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Returning error information for a system service call

When a system service is implemented, the function signature that we need to implement in Objective-C is:

- (void)myService:(NSPasteboard *)pboard userData:(NSString *)userData error:(NSString **)error;

In Swift, this turns out as:

func myService(pboard: NSPasteboard, userData: String, error: AutoreleasingUnsafeMutablePointer<NSString?>)

The pasteboard and userData parameters will not cause any problems, but the error information that we might want to return can take some puzzling when we encounter this for the first time.
We need to return the error information as a pointer to an NSString object.

In Objective-C this is implemented as follows:

*error = @"Failure, application did not start properly";

In Swift the syntax is:

error.memory = "Failure, application did not start properly"

Happy coding...

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