没有使用返回值时, 警告

swift:

@warn_unused_result func doSomething() -> Bool {

  return true

}

OC:

- (BOOL)doSomething __attribute__((warn_unused_result)) {

  return YES;

}

没有使用返回值时, 不警告

swift:

@discardableResult func doSomething() -> Bool {

  return true

}

OC:

To prevent the compiler from flooding us with warnings when importing Objective-C code the @discardableResult attribute is automatically added for all non-void functions that are not marked with the ((warn_unused_result)) attribute.