ARC环境

懒汉式

.h文件

@interface SingletonTool : NSObject

+ (instancetype)shareSingletonTool;

@end

.m文件

#import "SingletonTool.h"

@implementation SingletonTool
//必须使用static 否则会被其他类使用extern引用修改
static id _instance;

/**
 对外共享一个类方法，方便其他地方使用
 */
+ (instancetype)shareSingletonTool{
    if (_instance == nil) {//防止频繁加锁操作
        @synchronized (self) {//加锁是为了避免多线程同时使用
            if (_instance == nil) {
                _instance = [[self alloc] init];
            }
        }
    }
    return _instance;
}

/**
 重写此方法，避免开辟多个内存空间
 */
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (_instance == nil) {
        @synchronized (self) {
            if (_instance == nil) {
                _instance = [super allocWithZone:zone];
            }
        }
    }
    return _instance;
}

/**
 重写copy方法，避免copy时产生新的对象
 */
- (id)copyWithZone:(NSZone *)zone{
    //直接return是因为copy时，_instance已经有值了
    return _instance;
}
@end

饿汉式

饿汉式是让程序加载所有资源时就保证这个单利存在内存之中，所以此时不必考虑多线程的问题

@implementation SingletonTool
static id _instance;

//此方法是类被加载的第一次时调用，并且只会调用一次
+ (void)load{
    _instance = [[self alloc] init];
}

+ (instancetype)shareSingletonTool{
    return _instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (_instance == nil) {
        _instance = [super allocWithZone:zone];
    }
    return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
    return _instance;
}

@end

使用GCD

+ (instancetype)shareSingletonTool{
    static dispatch_once_t onceToken;
    //此方法的代码块的任务在程序运行到结束的过程中只运行一次，并且时线程安全的
    dispatch_once(&onceToken, ^{
        _instance = [[self alloc] init];
    });
    return _instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}

MRC环境

MRC环境多了处理release retain retainCount autorelease这四个方法

//release什么都不做，避免引用计数为0，对象被销毁
- (oneway void)release {
 }

//retain直接返回self
- (id)retain { 
    return self;
 }

//retainCount始终保持引用计数为1
- (NSUInteger)retainCount { 
    return 1;
}

- (id)autorelease { 
    return self;
}

单利宏

创建一个单利头文件，将单利实现代码封住成一个宏即可

// .h文件
#define HMSingletonH(name) + (instancetype)shared##name;

// .m文件
#if __has_feature(objc_arc)

#define HMSingletonM(name) \
static id _instace; \
\
+ (id)allocWithZone:(struct _NSZone *)zone { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instace = [super allocWithZone:zone]; \
    }); \
    return _instace; \
} \
\
+ (instancetype)shared##name { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
        _instace = [[self alloc] init]; \
    }); \
    return _instace; \
} \
\
- (id)copyWithZone:(NSZone *)zone { \
    return _instace; \
}

#else

#define HMSingletonM(name) \
static id _instace; \
\
+ (id)allocWithZone:(struct _NSZone *)zone { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
       _instace = [super allocWithZone:zone]; \
    }); \
    return _instace; \
} \
\
+ (instancetype)shared##name { \
    static dispatch_once_t onceToken; \
    dispatch_once(&onceToken, ^{ \
       _instace = [[self alloc] init]; \
    }); \
    return _instace; \
} \
\
- (id)copyWithZone:(NSZone *)zone { \
    return _instace; \
} \
\
- (oneway void)release { } \
- (id)retain { return self; } \
- (NSUInteger)retainCount { return 1;} \
- (id)autorelease { return self;}

#endif