【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                                          \
{                                                               \
}                                                               \

SingletonDefine

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

@interface SingletonClass : NSObject

DEFINE_SINGLETON_HEADER(SingletonClass)

@end

SingletonClass.h

#import "SingletonClass.h"

@implementation SingletonClass

DEFINE_SINGLETON_IMPLEMENTATION(SingletonClass)                                                        

@end

SingletonClass.m

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;                                                    \
}                                                                   \

SingletonDefine.h

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

@interface SingletonClass : NSObject

DEFINE_SINGLETON_HEADER(SingletonClass)

@end

SingletonClass.h

#import "SingletonClass.h"

@implementation SingletonClass

DEFINE_SINGLETON_IMPLEMENTATION(SingletonClass)

@end

SingletonClass.m