Adding method to instance of objective-c class at runtime

Question

I am wondering if there is a way to add an instance method to an "instance" of a class.

The scenario is that I am using EKEventEditViewController and there is a UITableView inside this class with a delegate(UITableViewDelegate) called "EKEventEditor" (non-public AFAIK). It does not implements the tableView:willDisplayCell:forRowAtIndexPath: method, which I am trying to use to disable some cell.

So I am trying to add a method to the instance, but all I can find in the Obj-C runtime is the class_addMethod call which adds method to the class but not the instance. With "EKEventEditor" being private, I can't just extend it and add that method myself.

Any hints?

Here is the code that I am using, the function that i am trying to add (willDisplayCell_) is not getting called.

- (void)navigationController:(UINavigationController *)navigationController 
  willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {

if ([navigationController isKindOfClass:[EKEventEditViewController class]]) {

UITableViewController *rootController = (UITableViewController*)[(UINavigationController*)navigationController visibleViewController];
if ([rootController isKindOfClass:[UITableViewController class]]) {
  UITableView *eventTableView = (UITableView*)[rootController view];

  class_addMethod([eventTableView.delegate class], 
                  @selector(tableView:willDisplayCell:forRowAtIndexPath:), 
                  (IMP)willDisplayCell_, "v@:@@@");
}}} 


void willDisplayCell_(id self, SEL cmd, UITableView *tableView, UITableViewCell *cell, NSIndexPath *indexPath) {
  NSLog(@"CALLED?");
}

jscs · Accepted Answer · 2012-07-05 23:59:11Z

3

That's the function you want. That will add the (instance) method to all instances of the class.* There's no way to add a method just to one particular instance.**

You might want to look into using a Category, however, which does let you extend a class whose @implementation you don't control. This will only work if you can import the class's primary @interface, though, and it sounds like that may not be the case.

*If you wanted to add a class method, you would call the function with the metaclass as the first argument.

**Barring some trickery with overriding forwardInvocation: and possibly libffi, which I'm working on.

edited Jul 5, 2012 at 23:59

answered Jul 5, 2012 at 23:52

jscs

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6 Comments

Chuck Over a year ago

You could add a method to one particular instance using the same subclass-and-swizzle trickery that KVO uses to dynamically override the accessors of individual objects.

Herman Over a year ago

@JoshCaswell I edited the question with my code, am I doing something wrong such that willDisplayCell_ is not called?

jscs Over a year ago

@Herman: I'm not sure. That looks right to me. It may be that the table view isn't sending that message, since it assumes that its delegate doesn't implement it.

Herman Over a year ago

@JoshCaswell sorry didn't see the comment until now. hm.... any trickery you think I can do to make this work?

jscs Over a year ago

@Herman: You should definitely put a breakpoint on [<#Whatever the class of the delegate is#> tableView:willDisplayCell:forRowAtIndexPath:] to check that out. If it isn't getting sent, I don't know what you could (reasonably) do. You'd have to swap the eventTableView out for your own object -- likely to be extremely difficult. That would probably be worth its own SO question, though; I'm sure someone would have an idea or two.

|

Alex Gray · Accepted Answer · 2013-06-25 07:52:37Z

This is totally possible, and amazingly useful... Think altering the functionality of an instance without subclassing, etc...

@interface NSObject (AZOverride)
/** Dynamically overrides the specified method on this particular instance.
* The block's parameters and return type must match those of the method you 
* are overriding. However, the first parameter is always "id _self", which 
* points to the object itself.
* You do have to cast the block's type to (__bridge void *), e.g.:
*
*    [self az_overrideSelector:@selector(viewDidAppear:)
*            withBlock:(__bridge void *)^(id _self, BOOL animated) { ... }];
*/
- (BOOL)az_overrideSelector:(SEL)selector withBlock:(void *)block;
/** To call super from the overridden method, do the following:
*    SEL sel = @selector(viewDidAppear:);
*    void (*superIMP)(id, SEL, BOOL) = [_self az_superForSelector:sel];
*    superIMP(_self, sel, animated);
* This first gets a function pointer to the super method and then you call it.
*/
- (void *)az_superForSelector:(SEL)selector;
@end

Kazki · Accepted Answer · 2013-02-14 14:24:44Z

2

Here is REKit for you. It gives you ability to add/override method at instance level.

With REKit, you can add the method to eventTableView.delegate like below:

[eventTableView.delegate
    respondsToSelector:@selector(tableView:willDisplayCell:forRowAtIndexPath:)
    withKey:nil
    usingBlock:^(id receiver, UITableView *tableView, UITableViewCell *cell, NSIndexPath *indexPath) {
        // Do something…
    }
];

edited Feb 14, 2013 at 14:24

answered Feb 14, 2013 at 5:41

Kazki

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