分配并初始化destinationViewController

Question

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    if ([segue.identifier isEqualToString:@“DoSomething”]) {
        if ([segue.destinationViewController isKindOfClass:[DoSomethingVC class]]) {
            DoSomethingVC *doVC = (DoSomethingVC *)segue.destinationViewController;
doVC.neededInfo = ...; }
} 
}

In the code above, I don't understand why you need the local variable doVC.

If your destinationViewController already exist in the heap, why can't you pass data to it as

seque.destinationViewController.neededInfo =...

If it doesn't exist in the heap, shouldn't you alloc and initialize it? Is that what this line of code is basically doing?

DoSomethingVC *doVC = (DoSomethingVC *)segue.destinationViewController;

You're allocating and instantiating an instance of DoSomethingVC class? If so, I don't understand how this local variable is going to be kept alive since it's local. Also the sourceVC will also die once the seque is complete.

I just don't understand how the compiler is manipulating the memory behind this process and syncing up the data from this local variable to the destinationViewController.

Answer 1

segue.destinationViewController is of type UIViewController (even though it is actually a DoSomethingVC which has a base type of UIViewController ) as it stands, so it wont know about your variable neededInfo that is why you need to cast it to your DoDomethingVC .

It keeps segue.destinationViewController as type UIViewController to be generic, since it cant really know at compile time what this destinationViewController is going to be.

Going on a hunch that your grasp on inheritance is a bit shaky (not trying to be mean) but an analogy would be, if UIViewController is say type Bird and DoSomethingVC is type Duck , this prepareForSegue: method knows that a Bird will be the destinationViewController, but it doesn't know at this point whether its going to be a Duck or some other kind of bird, so you have to tell it explicitly what it is going to be (and if you get it wrong its going to do bad things, or crash) also the framework doesnt know what kinds of birds there are (since you are making them), so it cant be factored in for you