Because the initialization method provided by the CocoaTouch frameworks can never be return nil value。However, custom initializers method can return nil value。
for example
// SomeViewController.h
@interface SomeViewController : UIViewController { }
- (id)initWithData:(MyData *)data;
@end
// SomeViewController.m
@implementation SomeViewController { }
- (id)initWithData:(MyData *)data {
return nil;
}
@end
or overwrite init method
@interface SomeViewController : UIViewController { }
- (id)init;
@end
// SomeViewController.m
@implementation SomeViewController { }
- (id)init {
return nil;
}
@end
Xcode BUG
func foo() -> UIViewController {
// returns nil will be crash!!!
// Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value
return SomeViewController(data: someData) // unsafe
}
