONNULL_BEGIN
-(void)doSomethingWithArgument1:(int) x error:(NSError **)error;
-(BOOL)doSomethingWithArgument2:(int) x error:(NSError **)error;
-(NSString *)doSomethingWithArgument3:(int) x error:(NSError **)error;
-(NSString * _Nullable)doSomethingWithArgument4:(int) x error:(NSError **)error;
-(BOOL)doSomething:(NSError **)error;
NS_ASSUME_NONNULL_END
@end
is mapped to Swift as
open class OClass : NSObject {
open func doSomethingWithArgument1(x: Int32, error: NSErrorPointer)
open func doSomethingWithArgument2(x: Int32) throws
open func doSomethingWithArgument3(x: Int32, error: NSErrorPointer) -> String
open func doSomethingWithArgument4(x: Int32) throws -> String
open func doSomething() throws
}
If you can change the interface of your method then you should add a boolean
return value to indicate success or failure.
Otherwise you would call it from Swift as
var error : NSError?
object.doSomethingWithArgument(argumentValue, error: &error)
if let theError = error {
print(theError)
}
Remark: At
I found that Clang has an attribute which forces a function to throw an error in Swift:
-(void)doSomethingWithArgument5:(int) x error:(NSError **)error
__attribute__((swift_error(nonnull_error)));
is mapped to Swift as
public func doSomethingWithArgument5(x: Int32) throws
and seems to work “as expected”. However, I could not find any official documentation
about this attribute, so it might not be a good idea to rely on it.