单例模式在iOS中的应用——三种创建单例方法对比

单例模式优点

1. 单例模式在内存中只有一个实例，减少了内存开支。特别是一个对象需要频繁的创建、销毁时，而创建与销毁的性能有无法优化，单例模式的优势就非常明显。
2. 单例模式只生成一个实例，减少了系统性能开销，当一个对象的产生需要比较多的资源时，如读取配置、产生其他依赖对象时，则可以通过在应用启动时直接产生一个单例对象，然后永久驻留内存的方式来解决。
3. 单例模式可以避免对资源的多重占用。
4. 单例模式可以在系统设置全局的访问点，优化和共享资源访问。

单例模式缺点

1. 单例模式一般没有接口，扩展很困难，除了修改代码基本上没有第二种途径实现。
2. 单例模式对测试是不利的。在并行开发环境中，如果单例模式没有完成，是不能进行测试的。
3. 单例模式与单一职责原则有冲突。

单例模式在iOS中的使用

单例模式在iOS开发中的使用还是蛮多的，许多Foundation、Cocoa和UIKit中的类都实现了单例模式，比如应用程序本身UIApplication、文件操作类NSFileManager、消息中心NSNotificitonCenter等系统都已经给我们实现单例，我们只需要使用就好了。在iOS中使用单例模式要使用类方法，通过类方法返回该类的唯一对象。

我知道的在iOS开发中实现单例模式主要有以下三种方式：

第一种

该方法是苹果的官方文档中写的一种方式，通过覆盖NSObject的部分方法实现，使该类无法alloc、retain、release。这是最麻烦的一种方法，也是最不好的一种方法

static Singleton *instance = nil;

+ (Singleton *)sharedInstance
{
    if (instance == nil) {
        instance = [[super allocWithZone:NULL] init];
    }
    return instance;
}

+ (id)allocWithZone:(NSZone *)zone
{
    return [[self sharedInstance] retain];
}

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

- (id)retain
{
    return self;
}

- (NSUInteger)retainCount
{
    return NSUIntegerMax;  //denotes an object that cannot be released
}

- (void)release
{
    //do nothing
}

- (id)autorelease
{
    return self;
}

可以看到这种方式，使用静态成员维持了一个永久存在的对象，而且覆盖了alloc方法（alloc方法会调用allocWithZone:方法），并且也覆盖了所有与引用技术有关的方法，这都使这个对象不会被销毁。这样看上去基本实现了我们需要的，但是写起来麻烦不说，还有很大的一个问题，那就是多线程问题，如果是在多线程中那么该种方法就不能保证只产生一个对象了。所以这种方式只是介绍一下，并不推荐使用。

第二种

第二种跟第一种差不多，也是通过覆盖NSObject的方法实现的，但是它在第一种的基础上增加了多线程的处理，所以即使在多线程下，该种方法创建的对象也是唯一的。这种方法已经有大牛为我们写好了，全都都是通过C的宏定义#define出来了。现给出该头文件：

//
//  SynthesizeSingleton.h
//
// Modified by Karl Stenerud starting 16/04/2010.
// - Moved the swizzle code to allocWithZone so that non-default init methods may be
//   used to initialize the singleton.
// - Added "lesser" singleton which allows other instances besides sharedInstance to be created.
// - Added guard ifndef so that this file can be used in multiple library distributions.
// - Made singleton variable name class-specific so that it can be used on multiple classes
//   within the same compilation module.
//
//  Modified by CJ Hanson on 26/02/2010.
//  This version of Matt's code uses method_setImplementaiton() to dynamically
//  replace the +sharedInstance method with one that does not use @synchronized
//
//  Based on code by Matt Gallagher from CocoaWithLove
//
//  Created by Matt Gallagher on 20/10/08.
//  Copyright 2009 Matt Gallagher. All rights reserved.
//
//  Permission is given to use this source code file without charge in any
//  project, commercial or otherwise, entirely at your risk, with the condition
//  that any redistribution (in part or whole) of source code must retain
//  this copyright and permission notice. Attribution in compiled projects is
//  appreciated but not required.
//

#ifndef SYNTHESIZE_SINGLETON_FOR_CLASS

#import <objc/runtime.h>


#pragma mark -
#pragma mark Singleton

/* Synthesize Singleton For Class
 *
 * Creates a singleton interface for the specified class with the following methods:
 *
 * + (MyClass*) sharedInstance;
 * + (void) purgeSharedInstance;
 *
 * Calling sharedInstance will instantiate the class and swizzle some methods to ensure
 * that only a single instance ever exists.
 * Calling purgeSharedInstance will destroy the shared instance and return the swizzled
 * methods to their former selves.
 *
 *
 * Usage:
 *
 * MyClass.h:
 * ========================================
 *      #import "SynthesizeSingleton.h"
 *
 *      @interface MyClass: SomeSuperclass
 *      {
 *              ...
 *      }
 *      SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(MyClass);
 *
 *      @end
 * ========================================
 *
 *
 *      MyClass.m:
 * ========================================
 *      #import "MyClass.h"
 *
 *      // This line is optional. Use it if you've enabled GCC_WARN_UNDECLARED_SELECTOR
 *      SYNTHESIZE_SINGLETON_FOR_CLASS_PROTOTYPE(MyClass);
 *
 *      @implementation MyClass
 *
 *      SYNTHESIZE_SINGLETON_FOR_CLASS(MyClass);
 *
 *      ...
 *
 *      @end
 * ========================================
 *
 *
 * Note: Calling alloc manually will also initialize the singleton, so you
 * can call a more complex init routine to initialize the singleton like so:
 *
 * [[MyClass alloc] initWithParam:firstParam secondParam:secondParam];
 *
 * Just be sure to make such a call BEFORE you call "sharedInstance" in
 * your program.
 */

#define SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(SS_CLASSNAME)     \
\
+ (SS_CLASSNAME*) sharedInstance;       \
+ (void) purgeSharedInstance;


#define SYNTHESIZE_SINGLETON_FOR_CLASS_PROTOTYPE(SS_CLASSNAME) \
@interface SS_CLASSNAME (SynthesizeSingletonPrivate)    \
- (NSUInteger)retainCountDoNothing;     \
- (NSUInteger)retainCountDoSomething;   \
- (void)releaseDoNothing;       \
- (void)releaseDoSomething;     \
- (id)autoreleaseDoNothing;     \
- (id)autoreleaseDoSomething; \
@end

#define SYNTHESIZE_SINGLETON_FOR_CLASS(SS_CLASSNAME)    \
\
static volatile SS_CLASSNAME* _##SS_CLASSNAME##_sharedInstance = nil;   \
\
+ (volatile SS_CLASSNAME*) sharedInstanceNoSynch        \
{       \
return (volatile SS_CLASSNAME*) _##SS_CLASSNAME##_sharedInstance;       \
}       \
\
+ (volatile SS_CLASSNAME*) sharedInstanceSynch  \
{       \
@synchronized(self)     \
{       \
if(nil == _##SS_CLASSNAME##_sharedInstance)     \
{       \
_##SS_CLASSNAME##_sharedInstance = [[self alloc] init]; \
}       \
}       \
return (volatile SS_CLASSNAME*) _##SS_CLASSNAME##_sharedInstance;       \
}       \
\
+ (volatile SS_CLASSNAME*) sharedInstance       \
{       \
return (volatile SS_CLASSNAME*)[self sharedInstanceSynch]; \
}       \
\
+ (id)allocWithZone:(NSZone*) zone      \
{       \
@synchronized(self)     \
{       \
if (nil == _##SS_CLASSNAME##_sharedInstance)    \
{       \
_##SS_CLASSNAME##_sharedInstance = [super allocWithZone:zone];  \
if(nil != _##SS_CLASSNAME##_sharedInstance)     \
{       \
Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceNoSynch));  \
method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod));     \
method_setImplementation(class_getInstanceMethod(self, @selector(retainCount)), class_getMethodImplementation(self, @selector(retainCountDoNothing)));  \
method_setImplementation(class_getInstanceMethod(self, @selector(release)), class_getMethodImplementation(self, @selector(releaseDoNothing)));  \
method_setImplementation(class_getInstanceMethod(self, @selector(autorelease)), class_getMethodImplementation(self, @selector(autoreleaseDoNothing)));  \
}       \
}       \
}       \
return (id)_##SS_CLASSNAME##_sharedInstance;    \
}       \
\
+ (void)purgeSharedInstance     \
{       \
@synchronized(self)     \
{       \
if(nil != _##SS_CLASSNAME##_sharedInstance)     \
{       \
Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceSynch));    \
method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod));     \
method_setImplementation(class_getInstanceMethod(self, @selector(retainCount)), class_getMethodImplementation(self, @selector(retainCountDoSomething)));        \
method_setImplementation(class_getInstanceMethod(self, @selector(release)), class_getMethodImplementation(self, @selector(releaseDoSomething)));        \
method_setImplementation(class_getInstanceMethod(self, @selector(autorelease)), class_getMethodImplementation(self, @selector(autoreleaseDoSomething)));        \
[_##SS_CLASSNAME##_sharedInstance release];     \
_##SS_CLASSNAME##_sharedInstance = nil; \
}       \
}       \
}       \
\
- (id)copyWithZone:(NSZone *)zone       \
{       \
return self;    \
}       \
\
- (id)retain    \
{       \
return self;    \
}       \
\
- (NSUInteger)retainCount       \
{       \
NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(NSUInteger)retainCount method did not get swizzled.", self); \
return NSUIntegerMax;   \
}       \
\
- (NSUInteger)retainCountDoNothing      \
{       \
return NSUIntegerMax;   \
}       \
- (NSUInteger)retainCountDoSomething    \
{       \
return [super retainCount];     \
}\
\
- (oneway void)release  \
{       \
NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(void)release method did not get swizzled.", self);   \
}       \
\
- (void)releaseDoNothing{}      \
\
- (void)releaseDoSomething      \
{       \
@synchronized(self)     \
{       \
[super release];        \
}       \
}       \
\
- (id)autorelease       \
{       \
NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(id)autorelease method did not get swizzled.", self); \
return self;    \
}       \
\
- (id)autoreleaseDoNothing      \
{       \
return self;    \
}       \
\
- (id)autoreleaseDoSomething    \
{       \
return [super autorelease];     \
}


#pragma mark -
#pragma mark Lesser Singleton

/* A lesser singleton has a shared instance, but can also be instantiated on its own.
 *
 * For a lesser singleton, you still use SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(),
 * but use SYNTHESIZE_LESSER_SINGLETON_FOR_CLASS() in the implementation file.
 * You must specify which creation methods are to initialize the shared instance
 * (besides "sharedInstance") via CALL_LESSER_SINGLETON_INIT_METHOD()
 *
 * Example:
 *
 * MyClass.h:
 * ========================================
 *      #import "SynthesizeSingleton.h"
 *
 *      @interface MyClass: SomeSuperclass
 *      {
 *              int value;
 *              ...
 *      }
 *      SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(MyClass);
 *
 *      + (void) initSharedInstanceWithValue:(int) value;
 *
 * - (id) initWithValue:(int) value;
 *
 *      @end
 * ========================================
 *
 *
 *      MyClass.m:
 * ========================================
 *      #import "MyClass.h"
 *
 *      // This line is optional. Use it if you've enabled GCC_WARN_UNDECLARED_SELECTOR
 *      SYNTHESIZE_SINGLETON_FOR_CLASS_PROTOTYPE(MyClass);
 *
 *      @implementation MyClass
 *
 *      SYNTHESIZE_LESSER_SINGLETON_FOR_CLASS(MyClass);
 *
 *      + (void) initSharedInstanceWithValue:(int) value
 *      {
 *              CALL_LESSER_SINGLETON_INIT_METHOD(MyClass, initWithValue:value);
 *      }
 *
 *      ...
 *
 *      @end
 * ========================================
 *
 *
 * Note: CALL_LESSER_SINGLETON_INIT_METHOD() will not work if your
 * init call contains commas. If you need commas (such as for varargs),
 * or other more complex initialization, use the PRE and POST macros:
 *
 *      + (void) initSharedInstanceComplex
 *      {
 *              CALL_LESSER_SINGLETON_INIT_METHOD_PRE(MyClass);
 *
 *              int firstNumber = [self getFirstNumberSomehow];
 *              _sharedInstance = [[self alloc] initWithValues:firstNumber, 2, 3, 4, -1];
 *
 *              CALL_LESSER_SINGLETON_INIT_METHOD_POST(MyClass);
 *      }
 */

#define SYNTHESIZE_LESSER_SINGLETON_FOR_CLASS(SS_CLASSNAME)     \
\
static volatile SS_CLASSNAME* _##SS_CLASSNAME##_sharedInstance = nil;   \
\
+ (SS_CLASSNAME*) sharedInstanceNoSynch \
{       \
return (SS_CLASSNAME*) _##SS_CLASSNAME##_sharedInstance;        \
}       \
\
+ (SS_CLASSNAME*) sharedInstanceSynch   \
{       \
@synchronized(self)     \
{       \
if(nil == _##SS_CLASSNAME##_sharedInstance)     \
{       \
_##SS_CLASSNAME##_sharedInstance = [[self alloc] init]; \
if(_##SS_CLASSNAME##_sharedInstance)    \
{       \
Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceNoSynch));  \
method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod));     \
}       \
}       \
}       \
return (SS_CLASSNAME*) _##SS_CLASSNAME##_sharedInstance;        \
}       \
\
+ (volatile SS_CLASSNAME*)sharedInstance        \
{       \
return (volatile SS_CLASSNAME*) [self sharedInstanceSynch]; \
}       \
\
+ (void)purgeSharedInstance     \
{       \
@synchronized(self)     \
{       \
Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceSynch));    \
method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod));     \
[_##SS_CLASSNAME##_sharedInstance release];     \
_##SS_CLASSNAME##_sharedInstance = nil; \
}       \
}


#define CALL_LESSER_SINGLETON_INIT_METHOD_PRE(SS_CLASSNAME) \
@synchronized(self)     \
{       \
if(nil == _##SS_CLASSNAME##_sharedInstance)     \
{


#define CALL_LESSER_SINGLETON_INIT_METHOD_POST(SS_CLASSNAME) \
if(_##SS_CLASSNAME##_sharedInstance)    \
{       \
Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceNoSynch));  \
method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod));     \
}       \
}       \
}


#define CALL_LESSER_SINGLETON_INIT_METHOD(SS_CLASSNAME,__INIT_CALL__) \
CALL_LESSER_SINGLETON_INIT_METHOD_PRE(SS_CLASSNAME); \
_##SS_CLASSNAME##_sharedInstance = [[self alloc] __INIT_CALL__];        \
CALL_LESSER_SINGLETON_INIT_METHOD_POST(SS_CLASSNAME)

#endif /* SYNTHESIZE_SINGLETON_FOR_CLASS */

使用时也非常方便，该头文件也已给出使用方法，在这里我在说一下，供那些E文不好的同学使用。

使用这种方式首先把该头文件加到我们的项目中，然后直接使用就可以了：

Singleton.h

#import <Foundation/Foundation.h>
#import "SynthesizeSingleton.h"

@interface Singleton : NSObject

SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(Singleton);

//定义该类的属性，方法等

@end
Singleton.m

@implementation Singleton

SYNTHESIZE_SINGLETON_FOR_CLASS(Singleton);

//属性方法的实现

@end

如此一来在使用时，通过[Singleton sharedInstance]就可以获得该类的单例对象了。 这种方法由于有了这个头文件的支持，所以使得使用单例方便多了，而且也避免了多线程的问题。

第三种

这是最后一种也是我最推荐的一种。iOS在4.0以后推出了block和GCD，这两个特性给iOS开发带来的很大的便利，也使开发变得更加趣味话。那么如何通过GCD+block来实现单例模式呢，这主要归功于dispatch_once(dispatch_once_t *predicate, ^(void)block)这个GCD的函数，他有两个参数第一参数是一个指向dispatch_once_t类型结构体的指针，用来测试block是否执行完成，该指针所指向的结构体必须是全局的或者静态的，第二个参数是一个返回值与参数均为空的block，在block体中进行对象的初始化即可。dispatch_once在程序的生命周期中保证只会被调用一次，所以在多线程中也不会有问题。 该种方法使用方法：

+ (Singleton *)sharedInstance
{
    static Singleton *instance = nil;

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[Singleton alloc]init];
    });

    return instance;
}

使用该种方法只需要这简单的几句代码就可以实现单例了。使用起来非常方便，但是这种创建单例的方法也不是完美的，它并不能阻止人们通过alloc方法来实例化一个对象，所以这并不是严格意义上的单例模式，但是一般程序都是我们自己写，我们自己记得就好了，这也没什么可担心的，从这一点上来说第二种方法又是比较好的，具体使用的时候呢，根据实际情况来吧，各取所需就好了。