iOS设计模式之单例

单例模式的意思就是这个类只有一个实例，这个类就是单例类。在iOS中有不少都是单例NSNull，NSFileManager ，UIApplication，NSUserDefaults ，UIDevice，还有一些第三方也有能用到了这种设计模式例如Afhttpmanger。。。

（1）单例模式的作用 :可以保证在程序运行过程，一个类只有一个实例，而且该实例易于供外界访问，从而方便地控制了实例个数，并节约系统资源。（2）单例模式的使用场合：在整个应用程序中，共享一份资源（这份资源只需要创建初始化1次），应该让这个类创建出来的对象永远只有一个。实现思路：创建一个一个全局的static的实例 static id _instance; 提供1个类方法让外界访问唯一的实例重写allocWithzone:方法，控制内存分配。因为alloc内部会调用该方法，每次调用allocWithzone:方法，系统都会创建一块新的内存空间。


// 实现copyWithZone:方法 
// AudioPlayer.m 
// 单例 
// 
// Created by 两好三坏 on 16/2/21. 
// Copyright © 2016年 ;. All rights reserved. 
// 
#import &amp;quot;AudioPlayer.h&amp;quot; 
> <span class="s2">@interface</spanAudioPlayer</span><span class="s3"> () 
@end 
@implementation</span> <span class="s4">AudioPlayer 
//创建一个一个全局的static的实例 static id _instance; 
> <span class="s2">static</span> <span class="s2">id</span><span class="s1"> _instance; 
//提供1个类方法让外界访问唯一的实例 
+(instancetype)shareAudioPlayer{ 
> <span class="s2">static</spandispatch_once_t</span><span class="s3"> onceToken; 
> <span class="s5">dispatch_once</span><span class="s1">(&amp;amp;onceToken, ^{ 
_instance = [[</span><span class="s2">self</span><span class="s1"> alloc] init]; 
}); 
> <span class="s2">return</span><span class="s1"> _instance; 
} 
//重写allocWithzone:方法，控制内存分配。因为alloc内部会调用该方法，每次调用allocWithzone:方法，系统都会创建一块新的内存空间。 
+(instancetype)allocWithZone:(</span><span class="s2">struct</span><span class="s1"> _</span><span class="s5">NSZone</span><span class="s1"> *)zone{ 
> <span class="s2">static</spandispatch_once_t</span><span class="s3"> onceToken; 
> <span class="s5">dispatch_once</span><span class="s1">(&amp;amp;onceToken, ^{ 
_instance = [</span><span class="s2">super</span><span class="s1"> allocWithZone:zone]; 
}); 
> <span class="s2">return</span><span class="s1"> _instance; 
> <span class="s1">} 
> <span class="s1">//实现copyWithZone:方法 
> <span class="s1">-(</span><span class="s2">id</span><span class="s1">)copyWithZone:(</span><span class="s5">NSZone</span><span class="s1"> *)zone{ 
> <span class="s2">return</span><span class="s1"> _instance; 
> <span class="s1">} 
> <span class="s1">@end 
> <span class="s1">在控制其中创建单例类的对象，打印地址： 
> <span class="s1">- (</span><span class="s2">void</span><span class="s1">)viewDidLoad { 
> <span class="s1">[</span><span class="s2">super</span><span class="s1"> viewDidLoad]; 
> <span class="s1">AudioPlayer *player1 = [AudioPlayer shareAudioPlayer]; 
> <span class="s1">AudioPlayer *player2 = [[AudioPlayer alloc] init]; 
> <span class="s1">AudioPlayer *player3 = [AudioPlayer new]; 
> <span class="s1">AudioPlayer *player4 = [player1 </span><span class="s2">copy</span><span class="s1">]; 
> <span class="s5">NSLog</span><span class="s1">(@&amp;quot;%p,%p,%p,%p&amp;quot;,player1,player2,player3,player4); 
> <span class="s1">} 
> <span class="s1">//打印结果 
> <span class="s1">2016</span><span class="s3">-</span><span class="s1">02</span><span class="s3">-</span><span class="s1">21</span> <span class="s1">23</span><span class="s3">:</span><span class="s1">27</span><span class="s3">:</span><span class="s1">13.990</span><span class="s3"> 单例[</span><span class="s1">2847</span><span class="s3">:</span><span class="s1">329685</span><span class="s3">] </span><span class="s1">0x7fb6e3e080a0</span><span class="s3">,</span><span class="s1">0x7fb6e3e080a0</span><span class="s3">,</span><span class="s1">0x7fb6e3e080a0</span><span class="s3">,</span><span class="s1">0x7fb6e3e080a0 
> <span class="s1">四个实例的内存地址是一样的，证明只创建了一个实例； 
> <span class="s1">MRC环境下通常需要在实现下面几个方法： 
> <span class="s1">- (</span><span class="s2">oneway</span> <span class="s2">void</span><span class="s1">)release {} 
> <span class="s1">- (</span><span class="s2">id</span><span class="s1">)retain { </span><span class="s2">return</span><span class="s1"> _instance; } 
> <span class="s1">- (</span><span class="s2">id</span><span class="s1">)autorelease { </span><span class="s2">return</span><span class="s1"> _instance; } 
> <span class="s1">- (</span><span class="s5">NSUInteger</span><span class="s1">)retainCount { </span><span class="s2">return</span> <span class="s5">UINT_MAX</span><span class="s1">; } 
> <span class="s1">[/objc] 
> <span class="s1">可以使用宏判断是否为mrc， 
> <span class="s1">#if __has_feature(objc_arc) 
> <span class="s1">// ARC 
> <span class="s1">#else 
> <span class="s1">// MRC 
> <span class="s1">#endif 
> &nbsp;
<span class="s1">当然还可以把单例也抽取成为宏，用起来还挺方便的；你们也可以试试试～～～</span>