Cannot find protocol declaration for 'UIViewController' when creating a protocol

Question

I am trying to define my protocol so that the class implementing it has to also be a UIViewController.

So i typed the following code into a header file:

#import <UIKit/UIKit.h>
#import <UIKit/UIViewController.h>
#import <EventKit/EventKit.h>
#import <EventKitUI/EventKitUI.h>

@protocol MySuperProtocol<UIViewController> // <-- here

@property(nonatomic, weak, nullable) id<EKEventViewDelegate> delegate;

@end

I always get the error saying that:

Cannot find protocol declaration for 'UIViewController'

If i replace UIViewController with NSObject, code compiles. If i remove the <> inheritance after the protocol, code compiles.

I tried all combinations of

#import <UIKit/UIKit.h>
#import <UIKit/UIViewController.h>

to no avail.

What am i doing wrong?

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

You are misunderstanding the syntax. @protocol MySuperProtocol<UIViewController> doesn't establish a constraint that implementers of MySuperProtocol must be UIViewControllers. It says that MySuperProtocol conforms to UIViewController

However, unlike NSObject, UIViewController is not a protocol; it is a class. A protocol cannot conform to a class, only to another protocol.

You can refer to the documentation:

Protocols inherit from other protocols

In the same way that an Objective-C class can inherit from a superclass, you can also specify that one protocol conforms to another.

There is no way to constrain protocol adoption in Objective C.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Mar 25, 2019 at 0:56

Paulw11

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