Understanding NSKeyedArchiver’s Encoding Process

As developers, we often rely on built-in classes like NSKeyedArchiver to serialize our objects into a format that can be easily stored or transmitted. However, sometimes the behavior of these classes may not always align with our expectations.

In this article, we will delve into the world of NSKeyedArchiver and explore what happens when it is called multiple times on the same object. We’ll examine the encoding process, identify potential issues, and provide practical examples to ensure you understand how to use NSKeyedArchiver effectively in your development projects.

Introduction to NSKeyedArchiver

NSKeyedArchiver is a part of Apple’s Foundation framework, which provides a way to serialize objects into a binary format that can be stored or transmitted. This class uses the Key-Value Coding (KVC) protocol to encode and decode objects.

When you create an instance of NSKeyedArchiver, you specify the types of objects that should be encoded by setting the objectTypes property. In our example, we are encoding objects with properties like title, newsItemId, author, description, imageUrl, thumbnailUrl, and createdAt.

The Encoding Process

When you call the encodeWithCoder: method on an object that has been set up for KVC, the following steps occur:

1. KVC Inspection: The code inspecting the object’s properties using KVC to determine which properties should be encoded.
2. Encoding: Each property is converted into a Key-Value Coding compliant format and stored in a dictionary called NSCodingContainer.
3. Encoding Completion: After all properties have been inspected, encoded, and stored, the encoder signals completion.

However, if you’re calling encodeWithCoder: multiple times on the same object, you might be concerned about potential issues with data consistency or performance.

The Problem: NSKeyedArchiver Being Called Twice

In our original question, we discussed an iPhone app shutting down while encoding objects. Upon further investigation, it appears that the issue lies in the fact that NSKeyedArchiver is being called twice on the same object.

Here’s a possible scenario:

1. Initial Encoding: The app starts and calls encodeWithCoder: on our NewsItem object to serialize its data.
2. App Terminates: When the app terminates, it triggers the NSKeyedArchiver encoding process again as part of the shutdown sequence.

This second call to encodeWithCoder: results in a duplicate serialization of our objects, which can lead to issues like:

• Data loss or corruption
• Inconsistent data across different storage locations

Solving the Problem: Avoiding Duplicate Encoding Calls

So, how do we prevent this from happening? Here are some potential solutions to avoid calling NSKeyedArchiver twice on the same object:

Solution 1: Set a Flag Indicating Initial Encoding

Create an instance variable (bool) to track whether the object has been encoded. When you call encodeWithCoder: for the first time, set this flag.

#import <Foundation/Foundation.h>

@interface NewsItem : NSObject <NSCoding>
@property (nonatomic) BOOL _hasBeenEncoded;
@end

@implementation NewsItem

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    if (!_hasBeenEncoded) {
        // Set encoding flag
        _hasBeenEncoded = YES;

        NSLog(@"News encode called %@", self.title);

        // ... rest of your encoding code ...

        // Images
        NSData *savedImageData = UIImagePNGRepresentation(self.savedImage);
        [aCoder encodeObject:savedImageData forKey:KNewsItemSavedImageKey];

        NSData *thumbnailImageData = UIImagePNGRepresentation(self.savedThumbnail);
        [aCoder encodeObject:thumbnailImageData forKey:KNewsItemSavedThumbnailKey];
    }
}

- (void)setNewsItemId:(NSString *)newsItemId {
    _newsItemId = newsItemId;
    // ... rest of your code ...
}

@end

Solution 2: Use a Locking Mechanism

Another approach is to use a locking mechanism, like @synchronized or NSLock, to prevent concurrent encoding calls on the same object.

#import <Foundation/Foundation.h>

@interface NewsItem : NSObject <NSCoding>
@property (nonatomic) NSLock *_lock;
@end

@implementation NewsItem

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    [_lock lock];
    if (!_hasBeenEncoded) {
        // Set encoding flag
        _hasBeenEncoded = YES;

        NSLog(@"News encode called %@", self.title);

        // ... rest of your encoding code ...

        // Images
        NSData *savedImageData = UIImagePNGRepresentation(self.savedImage);
        [aCoder encodeObject:savedImageData forKey:KNewsItemSavedImageKey];

        NSData *thumbnailImageData = UIImagePNGRepresentation(self.savedThumbnail);
        [aCoder encodeObject:thumbnailImageData forKey:KNewsItemSavedThumbnailKey];
    }
    [_lock unlock];
}

@end

Solution 3: Use a Serializable Object

Finally, consider re-designing your objects to be serializable from the start. This can simplify your encoding process and prevent potential issues with duplicate encoding calls.

#import <Foundation/Foundation.h>

@interface NewsItem : NSObject

@property (nonatomic) NSString *newsItemId;
@property (nonatomic) NSString *title;
@property (nonatomic) NSString *author;
@property (nonatomic) NSString *description;
@property (nonatomic) NSString *imageUrl;
@property (nonatomic) NSString *thumbnailUrl;
@property (nonatomic) NSDate *createdAt;

// Images
@property (nonatomic, strong) UIImage *savedImage;
@property (nonatomic, strong) UIImage *savedThumbnail;

@end

@implementation NewsItem

- (void)setNewsItemId:(NSString *)newsItemId {
    _newsItemId = newsItemId;
}

- (void)setTitle:(NSString *)title {
    _title = title;
}

- (void)setAuthor:(NSString *)author {
    _author = author;
}

- (void)setDescription:(NSString *)description {
    _description = description;
}

- (void)setImageUrl:(NSString *)imageUrl {
    _imageUrl = imageUrl;
}

- (void)setThumbnailUrl:(NSString *)thumbnailUrl {
    _thumbnailUrl = thumbnailUrl;
}

- (void)setCreatedAt:(NSDate *)createdAt {
    _createdAt = createdAt;
}

@end

Conclusion

NSKeyedArchiver provides a convenient way to serialize objects into a format that can be stored or transmitted. However, when dealing with duplicate encoding calls on the same object, potential issues like data loss or corruption arise.

By using one of the solutions outlined above (setting a flag indicating initial encoding, locking mechanisms, or serializable objects), you can prevent these issues and ensure consistent data across different storage locations.

Remember to consider your specific requirements and constraints when choosing an approach.

Last modified on 2025-05-04