Understanding Protocol Inheritance in Objective-C

Question

I'll appreciate if anyone can explain the logic behind protocol inheritance. e.g. what does the following mean (UITableView.h):

@protocol UITableViewDelegate<NSObject, UIScrollViewDelegate>

The following class implementation doesn't work. I have a class View1 (which inherits UIView), with an associated protocol. I have another class, View2 (which inhertits View1). Now i want to inherit the the protocol as well. Can anyone please point me in the right direction.

Class 1:

@protocol View1Delegate;

@interface View1 : UIView {
    id <View1Delegate> delegate;
    // . . .
}

@property (nonatomic, assign) id <View1Delegate> delegate; // default nil. weak reference

@end

@protocol View1Delegate <NSObject>
- (void)View1DelegateMethod;
@end

@implementation View1

@synthesize delegate;

// . . .
@end

Class 2:

@protocol View2Delegate;

@interface View2 : View1 {
    id <View2Delegate> delegate;
    // . . .
}

@property (nonatomic, assign) id <View2Delegate> delegate; // default nil. weak reference

@end

@protocol View2Delegate <NSObject>
- (void)View2DelegateMethod;
@end

@implementation View2

@synthesize delegate;

// . . .
@end

Pang · Accepted Answer · 2017-07-10 07:56:14Z

12

Think of it more as composition rather than inheritance.

@protocol UITableViewDelegate<NSObject, UIScrollViewDelegate> defines a protocol that includes all the methods of the NSObject protocol, the UIScrollViewDelegate protocol, as well as any methods defined for the UITableViewDelegate protocol. When you subclass and create a new property, you're overriding the type of the superclasses property. To make this work how I think you want, you should declare View2Delegate as @protocol View2Delegate <NSObject, View1Delegate>.

edited Jul 10, 2017 at 7:56

Pang

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answered Sep 20, 2010 at 13:37

Joshua Weinberg

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1 Comment

Mustafa Over a year ago

Thank you for your reply. I'm declaring the protocol as you've mentioned, but the problem i'm facing is with the declaration of id <...>delegate; In View1, I have id <View1Delegate> delegate; with it's associated @property and @synthesize. Since i'm inheriting View1 in View2, when i declare id <View2Delegate> delagete; with it's associated @property and @synthesize, i get an error: "Property 'delegate' attempting to use ivar 'delegate' declared in super class of 'View2'. What's the correct way to handle this situation. Please see the sample classes i've posted above.

user102008 · Accepted Answer · 2011-08-08 23:15:12Z

0

It's exactly the same as the inheritance of interfaces in Java (interface UITableViewDelegate extends NSObject, UIScrollViewDelegate), C# (interface UITableViewDelegate : NSObject, UIScrollViewDelegate), etc.

answered Aug 8, 2011 at 23:15

user102008

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