IPhone开发学习笔记：单例模式的实现

首先推荐非常棒的文章：http://www.duckrowing.com/2010/05/21/using-the-singleton-pattern-in-objective-c/

下面是实现代码：

  
    

   
     
   
     static
   
      MyGizmoClass 
   
     *
   
     sharedGizmoManager 
   
     =
   
      nil;


   
     +
   
      (MyGizmoClass
   
     *
   
     )sharedManager
{
 @synchronized(self) {
 
   
     if
   
      (sharedGizmoManager 
   
     ==
   
      nil) {
 [[self alloc] init]; 
   
     //
   
      assignment not done here
   
     

   
      }
 }
 
   
     return
   
      sharedGizmoManager;
}
 

   
     +
   
      (id)allocWithZone:(NSZone 
   
     *
   
     )zone
{
 @synchronized(self) {
 
   
     if
   
      (sharedGizmoManager 
   
     ==
   
      nil) {
 sharedGizmoManager 
   
     =
   
      [super allocWithZone:zone];
 
   
     return
   
      sharedGizmoManager; 
   
     //
   
      assignment and return on first allocation
   
     

   
      }
 }
 
   
     return
   
      nil; 
   
     //
   
     on subsequent allocation attempts return nil
   
     

   
     }
 

   
     -
   
      (id)copyWithZone:(NSZone 
   
     *
   
     )zone
{
 
   
     return
   
      self;
}
 

   
     -
   
      (id)retain
{
 
   
     return
   
      self;
}
 

   
     -
   
      (unsigned)retainCount
{
 
   
     return
   
      UINT_MAX; 
   
     //
   
     denotes an object that cannot be released
   
     

   
     }
 

   
     -
   
      (
   
     void
   
     )release
{
 
   
     //
   
     do nothing
   
     

   
     }
 

   
     -
   
      (id)autorelease
{
 
   
     return
   
      self;
}
  
    

简单解释下，详细的请看上面的那篇文章。

 

@synchronized(self) ： 这个就是同步语义，这里会自动加锁，以保证多线程的环境，不会new出多个实例来。
[[self alloc] 会默认调用 [self allocWithZone] 所以在sharedManager里没有将值给 sharedGizmoManager