【iOS】单例模式

单例模式在软件开发中经常用到，在iOS系统framework也很多地方用到单例模式，例如 [NSUserDefaults standardUserDefaults], [NSBundle mainBundle]等，下面演示一下iOS如何实现单例模式

MRC模式

SingletonClass.h

#import <Foundation/Foundation.h>

@interface SingletonClass : NSObject

+ (SingletonClass *)sharedInstance;

@end

SingletonClass.m

#import "SingletonClass.h"

@implementation SingletonClass

static SingletonClass *_singletonInstance = nil;                     
+ (instancetype)sharedInstance{                                 
    @synchronized(self){                                        
        if (!_singletonInstance) {                              
            _singletonInstance = [[self alloc] init];           
        }                                                       
    }                                                           
    return _singletonInstance;                                  
}                                                               

+ (id)allocWithZone:(NSZone *)zone{                             
    @synchronized(self){                                        
        if (!_singletonInstance) {                              
            _singletonInstance = [super allocWithZone:zone];    
        }                                                       
        return _singletonInstance;                              
    }                                                           
    return nil;                                                 
}                                                               

- (instancetype)copyWithZone:(NSZone *)zone;                    
{                                                               
    return self;                                                
}                                                               

- (instancetype)retain                                          
{                                                               
    return self;                                                
}                                                               

- (unsigned)retainCount                                         
{                                                               
    return UINT_MAX;                                            
}                                                               

- (instancetype)autorelease                                     
{                                                               
    return self;                                                
}                                                               

- (oneway void)release                                          
{                                                               
}                                                               

@end

懒人技巧：把单例的定义与实现定义成宏

//单例头宏
#define DEFINE_SINGLETON_HEADER(className)  
    + (className *)sharedInstance;          

//单例实现宏
#define DEFINE_SINGLETON_IMPLEMENTATION(className)              
static className *_singletonInstance = nil;                     
+ (instancetype)sharedInstance{                                 
    @synchronized(self){                                        
        if (!_singletonInstance) {                              
            _singletonInstance = [[self alloc] init];           
        }                                                       
    }                                                           
    return _singletonInstance;                                  
}                                                               
                                                                
+ (id)allocWithZone:(NSZone *)zone{                             
    @synchronized(self){                                        
        if (!_singletonInstance) {                              
            _singletonInstance = [super allocWithZone:zone];    
        }                                                       
        return _singletonInstance;                              
    }                                                           
    return nil;                                                 
}                                                               
                                                                
- (instancetype)copyWithZone:(NSZone *)zone;                    
{                                                               
    return self;                                                
}                                                               
                                                                
- (instancetype)retain                                          
{                                                               
    return self;                                                
}                                                               
                                                                
- (unsigned)retainCount                                         
{                                                               
    return UINT_MAX;                                            
}                                                               
                                                                
- (instancetype)autorelease                                     
{                                                               
    return self;                                                
}                                                               
                                                                
- (oneway void)release                                          
{                                                               
}                                                               

#import <Foundation/Foundation.h>
#import "SingletonDefine.h"

@interface SingletonClass : NSObject

DEFINE_SINGLETON_HEADER(SingletonClass)

@end

#import "SingletonClass.h"

@implementation SingletonClass

DEFINE_SINGLETON_IMPLEMENTATION(SingletonClass)                                                        

@end

ARC模式

SingletonClass.h

#import <Foundation/Foundation.h>

@interface SingletonClass : NSObject

+ (instancetype)sharedInstance;

//禁用alloc，init，new 创建对象，否则编译会报错
+(instancetype) alloc __attribute__((unavailable("alloc not available, call sharedInstance instead")));
-(instancetype) init __attribute__((unavailable("init not available, call sharedInstance instead")));
+(instancetype) new __attribute__((unavailable("new not available, call sharedInstance instead")));

@end

SingletonClass.m

#import "SingletonClass.h"

@implementation SingletonClass

+(instancetype) sharedInstance {                            
    static dispatch_once_t predicate;                       
    static SingletonClass *instance = nil;
    dispatch_once(&predicate, ^{                            
        instance = [[super alloc] initUniqueInstance];      
    });                                                     
    return instance;                                        
}                                                           

-(instancetype) initUniqueInstance {                        
    return [super init];                                    
}                                                           

- (instancetype)copyWithZone:(NSZone *)zone                 
{                                                           
    return self;                                            
}

@end

懒人模式 

//单例头宏(ARC)
#define DEFINE_SINGLETON_HEADER(className)                  
+ (instancetype)sharedInstance;                             


//单例实现宏(ARC)
#define DEFINE_SINGLETON_IMPLEMENTATION(className)                  
+(instancetype) sharedInstance {                                    
    static dispatch_once_t predicate;                               
    static className *_singletonInstance = nil;                     
    dispatch_once(&predicate, ^{                                    
        _singletonInstance = [[super alloc] init];                  
    });                                                             
    return _singletonInstance;                                      
}                                                                   
                                                                    
- (instancetype)copyWithZone:(NSZone *)zone                         
{                                                                   
    return self;                                                    
}                                                                   

#import <Foundation/Foundation.h>
#import "SingletonDefine.h"

@interface SingletonClass : NSObject

DEFINE_SINGLETON_HEADER(SingletonClass)

@end

#import "SingletonClass.h"

@implementation SingletonClass

DEFINE_SINGLETON_IMPLEMENTATION(SingletonClass)

@end