Understanding NSKeyedUnarchiver and Serializing Custom Objects

As a developer, it’s not uncommon to encounter the need to store complex data structures in memory. In iOS development, one common approach for serializing objects is using Apple’s NSKeyedArchiver class. However, when working with custom objects, things can get more complicated.

In this article, we’ll delve into the world of serialization and deserialization, focusing on how to restore an object from its archived form using NSKeyedUnarchiver.

Background: Serializing Objects with NSKeyedArchiver

NSKeyedArchiver is a part of Apple’s Foundation framework and provides a convenient way to serialize objects to NSData. When you use this class, it generates a unique binary format that can be stored on disk or transmitted over networks. This process involves several steps:

1. Encoding: The object being serialized is converted into its encoded form using the encodeWithCoder: method.
2. Archiving: The encoded data is then written to a buffer, creating an archive of the original object.
3. Writing: The archive is stored on disk or sent over networks.

Conversely, when deserializing an object, the process involves:

1. Reading: The archived data is read from storage or received over networks.
2. Decoding: The decoded data is converted back into its original form using the decodeWithDecoder: method.
3. Restoring: The restored object can now be used as needed.

However, there’s an important catch: when serializing custom objects with non-standard properties or methods, they need to conform to Apple’s NSCoding protocol.

Understanding NSCoding

NSCoding is a fundamental protocol in iOS development that provides the interface for encoding and decoding objects. Any object that conforms to this protocol must implement two key methods:

• encodeWithCoder:, which encodes an object into its binary form.
• initWithCoder:, which initializes an object from its encoded data.

In other words, when you create a custom class, it should conform to NSCoding by implementing these two methods. This ensures that the objects can be properly serialized and deserialized using NSKeyedArchiver.

Conforming to NSCoding

To give our example RssParser object an easy serialization experience we will implement the required methods:

- (void)encodeWithCoder:(NSCoder *)coder {
    [coder encodeObject:self.title forKey:@"title"];
    [coder encodeObject:self.url forKey:@"url"];
}

- (instancetype)initWithCoder:(NSCoder *)coder {
    self = [super init];
    if (self) {
        // Decode data
        self.title = [coder decodeObjectForKey:@"title"];
        self.url = [coder decodeObjectForKey:@"url"];
    }
    
    return self;
}

Now, when RssParser objects are serialized using NSKeyedArchiver, they’ll be able to conform to the required methods, making their data accessible during deserialization.

Using NSKeyedUnarchiver

As mentioned in the question’s answer, restoring an object from its archived form can be achieved by utilizing NSKeyedUnarchiver. The basic syntax is as follows:

NSData *data = [NSUserDefaults standardUserDefaults].dataForKey:@"Tweets";
RssParser *oldRssParser = (RssParser *)[NSKeyedUnarchiver unarchiveObjectWithData:data];

Here’s a step-by-step explanation of what happens when you use this code snippet:

• dataForKey:@"Tweets" retrieves the archived data associated with the specified key.
• The retrieved NSData object is then passed to unarchiveObjectWithData:, which converts it back into its original form using the RssParser class.
• The deserialized RssParser object is returned and can now be used as needed.

Best Practices for Serialization

When serializing custom objects, there are a few things to keep in mind:

• Be aware of memory management: When storing objects in memory or on disk, it’s essential to consider the potential implications for memory management. Avoid creating large amounts of serialized data that could lead to performance issues.
• Use data types efficiently: Make sure you’re using the most efficient data types possible when serializing your objects. Using NSData instead of a custom binary format can help improve performance, especially when dealing with small amounts of data.
• Consider security implications: When working with sensitive data, consider implementing encryption or other security measures to protect it during serialization and deserialization.

Conclusion

Serialization and deserialization are powerful tools in iOS development that allow you to store complex data structures in memory. By understanding how NSKeyedArchiver works, conforming to the NSCoding protocol, and using NSKeyedUnarchiver, you can efficiently serialize and deserialize custom objects.

Last modified on 2024-03-10