What does it mean to be compatible with NSCoding?

Question

So, I move on to this guide, and finally I figured out how to archive an object using NSCoding and also initialize it again from the file system using an initializer with an error.

// To encode the object in the first place func encode(with aCoder: NSCoder) { aCoder.encode(name, forKey: "name") } // To 're-initialize' the object required init?(coder aDecoder: NSCoder) { self.name = aDecoder.decodeObject(forKey: "name") as! String super.init() } 

However, I'm still a little unsure how the whole process works at a high level. Please tell me where my opinion is wrong.

1) If your object accepts the NSCoding protocol, you can use the encode function (c :) to make the NSCoder object pass through this function and execute the "encode" method, passing the property of the object instance (which the object itself) as the first argument and a string representing the key as the second value.

2) This is a recursive process, so essentially the reason why the property of the object instance (i.e. name) is passed is that the THAT property (which is the object) can be sent with an encoded message and so on and so forth then down down the line until he no longer reaches the NSCoding adopter.

3) The aDecoder object can also decode objects, so when initializing your custom object, you will want to use an initializer with the ability to initialize to decode any object that was set for the ambiguous string key used.

Here is what I really don't understand ...

How does an aDecoder object know which single object to use for a key set? For example, let's say I have 10 instances of objects for dogs. When the system passes aDecoder through, and I use the decodeObject method on it, and it sets self.name to the value of this decoded object by key, how aDecoder knows that this dog name was saved as "Jack", and not capture one of the other names Dog instances by accident, such as "Jodi"?

In other words, as soon as you encode the properties of the object, as the file system knows, so that the properties of the object of instance A are separated from the properties of the object of instance B, so that when the application loads back and the object A is initialized, it only captures the properties of object A ?

thanks

Answer 1

I think the part that you are missing (correct me if I am wrong) is that NSCoding is not a database. This is a protocol that defines how to serialize an object.

NSCoder, used for decoding, knows which object, because the one that was encoded into it. For example, if you used NSKeyedArchiver (a subclass of NSCoder and a common way to use NSCoding) to save your dog to a file on disk named "/tmp/jack.dog", you can later use NSKeyedUnarchiver to load "/tmp/jack.dog" and deserialize it back to the Dog instance. He “knows which object” because that (the only) that was saved in this file.

It should not be a file. Data can be saved anywhere .

Answer 2

So, you can use the archiver key by calling class func archivedData(withRootObject rootObject: Any) -> Data' , passing it some object and returning an instance of Data , which can then be saved to disk.

What kind of data is this? This is a representation of the object passed to rootObject , which includes the name of the object class and a set of keys and associated values ​​(where these related values ​​are similarly encoded). Thus, this is a hierarchical representation of the object graph with the corresponding object data.

Say that this object was your instance of Dog , he knows this class and all the attributes of the dog associated with it.

Instead, let's say that the object was an array of 10 Dog instances, now the array will store a set of encoded objects against its indices, each encoded object, knowing its class and the attributes of the dog associated with it, and the encoder saves that the root of the object is an array.

So, this is additional data that is controlled by the encoder (and used by the decoder), which allows it to know what to do when you pass it some data (for example, from a file) and ask it to decode.