Cannot find protocol declaration for

Remove this line from viewController1.h:

#import "viewController2.h"

The problem is that viewController2's interface is preprocessed before the protocol declaration.

The general structure of the file should be like this:

@protocol viewController1Delegate;@class viewController2;@interface viewController1@end@protocol viewController1Delegate <NSObject>@end

    A.h:    #import "B.h"  // A    @class A;    @protocol Delegate_A        (method....)    @end    @interface ViewController : A    @property(nonatomic,strong)id<ViewControllerDelegate> preViewController_B;(protocol A)    @end    B.h:    #import "A.h"  // A    @class B;    @protocol Delegate_B        (method....)    @end    @interface ViewController : B    @property(nonatomic,strong)id<ViewControllerDelegate> preViewController_A;(protocol B)    @end    A.m:    @interface A ()<preViewController_B>    @end    @implementation A    (implement protocol....)    end    B.m:    @interface B ()<preViewController_A>    @end    @implementation B    (implement protocol....)   @end

For those who might need it:

It's also possible to fix this by moving the importation of ViewController1.h in ViewController2's implementation file (.m) instead of the header file (.h).

Like so:

ViewController1.h

#import ViewController2.h@interface ViewController1 : UIViewController <ViewController2Delegate>@end

ViewController2.h

@protocol ViewController2Delegate;@interface ViewController2@end

ViewController2.m

#import ViewController2.h#import ViewController1.h@implementation ViewController2@end

This will fix the case where the error happens because ViewController1.h is imported in ViewController2.h before the protocol declaration.