Tutorial :Does it make sense that there may be more than one class that conforms to the UIApplicationDelegate protocol in an iPhone App?



Question:

I think I've understood what that Delegate is supposed to do. If a class conforms to that protocol, it tells the underlying system: "Hey man, I am the UIApplication object's delegate! Tell me what's up, and I may tell you what to do!".

What, if multiple classes implement that? Is that possible? Does that make any sense?


Solution:1

While you could implement multiple classes that conform to the UIApplicationDelegate protocol only one, the first, would receive these messages.

Implementing a protocol to create a delegate is only one part of the equation. That delegate then has to be registered with the code that's generating the messages and these systems generally only support one delegate.

In the case of UIApplication you can change the delegate using the 'delegate' property in the UIApplication shared class but this will replace the original delegate, not add an additional one.

If you need to broadcast UIApplication level messages to other systems then this is functionality you should add to your existing delegate.


Solution:2

You can implement multiple classes that adopt the UIApplicationDelegate protocol, but only one can be the actual delegate at any given time. It's set by [UIApplication sharedApplication].delegate, which is normally set up by the main NIB file by an outlet connection.


Solution:3

Just conforming to the protocol doesn't set your object as the delegate, you need to do that explicitly either in the nib or in code. As already mentioned, only one object can be a delegate at one time. Having multiple delegates may make sense in some cases-- for example if you have a table view that displays two sets of data, you could make two delegate and datasource objects for it, and switch between them as needed. It probably doesn't make sense to do this for the application's delegate though, since the code there is pretty specific.

Keep in mind that sometimes an object will send notifications in addition to calling delegate methods. A lot of time it looks like they're the same thing, since the object will automatically subscribe your delegate to the notification if it includes a certain method signature. The key difference though is that other objects besides the delegate can also subscribe to these notifications, so you can hook them up to multiple objects at once.


Solution:4

As Daniel Dickson stated:

You can implement multiple classes that adopt the UIApplicationDelegate protocol, but only one can be the actual delegate at any given time. It's set by [UIApplication sharedApplication].delegate, which is normally set up by the main NIB file by an outlet connection.

... but know that you can swap these out at runtime if you need to. I recently looked at using this technique as a way of merging two applications developed by different parties that could not share source code or refactor; yet needed to co-locate under a single icon on the device.


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