Handling app delegates and switching between views

问题:

I'm getting a warning about a semantic issue pertaining to passing a *const _strong to type id and cannot seem to fix it no matter what I change.

I have two views at the moment, and have written this code. In iPadSpeckViewController.m, here is the method that should switch between views:

-(IBAction) touchProducts {
    ProductsViewController *controller = [[ProductsViewController alloc]
            initWithNibName:@"Products" bundle:nil];
    controller.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
    controller.delegate = self;
    [self presentModalViewController:controller animated:YES];
}

And for ProductsViewController.h:

@interface ProductsViewController : UIViewController {
    id<ProductsViewControllerDelegate> delegate;
}
@property(nonatomic, retain)
    IBOutlet id<ProductsViewControllerDelegate> delegate;

ProductsViewController.m contains:

@synthesize delegate;

But the views do not switch... Thoughts?

EDIT: Here is the exact warning, as it appears on the line "controller.delegate = self;" in iPadSpeckViewController.m:

/Developer/iPadSpeckApp/iPadSpeckApp/iPadSpeckAppViewController.m:17:27:{17:27-17:31}: warning: passing 'iPadSpeckAppViewController *const __strong' to parameter of incompatible type 'id<ProductsViewControllerDelegate>' [3]

回答1:

This warning is oddly worded, but it is actually just a way of telling you that the class of self (whatever that class is) fails to conform to the ProductsViewControllerDelegate protocol. To get rid of the warning, you have two choices:

• Declare the class of self (whatever that class is), in its @interface statement, to conform to the protocol ProductsViewControllerDelegate:

  @interface MyClass : NSObject <ProductsViewControllerDelegate>;
  

• Suppress the warning by changing this:

  controller.delegate = self;
  

  to this:

  controller.delegate = (id)self;
  

The delegate property is typed as id<ProductsViewControllerDelegate>. But self is not. Under ARC you must make the cast explicit, so that the types formally agree. (I believe this is so that ARC can make absolutely certain it has sufficient information to make correct memory management decisions.)

回答2:

Got the same error when i tried to set the delegate of a UINavigationController to an object that implemented the wrong protocol (UINavigationBarDelegate instead of UINavigationControllerDelegate). This might be a simple typo.

回答3:

If you only want to switch views, you might want to try the code below. It works for me.

ProductsViewController *controller = [[ProductsViewController alloc] initWithNibName:@"Products" bundle:nil];
[self.navigationController pushViewController:controller animated:YES];

I used that to have my main menu in my app switch over to the game.

If you want a special animation (I think I saw Cross Dissolve?), however, I have no idea. I will try digging through the documentation to see, and I will tell you what I find.

As for the "*const_string to type id", although I don't know what you are trying to do with your app, I think the problem is the id <ProductsViewControllerDelegate> delegate in your view controller.