Objective-C中单例模式的实现
单例模式在Cocoa和Cocoa Touch中非常常见。比如这两个，[UIApplication sharedApplication]和[NSApplication sharedApplication]，大家应该都见过。但是我们应该如何在代码中实现一个单例模式呢？
1.如果你对苹果的文档很熟悉的话，你一定知道，在Cocoa Foundamentals Guide中有一段实现单例模式的示例代码。大致如下
/* Singleton.h */
#import <Foundation/Foundation.h>
@interface Singleton : NSObject
+ (Singleton *)instance;
@end
/* Singleton.m */
#import "Singleton.h"
static Singleton *instance = nil;
@implementation Singleton
+ (Singleton *)instance {
if (!instance) {
instance = [[super allocWithZone:NULL] init];
}
return instance;
}
+ (id)allocWithZone:(NSZone *)zone {
return [self instance];
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)init {
if (instance) {
return instance;
}
self = [super init];
return self;
}
- (id)retain {
return self;
}
- (oneway void)release {
// Do nothing
}
- (id)autorelease {
return self;
}
- (NSUInteger)retainCount {
return NSUIntegerMax;
}
@end 这是一种很标准的Singleton实现，中规中矩。不过这种实现并不是线程安全的。所以各路大神都各显神威，给出了多种单例模式的实现。
2.Matt Gallagher在博客中放出了一个Macro，用来实现单例模式。虽然是一个宏定义的代码，但是具体实现还是很清楚的。代码如下：
// SynthesizeSingleton.h
// 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.
//
#define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \
static classname *shared##classname = nil; \
+ (classname *)shared##classname \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [[self alloc] init]; \
} \
} \
\
return shared##classname; \
} \
\
+ (id)allocWithZone:(NSZone *)zone \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [super allocWithZone:zone]; \
return shared##classname; \
} \
} \
return nil; \
} \
\
- (id)copyWithZone:(NSZone *)zone \
{ \
return self; \
} \
\
- (id)retain \
{ \
return self; \
} \
\
- (NSUInteger)retainCount \
{ \
return NSUIntegerMax; \
} \
\
- (void)release \
{ \
} \
\
- (id)autorelease \
{ \
return self; \
} 是不是感觉这两种方法很拖沓，别担心，后面将介绍简单的实现单利的方法！
3.然而，eschaton则觉得这些实现都太繁琐了，他给出的实现如下：
@interface SomeManager : NSObject
+ (id)sharedManager;
@end
/* 非线程安全的实现 */
@implementation SomeManager
+ (id)sharedManager {
static id sharedManager = nil;
if (sharedManager == nil) {
sharedManager = [[self alloc] init];
}
return sharedManager;
}
@end
/* 线程安全的实现 */
@implementation SomeManager
static id sharedManager = nil;
+ (void)initialize {
if (self == [SomeManager class]) {
sharedManager = [[self alloc] init];
}
}
+ (id)sharedManager {
return sharedManager;
}
@end 关于为什么上述代码就能实现单例模式，以及关于线程安全问题的考量，请参考他的博客。
4.最后介绍一个比较现代的单例模式实现。为什么说现代呢？因为这种实现利用了GCD（Grand Central Dispatch）和ARC（Automatic Reference Counting）。核心代码如下：
+ (id)sharedInstance
{
static dispatch_once_t pred = 0;
__strong static id _sharedObject = nil;
dispatch_once(&pred, ^{
_sharedObject = [[self alloc] init]; // or some other init method
});
return _sharedObject;
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