Using Runtime to Implement Automatic Archiving and Unarchiving

The Pain of the Traditional Approach

Here’s what typical code looks like. Suppose there’s a Person class with name, age, and address:

@interface Person : NSObject <NSCoding>
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@property (nonatomic, copy) NSString *address;
@end

@implementation Person
- (void)encodeWithCoder:(NSCoder *)aCoder {
    [aCoder encodeObject:self.name forKey:@"name"];
    [aCoder encodeInteger:self.age forKey:@"age"];
    [aCoder encodeObject:self.address forKey:@"address"];
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super init]) {
        self.name = [aDecoder decodeObjectForKey:@"name"];
        self.age = [aDecoder decodeIntegerForKey:@"age"];
        self.address = [aDecoder decodeObjectForKey:@"address"];
    }
    return self;
}
@end

With three properties it’s manageable. By the time you hit ten, it’s a drag. And the keys must match the property names exactly — a typo and the data doesn’t load.

The Runtime Approach: Using class_copyIvarList to Get All Properties Automatically

The Runtime provides class_copyIvarList, which gets all member variables of a class — including private variables and the _xxx variables automatically generated by @property.

Ivar *class_copyIvarList(Class cls, unsigned int *outCount);

After getting all member variables, use ivar_getName to get the variable name as the key, then use KVC’s valueForKey: and setValue:forKey: to read and write values. You can completely avoid manually writing archiving and unarchiving code for each property.

- (void)encodeWithCoder:(NSCoder *)aCoder {
    unsigned int count = 0;
    Ivar *ivars = class_copyIvarList([self class], &count);
    for (unsigned int i = 0; i < count; i++) {
        Ivar ivar = ivars[i];
        NSString *key = [NSString stringWithUTF8String:ivar_getName(ivar)];
        id value = [self valueForKey:key];
        [aCoder encodeObject:value forKey:key];
    }
    free(ivars);  // Don't forget to release the memory
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super init]) {
        unsigned int count = 0;
        Ivar *ivars = class_copyIvarList([self class], &count);
        for (unsigned int i = 0; i < count; i++) {
            Ivar ivar = ivars[i];
            NSString *key = [NSString stringWithUTF8String:ivar_getName(ivar)];
            id value = [aDecoder decodeObjectForKey:key];
            [self setValue:value forKey:key];
        }
        free(ivars);
    }
    return self;
}

This removes repeated enumeration, but it does not remove schema maintenance. A new field should be reviewed before it is persisted, and long-lived archives need stable keys, type validation, and a migration strategy. The basic decoder below is suitable for explaining the Runtime loop; production code should prefer NSSecureCoding and explicit allowed classes.

Handling Superclass Properties: Traversing Up with a while Loop

The version above has a limitation: it only handles the current class’s member variables — it doesn’t handle superclass properties. If a class has a superclass, those properties should be archived and unarchived too.

The solution is to use a while loop to traverse from the current class all the way up to NSObject:

- (void)encodeWithCoder:(NSCoder *)aCoder {
    Class cls = self.class;
    while (cls && cls != [NSObject class]) {
        unsigned int count = 0;
        Ivar *ivars = class_copyIvarList(cls, &count);
        for (unsigned int i = 0; i < count; i++) {
            Ivar ivar = ivars[i];
            NSString *key = [NSString stringWithUTF8String:ivar_getName(ivar)];
            id value = [self valueForKey:key];
            [aCoder encodeObject:value forKey:key];
        }
        free(ivars);
        cls = [cls superclass];
    }
}

The unarchiving method works the same way. After this, all properties in the superclass are automatically archived and unarchived — no need to repeat implementations in each subclass.

Ignoring Certain Properties: Optional Filtering

In some cases, you might not want certain properties archived — like a temporary cache property, or a system object that can’t be archived. In those cases, add a filtering mechanism.

// Implement this method in the Model to return a list of property names to ignore
- (NSArray *)ignoredNames {
    return @[@"_tempCache", @"_session"];
}

- (void)encodeWithCoder:(NSCoder *)aCoder {
    Class cls = self.class;
    NSArray *ignored = [self respondsToSelector:@selector(ignoredNames)] 
                        ? [self ignoredNames] : nil;
    while (cls && cls != [NSObject class]) {
        unsigned int count = 0;
        Ivar *ivars = class_copyIvarList(cls, &count);
        for (unsigned int i = 0; i < count; i++) {
            Ivar ivar = ivars[i];
            NSString *key = [NSString stringWithUTF8String:ivar_getName(ivar)];
            if ([ignored containsObject:key]) continue;
            id value = [self valueForKey:key];
            [aCoder encodeObject:value forKey:key];
        }
        free(ivars);
        cls = [cls superclass];
    }
}

Wrapping in Macros: One Line for All Models

Even with this, writing encodeWithCoder: and initWithCoder: every time is still a bit repetitive. You can wrap it in an NSObject category and have the specific model call the category methods:

// NSObject+Archive.h
@interface NSObject (Archive)
- (void)autoEncodeWithCoder:(NSCoder *)aCoder;
- (void)autoDecodeWithCoder:(NSCoder *)aDecoder;
- (NSArray *)ignoredNames;  // Optional ignore list
@end

Then in the specific Model:

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [self autoEncodeWithCoder:aCoder];
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super init]) {
        [self autoDecodeWithCoder:aDecoder];
    }
    return self;
}

If even that feels like too much, define a macro that expands both methods.

Important Considerations

class_copyIvarList gets member variables, not properties. Member variable names in Objective-C typically have underscores, like _name and _age. When using KVC’s valueForKey: and setValue:forKey:, passing underscored keys is valid — KVC automatically matches them to the corresponding property.

C memory management: The array returned by class_copyIvarList needs to be freed with free() — it’s a C-level API not managed by ARC. Common practice is to call free(ivars) immediately after the loop to prevent memory leaks.

Superclass private variables: class_copyIvarList only gets member variables of the current class — superclass member variables are not included. So you need to use a while loop to traverse up the superclass chain.

Don’t archive NSObject‘s variables: NSObject doesn’t have any member variables worth archiving, so stop at cls != [NSObject class].

Summary

Runtime enumeration can reduce repetitive coding by discovering selected ivars and producing stable keys, while an explicit superclass boundary prevents framework internals from being archived. It is not automatically stable or complete: secure class allowlists, error handling, field review, and migration tests remain part of the design. The runnable project below demonstrates those safeguards rather than treating reflection as a substitute for a schema.