IOS设计模式之一：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。

#import <Foundation/Foundation.h>
 
 @interface Singleton : NSObject 
 +(Singleton *) instance;
@end
 
 @implementation Singleton
+(Singleton *) instance
 { 
   static Singleton *sharedSingleton_ = nil;
   @synchronized(self){  
       if(sharedSingleton_ == nil){
          sharedSingleton_ = [NSAllocateObject([self class], 0, NULL) init];
       }
   }
   return sharedSingleton_;
 }
 
 + (id) allocWithZone:(NSZone *)zone
 {
   return [[self sharedInstance] retain];
 }
 
 - (id) copyWithZone:(NSZone*)zone
 {
   return self;
 }
 
 - (id) retain
 {
   return self;
 }
 
 - (NSUInteger) retainCount
 {
   return NSUIntegerMax; 
 }
 
 -(void)release
 {
   [super release];
 }
 
 - (id) autorelease
 {
   return self;
 }
 
 @end

 当然,ios 5以上启用ARC就简单多了：

static RootViewController* sharedRootController = nil;
 
+(RootViewController *) sharedController{
    @synchronized(self){
        if (sharedRootController == nil) {
           sharedRootController = [[self alloc] init];
        }
    }
    return  singleController;
}