ios 单例模式(懒汉式)

1. 单例模式的作用

• 可以保证在程序运行过程，一个类只有一个实例，而且该实例易于供外界访问
• 从而方便地控制了实例个数，并节约系统资源

2. 单例模式的使用场合

• 在整个应用程序中，共享一份资源（这份资源只需要创建初始化1次）

3. ARC中，单例模式的实现

在.m中保留一个全局的static的实例

1.ARC

@interface HMDataTool : NSObject

+ (instancetype)sharedDataTool;

@end

@implementation HMDataTool

// 用来保存唯一的单例对象

static id _instace;

//重载了allocWithZone:，保持了从sharedInstance方法返回的单例对象，使用者哪怕使用alloc时也会返回唯一的 实例（alloc方法中会先调用allocWithZone:创建对象）。而retain等内存管理的函数也被重载了，这样做让我们有了把 Singleton类变得“严格”了。

+ (id)allocWithZone:(struct _NSZone *)zone

{

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

_instace = [super allocWithZone:zone];

});

return _instace;

}

+ (instancetype)sharedDataTool

{

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

_instace = [[self alloc] init];

});

return _instace;

}

- (id)copyWithZone:(NSZone *)zone

{

return _instace;

}

@end

简化单例模式

定义一个.h文件，只要包含该头文件，就可以用宏直接定义单例内容

// .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 ;} \
    - (id)autorelease { return self;}

#endif

调用时：

@interface HMMusicTool : NSObject
HMSingletonH(MusicTool)
@end
@implementation HMMusicTool
HMSingletonM(MusicTool)
@end

2.非ARC

@interface HMDataTool : NSObject

+ (instancetype)sharedDataTool;

@end

@implementation HMDataTool

// 用来保存唯一的单例对象

static id _instace;

+ (id)allocWithZone:(struct _NSZone *)zone

{

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

_instace = [super allocWithZone:zone];

});

return _instace;

}

+ (instancetype)sharedDataTool

{

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;

}

@end