iOS单例初步理解

在iOS开发中，系统自带的框架中使用了很多单例，非常方便用户（开发者，使用比如[NSApplication sharedApplication] 等），在实际的开发中，有时候也需要设计单例对象，为保证每次获取的对象都为同一个对象。
在iOS开发中创建单例具体步骤：
1.提供一个类方法：+ (instancetype)sharedXXXX;
2.创建一个全局静态变量static id _instance;
3.重写allocWithZone
4.重写copyWithZone

特举例子如下：
@interface MusicTool : NSObject
+ (instancetype)sharedMusicTool;
@end
static id _instance; // 全局变量
/**
* alloc方法内部会调用allocWithZone
*/
+ (id)allocWithZone:(struct _NSZone *)zone {
if (_instance == nil) {
@synchronized(self) {
if (_instance == nil) {
_instance = [super allocWithZone:zone];
}
}
}
return _instance;
}
/**
* 重写copy方法，防止copy出错
*/
- (instancetype)copyWithZone:(NSZone *)zone {
return _instance;
}

• (instancetype)sharedMusicTool {
  if (_instance == nil) {
  @synchronized(self) {
  if (_instance == nil) {
  _instance = [[self alloc] init];
  }
  }
  }
  return _instance;
  }

第二种使用GCD创建单例方法
@interface DataTool : NSObject
+ (instancetype)shareDataTool;
@end
static id _instance;
+ (id)allocWithZone:(struct _NSZone *)zone {
if (_instance == nil) {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [super allocWithZone:zone];
});
}
return _instance;
}
- (instancetype)copyWithZone:(NSZone *)zone {
return _instance;
}

• (instancetype)shareDataTool {
  if (_instance == nil) {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _instance = [[self alloc]init];
  });
  }
  return _instance;
  }
  第三种使用饿汉模式
  @interface SoundTool : NSObject
• (instancetype)sharedSoundTool;
  @end

static id _instance;
+ (void)load {
_instance = [[self alloc]init];
}

• (instancetype)allocWithZone:(struct _NSZone *)zone {
  if (_instance == nil) {
  _instance = [super allocWithZone:zone];
  }
  return _instance;
  }

• (instancetype)sharedSoundTool {
  return _instance;
  }

• (instancetype)copyWithZone:(NSZone *)zone {
  return _instance;
  }

为保证兼容MRC还需要重写
static id _instace;

• (id)allocWithZone:(struct _NSZone *)zone
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _instace = [super allocWithZone:zone];
  });
  return _instace;
  }

• (instancetype)sharedDataTool
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _instace = [[self alloc] init];
  });
  return _instace;
  }

• (id)copyWithZone:(NSZone *)zone
  {
  return _instace;
  }

• (oneway void)release { } //重写release

• (id)retain { return self; } //重写retain
• (NSUInteger)retainCount { return 1;} //重写retainCount
• (id)autorelease { return self;} //重写autorelease