If I catch your point exactly, I feel you are mixing 2 different things :
- the data stored on a file (using NSCoding) : it is visible to the users : it's users' business
- the instances your app create : they cannot be "manipulated" by users, it's your app business
So if you want to add a new property tou your class, your are free to do it.
Question is : what about existing users' data.
This was for OSX App, but should be very similar for IOS.
For the following, let's assume you have defined a new property in the class:
var newProperty: Bool
the key for coding is
let newPropertyKey = "newpropertyKey"
What you have to do in the new release:
1. In required init(coder decoder: NSCoder), add the decoding for the new property.
Here you need to take care and test for the following :
- once data have been saved with the new version, everything is OK
- but first time you read data created by the old version, the field for the key does not exist yet.
You need also to take care wether the field was created with Swift 2 version of Swift 3.
Here is how I handle it (Swift3)
required init(coder decoder: NSCoder) {
if decoder.containsValue(forKey: newPropertyKey) { // is it already stored ?
self.newProperty = decoder.decodeObject(forKey: newPropertyKey) as? Bool ?? decoder.decodeBool(forKey: newPropertyKey)
// was it created as an object (in Swift2) or as a Bool (in Swift3)
} else { // not yet saved ; will be created at next encode
self.newProperty = false // or true if you decide so
}
}
2. In encode(with coder: NSCoder), you simply add the new p^roperty
func encode(with coder: NSCoder) {
// encode all previous properties
coder.encode(self.newProperty, forKey: newPropertyKey) // the new one
// Note that if newPropoerty is optional (Bool?), you need to encode newPropertyKey!
}
}
3. Of course handle code to handle the new property.
