Swizzled method for NSMutableDictionary is not getting called

Question

I'm trying to swizzle NSMutableDictionary. What am I doing wrong here? I'm trying to override setObject:forKey: for NSMutableDictionary.

#import "NSMutableDictionary+NilHandled.h"
#import <objc/runtime.h>


@implementation NSMutableDictionary (NilHandled)

+ (void)load{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];

        SEL originalSelector = @selector(setObject:forKey:);
        SEL swizzledSelector = @selector(swizzledSetObject:forKey:);

        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

        class_replaceMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));

        BOOL didAddMethod = class_addMethod(class,originalSelector,method_getImplementation(swizzledMethod),method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            class_replaceMethod(class,swizzledSelector,method_getImplementation(originalMethod),method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (void) swizzledSetObject:(id)anObject forKey:(id<NSCopying>)aKey
{
    [self swizzledSetObject:anObject?anObject:@"Nil" forKey:aKey];
}

@end

Apart from things not working, replacing nil values with @"Nil" is a very, very bad idea. [NSNull null] is meant to be used for exactly that purpose. — gnasher729, May 28 '16 at 12:16
Last time I came up with this kind of task, I followed through [Advice on how to catch “attempt to insert nil object” from a device needed](http://stackoverflow.com/a/23789405/1492173) SO answer. — Yevhen Dubinin, May 28 '16 at 14:38

score 7 · Accepted Answer · answered May 28 '16 at 08:01

NSMutableDictionary is a class cluster. It is an abstract base class for a number of Apple-private concrete subclasses that implement the real work. As subclasses, they override -setObject:forKey:. There is no real implementation of that method on NSMutableDictionary itself and nothing attempts to call it. Therefore, nothing calls your replacement implementation.

Princekin · Answer 2 · 2020-03-17T08:44:32.603

change the code:

Class class = [self class];

to:

Class class = NSClassFromString(@"__NSDictionaryM");

worked for me,you can try it!

here is my whole code:

@implementation NSMutableDictionary (Category)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    [super load];
 Class class = NSClassFromString(@"__NSDictionaryM");
 SEL originalSelector = @selector(setObject:forKey:);
 SEL swizzledSelector = @selector(fu_setObject:forKey:);

    Method originalMethod = class_getInstanceMethod(class,originalSelector);
 Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
 BOOL didAddMethod =
  class_addMethod(class,
   originalSelector,
   method_getImplementation(swizzledMethod),
   method_getTypeEncoding(swizzledMethod));

 if (didAddMethod) {
  class_replaceMethod(class,
   swizzledSelector,
   method_getImplementation(originalMethod),
   method_getTypeEncoding(originalMethod));
 } else {
  method_exchangeImplementations(originalMethod, swizzledMethod);
 }
    });
}

-(void)fu_setObject:(id)anObject forKey:(id<NSCopying>)aKey{
   [self fu_setObject:anObject forKey:aKey];
   
}
@end

score 0 · Answer 3 · answered May 28 '16 at 12:17

After being told why it doesn't work: Just add a new method to NSMutableDictionary like -(void)setObject:(id)object orNullForKey:(NSString*)key.

Possibly more useful is a method that doesn't try to add, or even removes, the object if it is nil. That way, objectForKey will return nil, the same that you passed in.

Swizzled method for NSMutableDictionary is not getting called

3 Answers3

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