單例模式優點
- 單例模式在內存中只有一個實例,減少了內存開支。特別是一個對象需要頻繁的創建、銷燬時,而創建與銷燬的性能有無法優化,單例模式的優勢就非常明顯。
- 單例模式只生成一個實例,減少了系統性能開銷,當一個對象的產生需要比較多的資源時,如讀取配置、產生其他依賴對象時,則可以通過在應用啓動時直接產生一個單例對象,然後永久駐留內存的方式來解決。
- 單例模式可以避免對資源的多重佔用。
- 單例模式可以在系統設置全局的訪問點,優化和共享資源訪問。
單例模式缺點
- 單例模式一般沒有接口,擴展很困難,除了修改代碼基本上沒有第二種途徑實現。
- 單例模式對測試是不利的。在並行開發環境中,如果單例模式沒有完成,是不能進行測試的。
- 單例模式與單一職責原則有衝突。
單例模式在iOS中的使用
單例模式在iOS
開發中的使用還是蠻多的,許多Foundation
、Cocoa
和UIKit
中的類都實現了單例模式,比如應用程序本身UIApplication
、文件操作類NSFileManager
、消息中心NSNotificitonCenter
等系統都已經給我們實現單例,我們只需要使用就好了。在iOS
中使用單例模式要使用類方法,通過類方法返回該類的唯一對象。
我知道的在iOS
開發中實現單例模式主要有以下三種方式:
第一種
該方法是蘋果的官方文檔中寫的一種方式,通過覆蓋NSObject
的部分方法實現,使該類無法alloc
、retain
、release
。這是最麻煩的一種方法,也是最不好的一種方法
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;
}
可以看到這種方式,使用靜態成員維持了一個永久存在的對象,而且覆蓋了alloc
方法(alloc
方法會調用allocWithZone:
方法),並且也覆蓋了所有與引用技術有關的方法,這都使這個對象不會被銷燬。這樣看上去基本實現了我們需要的,但是寫起來麻煩不說,還有很大的一個問題,那就是多線程問題,如果是在多線程中那麼該種方法就不能保證只產生一個對象了。所以這種方式只是介紹一下,並不推薦使用。
第二種
第二種跟第一種差不多,也是通過覆蓋NSObject
的方法實現的,但是它在第一種的基礎上增加了多線程的處理,所以即使在多線程下,該種方法創建的對象也是唯一的。這種方法已經有大牛爲我們寫好了,全都都是通過C
的宏定義#define
出來了。現給出該頭文件:
//
// 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 */
使用時也非常方便,該頭文件也已給出使用方法,在這裏我在說一下,供那些E文不好的同學使用。
使用這種方式首先把該頭文件加到我們的項目中,然後直接使用就可以了:
Singleton.h
#import <Foundation/Foundation.h>
#import "SynthesizeSingleton.h"
@interface Singleton : NSObject
SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(Singleton);
//定義該類的屬性,方法等
@end
Singleton.m
@implementation Singleton
SYNTHESIZE_SINGLETON_FOR_CLASS(Singleton);
//屬性方法的實現
@end
如此一來在使用時,通過
[Singleton sharedInstance]
就可以獲得該類的單例對象了。
這種方法由於有了這個頭文件的支持,所以使得使用單例方便多了,而且也避免了多線程的問題。
第三種
這是最後一種也是我最推薦的一種。iOS
在4.0以後推出了block
和GCD
,這兩個特性給iOS
開發帶來的很大的便利,也使開發變得更加趣味話。那麼如何通過GCD
+block
來實現單例模式呢,這主要歸功於dispatch_once(dispatch_once_t
*predicate, ^(void)block)
這個GCD
的函數,他有兩個參數第一參數是一個指向dispatch_once_t
類型結構體的指針,用來測試block
是否執行完成,該指針所指向的結構體必須是全局的或者靜態的,第二個參數是一個返回值與參數均爲空的block
,在block
體中進行對象的初始化即可。dispatch_once
在程序的生命週期中保證只會被調用一次,所以在多線程中也不會有問題。
該種方法使用方法:
+ (Singleton *)sharedInstance
{
static Singleton *instance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
instance = [[Singleton alloc]init];
});
return instance;
}
使用該種方法只需要這簡單的幾句代碼就可以實現單例了。使用起來非常方便,但是這種創建單例的方法也不是完美的,它並不能阻止人們通過
alloc
方法來實例化一個對象,所以這並不是嚴格意義上的單例模式,但是一般程序都是我們自己寫,我們自己記得就好了,這也沒什麼可擔心的,從這一點上來說第二種方法又是比較好的,具體使用的時候呢,根據實際情況來吧,各取所需就好了。