Dynamic Getters and Setters with Objective C

Question

I am in a situation where I want to dynamically generate getters and setters for a class at runtime (in a similar manner to what NSManagedObject does behind the scenes). From my understanding, this is possible using resolveInstanceMethod: on a specific class. At this point, you would have to use class_addMethod to dynamically add the method based on the selector. I understand this at a theoretical level, but I haven't delved much into the obj-c runtime, so I was curious if there were any great examples of how to do this. Most of my knowledge comes from this article:

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtDynamicResolution.html

Any thoughts / examples?

Answer 1

The only nice discussion I know is at Mike Ash's blog post. It's not that hard, actually.

I once needed to split a big NSManagedObject subclass into two, but decided to keep the fact an implementation detail so that I don't have to rewrite other parts of my app. So, I needed to synthesize getter and setter which sends [self foo] to [self.data foo], automatically.

To achieve that, I did the following:

1. Prepare the new method, already in my class.

   - (id)_getter_
   {
       return objc_msgSend(self.data, _cmd);
   }
   
   - (void)_setter_:(id)value 
   {
       objc_msgSend(self.data, _cmd,value);
   }
   

   Note that _cmd has the selector in it. So, usually, _cmd is either @selector(_getter_) or @selector(_setter_) in these methods, but I'm going to plug the implementation of _getter_ as the implementation of foo. Then, _cmd contains @selector(foo), and thus calls self.data's foo.

2. Write a generic synthesizing method:

   +(void)synthesizeForwarder:(NSString*)getterName
   {
       NSString*setterName=[NSString stringWithFormat:@"set%@%@:",
             [[getterName substringToIndex:1] uppercaseString],[getterName substringFromIndex:1]];
       Method getter=class_getInstanceMethod(self, @selector(_getter_));
       class_addMethod(self, NSSelectorFromString(getterName), 
                       method_getImplementation(getter), method_getTypeEncoding(getter));
       Method setter=class_getInstanceMethod(self, @selector(_setter_:));
       class_addMethod(self, NSSelectorFromString(setterName), 
                       method_getImplementation(setter), method_getTypeEncoding(setter));
   }
   

   Note that this is a class method. So self stands for the class. Note also that I didn't hardcode type encodings (which tells Objective-C runtime what the arguments of the particular method are). The syntax of type encodings is documented, but constructing by hand is very error-prone; I wasted a few days that way until Mike Ash told me to stop it. Generate it using an existing method.

3. Generate forwarders at the earliest possible time:

    +(void)load  
    {
        for(NSString*selectorName in [NSArray arrayWithObjects:@"foo", @"bar", @"baz",nil]){
           [self synthesizeForwarder:selectorName];
        }
    }
   

   This generates foo, setFoo:, bar, setBar:, and baz, setBaz:.

Hope this helps!

Answer 2

Another example is one I wrote, called DynamicStorage, available here:

https://github.com/davedelong/Demos

The primary impetus behind it was this question, which was asking how to use an NSMutableDictionary as the backing store for any object ivar. I wrote a class that will generate getters and setters for any @property, respecting things like a custom getter/setter name, the object memory management policy, etc. The neat thing about it is that it's using imp_implementationWithBlock() so that it only has to calculate the appropriate property name once (and then captures and saves it as part of the block).