conformsToProtocol不起作用

Question

Hi I have a problem with one protocol

  #import <Foundation/Foundation.h>

  @protocol StandardQuality <NSObject>

  - (NSString *)color;
  - (NSString *)fabric;
  -( NSString *)store;
  - ( NSString *)price;

  @end

I implement this protocol in a class

  @interface Hat : NSObject <StandardQuality> 

  @property NSString *color;
  @property NSString *fabric;
  @property NSString *store;

  @end

It is happens what I expected . I received a warning message

method "price" in protocol not implemented.

Anyway.I tested in main.h

    [myHat setShop:@"Unirea"];
    NSLog(@" The  hat is selling at %@", myHat.store);

    if([myHat conformsToProtocol:@protocol(StandardQuality)]== YES)
    {
        NSLog(@"My hat respects the protocol");
    }
    else {
         NSLog(@"My hat doesn't respect the protocol");

    }


    if([myHat respondsToSelector:@selector(price)]== YES)
    {
        NSLog(@"My hat has price");
    }
    else {
        NSLog(@"My hat doesn't have price");

    }

Strangely, I recieved

My hat respects the protocol

My hat doesn't have price

My second question is : It is a way to block the compiler to compile your program if a protocol is not respected , not just to receive a warning message?

Answer 1

Strangely, I recieved

My hat respects the protocol

My hat doesn't have price

What's strange? You get back exactly what you did. Your class conforms to the protocol, but doesn't implement the price method.

It is a way to block the compiler to compile your program if a protocol is not respected , not just to receive a warning message?

I assume you wanted to ask if there is a way. Well, you can turn on the -Werr compiler switch which makes the compiler bail out if a warning is encountered, then possibly turn off warnings only for unimplemented methods in a protocol (google for the appropriate compiler flag).

Answer 2

You can convert the warning for an incomplete protocol into an error by adding -Werror=incomplete-implementation to the "Other C Flags" in the "Language" section of your Xcode projects "Build Settings" (or you can pass that option to the compiler if invoking from the command line).

If you'd like to check at runtime whether a class actually implements a protocol, as opposed to simply declaring that it does, see addendum to this answer .

Answer 3

From the API docs :

This method determines conformance solely on the basis of the formal declarations in header files, as illustrated above. It doesn't check to see whether the methods declared in the protocol are actually implemented—that's the programmer's responsibility.

Answer 4

From the documentation your class conforms to the protocol because you are declaring it as such in it's @interface

@interface Hat : NSObject <StandardQuality>

However, you haven't implemented the price method.

There is the concept of @required and @optional protocol methods. If you absolutely require an class to implement all or some of the methods of a protocol you should specify it like this:

@protocol StandardQuality <NSObject>

@required
- (NSString *)color;
- (NSString *)fabric;
- (NSString *)store;

@optional
- (NSString *)price;

@end

In which case you should check that the object implementing a protocol actually implements the optional methods without calling them.

I think is more what you are looking for.