Invoking a method on a delegate after delay, from an object at the end of it's lifetime

Question

I have a modal view controller that creates core data changes in it's own context, and when I click done, it saves the changes (that dispatches the merge changes notification), notifies the delegate and dismisses.

My problem is that I need the delegate to receive the message after my main context has merged with the changes of the editing context. I want the delegate call to take place on the next run loop but I'm having problems with object lifetimes. I've thought of the following:

1. Make call to [delegate performSelector:withObject:afterDelay:] however it seems that that message is not recognised. My delegate conforms to the NSObject protocol but that doesn't include the perform selector with delay.
2. Create a method in my view controller: informDelegateWithObject: that calls the delegate method, and call that method after a delay. I.e. [self performSelector:@selector(informDelegateWithObject:) withObject:.. afterDelay:..]. This could work, however, as my view controller is being dismissed, if the delay is several seconds then it would have been released from memory and wouldn't that cause a crash when it comes to invoking?
3. Create an instance of NSInvocation. I have thought about this, however, what is the lifetime of this object? If I create it using [NSInvocation invocationWithMethodSignature:] then wouldn't the NSInvocation object be autoreleased, and not be around for the next run loop? Let alone several seconds. And as my modal view controller is being dismissed and released, I can't store the invocation object in my view controller.

Any suggestions?

Answer 1

You should merge contexts into the delegate.

Say that you press Save into you modal controller: you will send a myViewController:didFinishSaving: to the delegate.

This delegate into myViewController:didFinishSaving: implementation will save, merge and dismiss the modal view controller.

I hope I have understood your problem. Bye! :)

Answer 2

You might look at Apple's Core Data Books tutorial which works along the lines that muccy describes. Saving happens after the modal view is dismissed and control is returned to the parent view controller. The parent contains the update code and fires notifications required to merge changes (whether that happens in the delegate or elsewhere).

Answer 3

To question #1: performSelector:withObject:afterDelay: is defined in the NSObject class, not the NSObject protocol. Any object you are using is probably an instance of NSObject. You are probably referring to a compiler warning resulting from static type checking. (Technically, it's possible for an object that conforms to the NSObject protocol to not be an NSObject; NSProxy is one example. But any object you normally use will be an NSObject.) You can ignore this warning (in Objective-C, you can try to send any message to any object). Or, if you want, you can cast it to either id (which allows you to send any message without any warnings) or NSObject *.

To question #2: "if the delay is several seconds then it would have been released from memory" No, the documentation for performSelector:withObject:afterDelay: says "This method retains the receiver and the anArgument parameter until after the selector is performed."

Answer 4

You can also declare your delegate like this:

    NSObject <MyClassDelegateProtocol> *delegate;

Then your delegate will also be an NSObject that conforms to your protocol.