单例模式怎么定义的，可能在不同的语言，不同的书中不完全一样，但是概况开来都应该是:一个类有且仅有一个实例，并且自行实例化向整个系统提供。
因此，首先你可能需要确定你是真的需要一个单例类，还是说仅仅是需要一个方便调用的实例化方法。如果你是真的需要一个单例类，那么你就应该确保这个单例类，有且仅有一个实例（不管怎么操作都只能获取到这个实例）。
最近看到一些github上的单例使用，别人的用法，有一些思考，然后写demo测试了下，就这个简单的单例也有一些坑呢，希望能给他人一些提醒。
Objective-C中的单例
我们通常在OC中实现一个单例方法都是这样：
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static HLTestObject *instance = nil;+ (instancetype)sharedInstance{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [[[self class] alloc] init]; }); return instance;} |
可是这样就可以了么？我做了如下测试：
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HLTestObject *objct1 = [HLTestObject sharedInstance];NSLog(@"%@",objct1);HLTestObject *objc2 = [[HLTestObject alloc] init];NSLog(@"%@",objc2);HLTestObject *objc3 = [HLTestObject new];NSLog(@"%@",objc3); |
看到这个测试，你想到打印结果了么？结果是这样的：
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2016-05-23 12:52:57.095 PractiseProject[3579:81998] <hltestobject: 0x7fcf39515510>2016-05-23 12:52:57.095 PractiseProject[3579:81998] <hltestobject: 0x7fcf395c4b70>2016-05-23 12:52:57.095 PractiseProject[3579:81998] <hltestobject: 0x7fcf395c6890></hltestobject: 0x7fcf395c6890></hltestobject: 0x7fcf395c4b70></hltestobject: 0x7fcf39515510> |
很明显，通过三种方式创建出来的是不同的实例对象，这就违背了单例类有且仅有一个实例的定义。
为了防止别人不小心利用alloc/init方式创建示例，也为了防止别人故意为之，我们要保证不管用什么方式创建都只能是同一个实例对象，这就得重写另一个方法，实现如下：
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+ (instancetype)allocWithZone:(struct _NSZone *)zone{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [super allocWithZone:zone]; }); return instance;} |
再次用上面的测试代码，结果是这样的：
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2016-05-23 12:57:37.396 PractiseProject[3618:83975] <hltestobject: 0x7f88b9488ac0>2016-05-23 12:57:37.396 PractiseProject[3618:83975] <hltestobject: 0x7f88b9488ac0>2016-05-23 12:57:37.396 PractiseProject[3618:83975] <hltestobject: 0x7f88b9488ac0></hltestobject: 0x7f88b9488ac0></hltestobject: 0x7f88b9488ac0></hltestobject: 0x7f88b9488ac0> |
好像用不同的构造方法，获取的都是同一个对象，你以为这样就完了？还早着呢！
一般我们的类里肯定都会有一些属性，然后我就添加了两个property：
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@property (assign, nonatomic) int height;@property (strong, nonatomic) NSObject *object;@property (strong, nonatomic) NSMutableArray *arrayM; |
而一些对象类的初始化，或者基础类型的默认值设置都是在init方法里，就像这样：
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- (instancetype)init{ self = [super init]; if (self) { _height = 10; _object = [[NSObject alloc] init]; _arrayM = [[NSMutableArray alloc] init]; } return self;} |
我重写了HLTestObject类的description方法：
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- (NSString *)description{ NSString *result = @""; result = [result stringByAppendingFormat:@"<%@: %p>",[self class], self]; result = [result stringByAppendingFormat:@" height = %d,",self.height]; result = [result stringByAppendingFormat:@" arrayM = %p,",self.arrayM]; result = [result stringByAppendingFormat:@" object = %p,",self.object]; return result;} |
还是用上面的测试代码，测试结果是这样的：
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2016-05-23 13:14:43.684 PractiseProject[3781:92758] <hltestobject: 0x7f8a5b458450> height = 20, arrayM = 0x7f8a5b422940, object = 0x7f8a5b4544e0,2016-05-23 13:14:43.684 PractiseProject[3781:92758] <hltestobject: 0x7f8a5b458450> height = 10, arrayM = 0x7f8a5b4552e0, object = 0x7f8a5b45a710,2016-05-23 13:14:43.684 PractiseProject[3781:92758] <hltestobject: 0x7f8a5b458450> height = 10, arrayM = 0x7f8a5b459770, object = 0x7f8a5b4544e0,</hltestobject: 0x7f8a5b458450></hltestobject: 0x7f8a5b458450></hltestobject: 0x7f8a5b458450> |
可以看到，尽管使用的是同一个示例，可是他们的property值却不一样。
因为尽管没有为示例重新分配内存空间，但是因为又执行了init方法，会导致property被重新初始化。
所以我们需要修改单例的实现。
第一种：
可以将property的初始化或者默认值设置放到dispatch_once 的block内部：
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static HLTestObject *instance = nil;+ (instancetype)sharedInstance{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [[[self class] alloc] init]; instance.height = 10; instance.object = [[NSObject alloc] init]; instance.arrayM = [[NSMutableArray alloc] init]; }); return instance;}+ (instancetype)allocWithZone:(struct _NSZone *)zone{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [super allocWithZone:zone]; }); return instance;}- (NSString *)description{ NSString *result = @""; result = [result stringByAppendingFormat:@"<%@: %p>",[self class], self]; result = [result stringByAppendingFormat:@" height = %d,",self.height]; result = [result stringByAppendingFormat:@" arrayM = %p,",self.arrayM]; result = [result stringByAppendingFormat:@" object = %p,",self.object]; return result;} |
来看看测试结果：
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2016-05-23 13:29:14.856 PractiseProject[3909:99058] <hltestobject: 0x7fa72270c570> height = 20, arrayM = 0x7fa722716c10, object = 0x7fa7227140e0,2016-05-23 13:29:14.856 PractiseProject[3909:99058] <hltestobject: 0x7fa72270c570> height = 20, arrayM = 0x7fa722716c10, object = 0x7fa7227140e0,2016-05-23 13:29:14.856 PractiseProject[3909:99058] <hltestobject: 0x7fa72270c570> height = 20, arrayM = 0x7fa722716c10, object = 0x7fa7227140e0,</hltestobject: 0x7fa72270c570></hltestobject: 0x7fa72270c570></hltestobject: 0x7fa72270c570> |
第二种：
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static HLTestObject *instance = nil;+ (instancetype)sharedInstance{ return [[self alloc] init];}- (instancetype)init{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [super init]; instance.height = 10; instance.object = [[NSObject alloc] init]; instance.arrayM = [[NSMutableArray alloc] init]; }); return instance;}+ (instancetype)allocWithZone:(struct _NSZone *)zone{ static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ instance = [super allocWithZone:zone]; }); return instance;}- (NSString *)description{ NSString *result = @""; result = [result stringByAppendingFormat:@"<%@: %p>",[self class], self]; result = [result stringByAppendingFormat:@" height = %d,",self.height]; result = [result stringByAppendingFormat:@" arrayM = %p,",self.arrayM]; result = [result stringByAppendingFormat:@" object = %p,",self.object]; return result;} |
测试结果：
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2016-05-23 13:31:44.824 PractiseProject[3939:100662] <hltestobject: 0x7fa9da711a70> height = 20, arrayM = 0x7fa9da707ca0, object = 0x7fa9da70a940,2016-05-23 13:31:44.825 PractiseProject[3939:100662] <hltestobject: 0x7fa9da711a70> height = 20, arrayM = 0x7fa9da707ca0, object = 0x7fa9da70a940,2016-05-23 13:31:44.825 PractiseProject[3939:100662] <hltestobject: 0x7fa9da711a70> height = 20, arrayM = 0x7fa9da707ca0, object = 0x7fa9da70a940,</hltestobject: 0x7fa9da711a70></hltestobject: 0x7fa9da711a70></hltestobject: 0x7fa9da711a70> |
注意：
以上代码均是使用ARC的方式管理内存，如果你还在使用MRC（这也太不与时俱进了）。那你还需要重写 retain 和release方法，防止示例引用计数的改变。
Swift中的单例
利用Swift中的一些特性，Swift中的单例可以超级简单，like this:
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class HLTestObject: NSObject { static let sharedInstance = HLTestObject();} |
可是这样就完了么？同样写一段测试代码：
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let object1 = HLTestObject.sharedInstance;print(object1);let object2 = HLTestObject();print(object2); |
打印结果却是这样的：
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<swiftproject.hltestobject: 0x7f90ebc74e50><swiftproject.hltestobject: 0x7f90ebe5cf40></swiftproject.hltestobject: 0x7f90ebe5cf40></swiftproject.hltestobject: 0x7f90ebc74e50> |
所以，我们必须禁用到构造方法：
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class HLTestObject: NSObject { static let sharedInstance = HLTestObject(); private override init() { }} |
如果有实例属性需要初始化，就可以这样：
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class HLTestObject: NSObject { var height = 10; var arrayM: NSMutableArray var object: NSObject static let sharedInstance = HLTestObject(); private override init() { object = NSObject() arrayM = NSMutableArray() super.init() }} |
当然，由于Swift的特性，在Swift中创建单例的方式也不止一种，需要注意的是要确保该类有且仅有一个实例就OK了。
原文：http://www.cnblogs.com/graveliang/p/5719042.html