objectc 单例模式

@interface MySingleton : NSObject {
 
        // ...
 
}
 
+ (MySingleton *)sharedInstance;
 
// Interface
- (NSString *)helloWorld;
 
@end#import "MySingleton.h"
 
static MySingleton *sharedInstance = nil;
 
 
@implementation MySingleton
 
 
#pragma mark Singleton methods
 
+ (MySingleton *)sharedInstance {
 
    @synchronized(self) {
        if (sharedInstance == nil) {
                        sharedInstance = [[MySingleton alloc] init];
                }
    }
 
    return sharedInstance;
}
 
+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedInstance == nil) {
            sharedInstance = [super allocWithZone:zone];
            return sharedInstance;  // 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;
}
 
#pragma mark -
#pragma mark NSObject methods
 
- (id)init {
        if (self = [super init]) {
                // ...
        }
        return self;
}
 
- (void)dealloc {
        // ...
 
        [super dealloc];
}
 
#pragma mark -
#pragma mark Implementation
 
- (NSString *)helloWorld {
        return @"Hello World!";
}
 
@end#import "MySingleton.h"
 
// ...
 
NSLog(@"Result for singleton method helloWorld: %@", [[MySingleton sharedInstance] helloWorld]);