问题
I'm attempting to programmatically update the subtitle within MKAnnotation. The following lines of code produce a cannot assign subtitle in annotation error:
let annotation: MKAnnotation = mapView.annotations[0] as! MKAnnotation
annotation.subtitle = nil
Could anyone help lead me in a direction where I can update the subtitle after the MKAnnotation is shown within the map?
回答1:
MKAnnotation is a protocol. In that protocol, subtitle is read-only. That means it cannot be set.
So, because you have cast this annotation to an MKAnnotation, you cannot set the subtitle.
If you know know what class the annotation really is, then you can cast to that class, and presumably it will have a subtitle which is read/write so you can set it. For example, it might be an MKPointAnnotation, or you might have your own custom annotation class. Then you'll be able to cast to that, and now you can set the subtitle.
来源：https://stackoverflow.com/questions/30086886/swift-cannot-assign-subtitle-in-mkannotation