iOS之单例模式

单例模式的意思就是只有一个实例。单例模式确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。这个类称为单例类。

1.单例模式的要点：

　　显然单例模式的要点有三个；一是某个类只能有一个实例；二是它必须自行创建这个实例；三是它必须自行向整个系统提供这个实例。

2.单例模式的优点：

　　1.实例控制：Singleton
会阻止其他对象实例化其自己的 Singleton
对象的副本，从而确保所有对象都访问唯一实例。

　　2.灵活性：因为类控制了实例化过程，所以类可以更加灵活修改实例化过程

IOS中的单例模式

　　在objective-c中要实现一个单例类，至少需要做以下四个步骤：

　　1、为单例对象实现一个静态实例，并初始化，然后设置成nil，

　　2、实现一个实例构造方法检查上面声明的静态实例是否为nil，如果是则新建并返回一个本类的实例，

　　3、重写allocWithZone方法，用来保证其他人直接使用alloc和init试图获得一个新实力的时候不产生一个新实例，

　　4、适当实现allocWitheZone，copyWithZone，release和autorelease。

以下转载自：http://www.galloway.me.uk/tutorials/singleton-classes/

One of my most used design patterns when developing for iOS is the singleton pattern. It’s an extremely powerful way to share data between different parts of code without
having to pass the data around manually. More about the singleton pattern and other patterns can be found in this excellent book:

Background

Singleton classes are an important concept to understand because they exhibit an extremely useful design pattern. This idea is used throughout the iPhone SDK, for example, UIApplication has a method called sharedApplication which when called from anywhere will
return the UIApplication instance which relates to the currently running application.

How to implement

You can implement a singleton class in Objective-C using the following code:
MyManager.h

1 2 3 4 5 6 7 8 9 10 11

#import <foundation/Foundation.h> @interface MyManager : NSObject { NSString *someProperty; } @property (nonatomic, retain) NSString *someProperty; + (id)sharedManager; @end

MyManager.m

1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

#import "MyManager.h" @implementation MyManager @synthesize someProperty; #pragma mark Singleton Methods + (id)sharedManager { static MyManager *sharedMyManager = nil; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ sharedMyManager = [[self alloc] init]; }); return sharedMyManager; } - (id)init { if (self = [super init]) { someProperty = [[NSString alloc] initWithString:@"Default Property Value"]; } return self; } - (void)dealloc { // Should never be called, but just here for clarity really. } @end

What this does is it defines a static variable (but only global to this translation unit)) called 

sharedMyManager

 which
is then initialised once and only once in 

sharedManager

.
The way we ensure that it’s only created once is by using the 

dispatch_once

 method
from Grand Central Dispatch (GCD). This is thread safe and handled entirely by the OS for you so
that you don’t have to worry about it at all.

However, if you would rather not use GCD then you should use the following code for 

sharedManager

:
Non-GCD based code

1 2 3 4 5 6 7 8

+ (id)sharedManager { static MyManager *sharedMyManager = nil; @synchronized(self) { if (sharedMyManager == nil) sharedMyManager = [[self alloc] init]; } return sharedMyManager; }

Then you can reference the singleton from anywhere by calling the following function:

MyManager *sharedManager = [MyManager sharedManager];

I’ve used this extensively throughout my code for things such as creating a singleton to handle CoreLocation or CoreData functions.

Non-ARC code

Not that I recommend it, but if you are not using Automatic Reference Counting (ARC), then you should use the following code:
MyManager.h non-ARC

1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

#import "MyManager.h" static MyManager *sharedMyManager = nil; @implementation MyManager @synthesize someProperty; #pragma mark Singleton Methods + (id)sharedManager { @synchronized(self) { if(sharedMyManager == nil) sharedMyManager = [[super allocWithZone:NULL] init]; } return sharedMyManager; } + (id)allocWithZone:(NSZone *)zone { return [[self sharedManager] retain]; } - (id)copyWithZone:(NSZone *)zone { return self; } - (id)retain { return self; } - (unsigned)retainCount { return UINT_MAX; //denotes an object that cannot be released } - (oneway void)release { // never release } - (id)autorelease { return self; } - (id)init { if (self = [super init]) { someProperty = [[NSString alloc] initWithString:@"Default Property Value"]; } return self; } - (void)dealloc { // Should never be called, but just here for clarity really. [someProperty release]; [super dealloc]; } @end

Updates

EDIT: Added property to MyManager.

EDIT: Updated as per Apple’s
guidelines to pass static analysis.

EDIT: Updated to support ARC.

EDIT: Switched to use the more common GCD approach.