iOS 中的单例模式

http://www.jianshu.com/p/e535c7575d72

在iOS中有很多单例对象，比如UIApplication,UIScreen等等，那我们自己可以实现单例吗？答案是肯定的，请往下看

ARC中的单例实现

//.h文件#import <Foundation/Foundation.h>@interface Singleton : NSObject//单例方法+(instancetype)sharedSingleton;@end//.m文件#import "Singleton.h"@implementation Singleton//全局变量static id _instance = nil;//单例方法+(instancetype)sharedSingleton{    return [[self alloc] init];}////alloc会调用allocWithZone:+(instancetype)allocWithZone:(struct _NSZone *)zone{    //只进行一次    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [super allocWithZone:zone];    });    return _instance;}//初始化方法- (instancetype)init{    // 只进行一次    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [super init];    });    return _instance;}//copy在底层 会调用copyWithZone:- (id)copyWithZone:(NSZone *)zone{    return  _instance;}+ (id)copyWithZone:(struct _NSZone *)zone{    return  _instance;}+ (id)mutableCopyWithZone:(struct _NSZone *)zone{    return _instance;}- (id)mutableCopyWithZone:(NSZone *)zone{    return _instance;}@end

• 测试代码及运行结果

#import <Foundation/Foundation.h>#import "Singleton.h"int main(int argc, const char * argv[]) {    @autoreleasepool {        Singleton *s1 = [[Singleton alloc]init];        Singleton *s2 = [Singleton sharedSingleton];        Singleton *s3 = [[Singleton alloc]init];        Singleton *s4 = [Singleton sharedSingleton];        Singleton *s5 = [s4 copy];        NSLog(@"%p %p %p %p %p",s1,s2,s3,s4, s5);    }    return 0;}运行结果2015-08-29 21:07:01.869 Singleton-ARC[42202:784939] 0x100114760 0x100114760 0x100114760 0x100114760 0x100114760

非ARC中的单例实现

• 因为在非ARC中，需要我们自己管理内存，所以实现稍微负责一点

//.m文件#import <Foundation/Foundation.h>//单例方法@interface Singleton : NSObject+ (instancetype)sharedSingleton;@end//.m文件#import "Singleton.h"@implementation Singleton//全局变量static id _instance = nil;//单例方法+(instancetype)sharedSingleton{    //系统的大多数类方法都有做autorelease，所以我们也需要做一下     return [[[self alloc] init] autorelease];}//alloc会调用allocWithZone:+(instancetype)allocWithZone:(struct _NSZone *)zone{    //只进行一次    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [super allocWithZone:zone];    });    return _instance;}//初始化方法-(instancetype)init{    // 只进行一次    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [super init];    });    return _instance;}- (oneway void)release{    //什么都不做 保证单例对象不被销毁}//返回本身 保证只有一个单例对象- (instancetype)retain{    return _instance;} //计数器为1 保证只有一个单例对象- (NSUInteger)retainCount{    return 1;}//copy在底层 会调用copyWithZone:+ (id)copyWithZone:(struct _NSZone *)zone{    return  _instance;}- (id)copyWithZone:(NSZone *)zone{    return _instance;}+ (id)mutableCopyWithZone:(struct _NSZone *)zone{    return _instance;}- (id)mutableCopyWithZone:(NSZone *)zone{    return _instance;}@end

• 因为要保证单例对象在程序运行期间一直存在，所以不需要实现release方法和dealloc方法
• 测试代码及运行结果

#import <Foundation/Foundation.h>#import "Singleton.h"int main(int argc, const char * argv[]) {    @autoreleasepool {        Singleton *s1 = [[Singleton alloc]init];        Singleton *s2 = [Singleton sharedSingleton];        Singleton *s3 = [[Singleton alloc]init];        Singleton *s4 = [Singleton sharedSingleton];        Singleton *s5 = [s4 copy];        NSLog(@"%p %p %p %p %p",s1,s2,s3,s4, s5);        [s1 release];        [s3 release];        [s5 release];    }    return 0;}运行结果2015-08-29 21:01:43.770 Singleton-非ARC[42192:782473] 0x100206880 0x100206880 0x100206880 0x100206880 0x100206880

• 也可以把上面的代码抽出一个宏，以ARC为例

//.h文件   ##表示拼接字符#define SingletonH(methodName) +(instancetype)shared##methodName;//.m文件#define SingletonM(methodName)\\static id _instance = nil;\+(instancetype)sharedSingleton{\    return [[self alloc] init];\}\+(instancetype)allocWithZone:(struct _NSZone *)zone{\    static dispatch_once_t onceToken;\    dispatch_once(&onceToken, ^{\        _instance = [super allocWithZone:zone];\    });\    return _instance;\}\- (instancetype)init{\    static dispatch_once_t onceToken;\    dispatch_once(&onceToken, ^{\        _instance = [super init];\    });\    return _instance;\}\- (id)copyWithZone:(NSZone *)zone{\    return  _instance;\}\+ (id)copyWithZone:(struct _NSZone *)zone{\    return  _instance;\}

文／奋斗的菜鸟（简书作者）
原文链接：http://www.jianshu.com/p/e535c7575d72
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