如何创建接口的实例符合类型 id<..> 的协议

Question

I am leaning objective-c, and I am trying to learn the @protocol. I developed the below example. But what is not clear to me is The code in main_1 and main_2. For main_1, I thought it might be more or less like java for example, *sampleClass is instance of SampleClass, hence, it should not have access to ProcessComplete method. However, in Objective-c, the code works….

For main_2, I am trying to instantiate object of SampleClass that contains SampleClass as implementor of the protocol. Unlike the code in main_1, I supposed or thought that in the code in main_1 is provide instance of SampleClass but NOT as implementor of the protocol.

1-Please correct me if I am not right. 2-please let me know why in code main_2, I received the following errors:

Implicit conversion of an Objective-C pointer to '__strong id<PrintClientProtocol> *' is disallowed with ARC
Pointer to non-const type 'id<PrintClientProtocol>' with no explicit ownership
Incompatible pointer types initializing '__strong id<PrintClientProtocol> *' with an expression of type 'SampleClass *'

main_1

    SampleClass *sampleClass = [[SampleClass alloc] init];
    [sampleClass startAction];
    [sampleClass processCompleted];

main_2

    id<PrintClientProtocol> *sampleClass2 = [[SampleClass alloc] init];
    [sampleClass2 startAction];
    [sampleClass2 processCompleted];

printClass.h

#ifndef PrintClass_h
#define PrintClass_h
#endif /* PrintClass_h */

//The class/interface that to execute the protocols in the client contract. execution is to be done according
//to the DELEGATE Class/Interface
@interface PrintClass :NSObject {
   id implementor;//instance of the DELEGATE/implementor and executor class/interface
    //change id to SampleClass for testing
}

- (void) printDetails;
- (void) setDelegate: (id) implementor;

@end

printClass.m

#import <Foundation/Foundation.h>
#import "PrintClass.h"
#import "SampleClass.h"

@implementation PrintClass

- (void) printDetails {
    NSLog(@"Printing Details");
    [implementor processCompleted];
}

- (void) setDelegate:(id)newDelegate {
    implementor = newDelegate;
}

@end

SampleClass.h

#ifndef SampleClass_h
#define SampleClass_h


#endif /* SampleClass_h */
#import "PrintClientProtocol.h"
//the class/interface that provide implementions and execution of it, which is the "DELEGATE"
@interface SampleClass : NSObject<PrintClientProtocol>
- (void)startAction;
@end

SampleClass.m

#import <Foundation/Foundation.h>
#import "SampleClass.h"
#import "PrintClass.h"

@implementation SampleClass

-(void)startAction {
    PrintClass *printClass = [[PrintClass alloc] init];
    [printClass setDelegate: self];
    [printClass printDetails];
}

-(void) processCompleted {
    NSLog(@"PROCESS_COMPLETED.....");
}

@end

PrintClientProtocol :

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

//the protocol that demands or needs implementor and executor "Client/Delegatee Protocol"
@protocol PrintClientProtocol <NSObject>
- (void) processCompleted;
@end

NS_ASSUME_NONNULL_END

Answer 1

The declaration of the class SampleClass says that it conforms to the protocol PrintClientProtocol . That means it automatically inherits all the members declared in the protocol PrintClientProtocol , including -processCompleted . This is the same as what you would expect in Java.

In main_2 , the problem is that the type id<PrintClientProtocol> is already a pointer-to-object type, akin to SampleClass * . By writing id<PrintClientProtocol> * , you are writing a pointer-to-pointer-to-object type, which is not compatible with the right side, which has a pointer-to-object type.