You said:
I want to be able to determine what type of classes should go in my array so I'd like to annotate it somehow to say so. Then later be able to access that annotation via something like the runtime library where I can access lists of properties and their names.
There are a few ways to do this sort of thing in Objective-C. Apple's frameworks do this sort of thing by adding a class method that returns the required information. Examples: dependent keys in KVO, +[CALayer needsDisplayForKey:] and related methods.
So, let's create a class method that returns an array of classes that can go into your container property, given the property name. First, we'll add a category to NSObject to implement a generic version of the method:
@interface NSObject (allowedClassesForContainerProperty)
+ (NSArray *)allowedClassesForContainerPropertyWithName:(NSString *)name;
@end
@implementation NSObject (allowedClassesForContainerProperty)
+ (NSArray *)allowedClassesForContainerPropertyWithName:(NSString *)name {
if (class_getProperty(self, name.UTF8String)) {
return @[ [NSObject class] ];
} else {
[NSException raise:NSInvalidArgumentException
format:@"%s called for non-existent property %@", __func__, name];
abort();
}
}
@end
As you can see, this default version of the method doesn't do anything particularly useful. But adding it to NSObject means we can send the message to any class without worrying about whether that class implements the method.
To make the message return something useful, we override it in our own classes. For example:
@implementation MyViewController
+ (NSArray *)allowedClassesForContainerPropertyWithName:(NSString *)name {
if ([name isEqualToString:@"myArray"]) {
return @[ [UIButton class], [UIImageView class] ];
} else {
return [super allowedClassesForContainerPropertyWithName:name];
}
}
...
We can use it like this:
SomeViewController *vc = ...;
SomeObject *object = ...;
if ([[vc.class allowedClassesForContainerPropertyWithName:@"bucket"] containsObject:object.class]) {
[vc.bucket addObject:object];
} else {
// oops, not supposed to put object in vc.bucket
}
