Single-instance mode in iOS: MRC and ARC+GCD implementations

The implementation of the Singleton pattern in iOS is generally divided into two types: MRC and ARC+GCD

1.MRC (non-ARC)

Non-Arc single-instance implementations:

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<span style="font-family: 仿宋; font-size: 15px;">#import <Foundation/Foundation.h> @interface NoARCSingleton:NSObject <span>//这个属性在后面调试有用处，而且也不要苦恼为什么是retain？不应该是copy么？请继续看下去，后面自然明白了<br></span>@property (nonatomic,retain) NSString *tempProperty; +(NoARCSingleton *)sharedInstance; @end @implementation NoARCSingleton static NoARCSingleton  *sharedInstance  = nil; //获取一个单例对象 + (BVNonARCSingleton *)sharedInstance {<br>//--------------------------------------------------<br>  if (sharedInstance == nil) {         sharedInstance = [[super allocWithZone:NULL] init];     }    return sharedInstance;<br>//-------------------------------------------------- } //当第一次使用这个单例的时候，会调用这个init方法。 -(id)init {    self = [super init];   if(self){    //通常在这里做一些相关的初始化任务   return self;     }      } //这个delloc方法永远都不会被调用，因为在程序的生命周期内，该单例都必须存在，该方法可以不用实现 -(void)delloc {   [super dealloc]; }//通过返回当前的单例对象的方式来防止实例化新的对象 + (id)allocWithZone:(NSZone*)zone {<br>　　//线程相关的操作安全有</span>sharedInstance处理

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<span style="font-family: 仿宋; font-size: 15px;">return [[self sharedInstance] retain]; } <br>Similarly, you do not want to generate multiple copies of the singleton, you must override <br>-(ID) Copywithzone: (Nszone *) zone {return self;} <br>// None of the operations in this method need a reference count (retain counter) <br>-(ID) retain {return self;} <br>//Replace the reference counter, which will never release this singleton <br>-(Nsuinteger) Retaincount {return nsuintegermax;} <br>// The method is empty-do not want the user to release the object. <br>-(OneWay void) release{}<br>//In addition to returning the single exception, do nothing. <br>-(ID) autorelease {return self;} <br> @end <br>//@synchronized's role is to create a mutex, Ensure that no other threads are modifying the Self object at this time. This is a lock token for objective-c, which prevents the self object from being accessed by other threads at the same time, and acts as a thread protection. Generally used in the case of common variables, such as singleton mode or the static variable of an operation class. <br>//Non-ARC implementation of the Singleton method is thread insecure, if there are multiple threads calling the <span>sharedinstance method at the same time to get an instance, <span> Sharedinstance takes 1-2s of time, the <span>nonarcsingleton init method may be called multiple times, that is, multiple threads may not be a singleton, and the way to solve this thread is unsafe by using <a href= "https://developer.apple.com/library/mac/#documentation/cocoa/conceptual/multithreading/threadsafety /threadsafety.html "target=" _blank "><span class=" Edui-filter-underline "> @synchronized &LT;/span></a><span> to create a mutex. <br>//<span> of course, this method does not guarantee that the invocation of all methods in this singleton is thread-safe </span><br></span></span></span ></span>//so the code between the lines above should be replaced with the following code @synchronized (self)</span> {      if(sharedInstance == nil)     {        sharedInstance = [[super allocWithZone:NULL] init]; }<br>#param mark <span style="font-family: 仿宋; font-size: 15px;">-提醒：在iOS中，一般不建议使用非ARC来实现单例模式。更好的方法是使用ARC+GCD来实现。</span>

　　How arc is implemented

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<span style="font-family: 仿宋; font-size: 15px;">#import "ARCSingleton.h" @interface ARCSingleton : NSObject<br>//调试之用和上面的代码作用类似的，这里为什么用weak呢？请继续看下去~<br><span>@property  ( nonatomic, weak) NSString  *tempProperty;</span> + (ARCSingleton *)sharedInstance; @end @implementation ARCSingleton + (ARCSingleton *) sharedInstance {<br><span style="line-height: 1.5;">      staticARCSingleton *sharedInstance = nil ;</span></span>

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<span style="font-family: 仿宋; font-size: 15px;">     staticdispatch_once_t onceToken = 0;  // 锁      dispatch_once (&onceToken, ^ {     // 最多调用一次        sharedInstance = [[self alloc] init];     });     returnsharedInstance; } //当第一次使用这个单例时，会调用这个init方法 -(id)init {     self = [super init];     if(self){        //初始化单例     }       return self;   } @end <br>//<span>在上面的代码中，调用</span><span class="edui-filter-underline"><a href="http://developer.apple.com/library/ios/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html" target="_blank">Grand Central Dispatch <br>(GCD)</a></span><span>中的</span><span class="edui-filter-underline"><a href="https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_once" target="_blank">dispatch_once</a></span><span>方法就可以确保ARCSingleton只被实例化一次。并且该方法是线程安全的，我们不用担心在不同的线程中，会获得不同的实例。（当然，该方法同样不能保证该单例中所有方法的调用都是线程安全的）。</span><br> </span>

　　Of course, in the arc, without GCD can also be thread-safe, with the previous non-arc code using @synchronized, the following code:

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<span style="font-family: 仿宋; font-size: 15px;"> // 不使用GCD，通过@synchronized @synchronized (self) {    if(sharedInstance == nil)     {         sharedInstance = [[self alloc] init];     } } </span>

　　To simplify the use of ARC+GCD to create a singleton, you can use the following macro

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#define DEFINE_SHARED_INSTANCE_USING_BLOCK(block) \static dispatch_once_t onceToken = 0; \  static id sharedInstance = nil; \dispatch_once(&onceToken, ^{ \sharedInstance = block(); \}); \return sharedInstance; \<br><span style="font-family: 仿宋; font-size: 15px;">//如果对于macro有问题的话强烈建议去脑补下猫神的文章http://onevcat.com/2014/01/black-magic-in-macro/<br>//另外猫神的很多文章都是很深入的建议有时间多去细细品味</span>

　After the macro has been written, then easy!

Instantiation method Implementation:

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+ (BVARCSingleton *) sharedInstance {    DEFINE_SHARED_INSTANCE_USING_BLOCK(^{        return [[self alloc] init];     });}<br>//Done

　　Single use: The use of a single example is very simple, anywhere in the code, such as the following can be used:

to add a header file in BVAPPDELEGATE.M:
#import "NonARCSingleton.h"
#import "ARCSingleton.h"

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- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {     　　[NonARCSingleton sharedInstance].tempProperty = @"非ARC单例的实现";     　　NSLog(@"%@", [BVNonARCSingleton sharedInstance].tempProperty);    　　[ARCSingleton sharedInstance].tempProperty = @"ARC单例的实现";      　　NSLog(@"%@", [ARCSingleton sharedInstance].tempProperty);      　　return YES; }

　　　Additional Dry Goods

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