在目标c中访问类方法.使用的是self还是classname? [英] Accessing class method in objective c. Using self or classname?

问题描述

我正在学习iOS编程，并且对以下有关关键字self的使用的代码感到困惑.

I am learning iOS programming and am confused by the following code regarding the use of keyword self.

根据我的理解， self 就像Java的 this .它指的是当前实例.当我想调用类方法时，通常的方法应该类似于 [PlayingCard validSuits]; ，但是也可以在实例上入侵类方法，对吗?就像 [self validSuits]; (我在课程中，self指的是PlayingCard的实例)

From my understanding, self is like Java's this. It refers to the current instance. When I want to call a class method, the usual way should be like [PlayingCard validSuits]; But it's also OK to invade a class method on an instance, right? Like [self validSuits]; (I am in the class so self refers to an instance of PlayingCard)

但是在下面的代码中，它在某处给出错误，但在其他地方看起来还可以.(由3条注释指出，这在Xcode 5.1中)

But in the following code, it gives error somewhere but looks ok elsewhere.(Pointed out by 3 comments, this is within Xcode 5.1)

我错过了什么吗?

(PS，我认为我遇到的问题与

(P.S. I think I am having the similar problem as here, which no one answered yet. He got the same error even using [PlayingCard validSuits]. )

//  PlayingCard.m

#import "PlayingCard.h"

@implementation PlayingCard
@synthesize suit = _suit; 

+ (NSArray *)validSuits {
    return @[@"♠︎", @"♣︎", @"♥︎", @"♦︎"];
}

+ (NSArray *)rankStrings {
    return @[@"?", @"A", @"2", @"3", @"4",@"5",@"6",@"7",@"8",@"9",@"10",@"J",@"Q",@"K"];

}

+ (NSUInteger)maxRank {
    return [[PlayingCard rankStrings] count] -1; 
          //1. [self rankStrings] works fine.** 
}


//override super class's method
- (NSString *)contents {

    NSArray *rankStrings = [PlayingCard rankStrings];  
           //2. if change rankStrings to self, then error:
           //No visible @interface for 'PlayingCard' declares the selector 'rankStrings'

    return [rankStrings[self.rank] stringByAppendingString:self.suit];
}

- (void) setSuit:(NSString *)suit {
    if ( [[PlayingCard validSuits] containsObject:suit]) {  
        //3.error when changed to [self validsuits] 
        //No visible @interface for 'PlayingCard' declares the selector 'validsuits'**
       _suit = suit;
    }

}

- (NSString *) suit {
    return _suit ? _suit : @"?";
}

@end

头文件:

//  PlayingCard.h

#import "Card.h"

@interface PlayingCard : Card

@property (nonatomic, strong) NSString *suit;
@property (nonatomic) NSUInteger rank;

+ (NSArray *) validSuits;
+ (NSUInteger) maxRank;

@end

推荐答案

如果要从(同一类的)类方法中调用另一个类方法，则可以使用 [self classMethod] .但是，如果您在实例方法中，并且需要调用该类的类方法，则可以使用 [[[self class] classMethod]

If you are calling another class method from inside a class method (of the same class) you can just use [self classMethod]. If however you are in an instance method and you need to call that classes class method you can use [[self class] classMethod]

@Martin R 指出的-如果您将 PlayingCard 子类化，则调用那么，在类方法中的 self 将是该子类，而不是 PlayingCard .

As pointed out by @Martin R - if you subclass PlayingCard, calling self in a class method will then be that subclass and not PlayingCard.

为完整起见，您需要执行以下操作:

For completeness you need to do:

//  PlayingCard.m

#import "PlayingCard.h"

@implementation PlayingCard
@synthesize suit = _suit; 

+ (NSArray *)validSuits {
    return @[@"♠︎", @"♣︎", @"♥︎", @"♦︎"];
}

+ (NSArray *)rankStrings {
    return @[@"?", @"A", @"2", @"3", @"4",@"5",@"6",@"7",@"8",@"9",@"10",@"J",@"Q",@"K"];

}

+ (NSUInteger)maxRank {
    return [[self rankStrings] count] -1; 
}

//override super class's method
- (NSString *)contents {

    NSArray *rankStrings = [[self class] rankStrings];  

    return [rankStrings[self.rank] stringByAppendingString:self.suit];
}

- (void) setSuit:(NSString *)suit {
    if ( [[[self class] validSuits] containsObject:suit]) {  
       _suit = suit;
    }
}

- (NSString *) suit {
    return _suit ? _suit : @"?";
}

@end

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