iOS的单例:

//Singleton.h
@interface Singleton : NSObject
  + (Singleton *) instance;
@property (nonatomic,strong) NSString *singletonData;
@end

//Singleton.m
#import "Singleton.h"
@implementation Singleton
@synthesize singletonData = _singletonData;
static Singleton *instance =nil;
+ (Singleton *) instance{
  static dispatch_once_t once;
  dispatch_once(&once,^{
  instance = [[self alloc] init];
});
  return instance;
}

其中 static Singleton instance 为静态变量 ,类方法为 + (Singleton ) instance instance 方法采用了 GCD(Grand Central Dispatch)技术,这是一种基于C语言的多线程访问技术,在上述代码中dispatch_once函数就是由GCD提供的.它只执行一次, stringByAddingPercentEscapesUsingEncoding 转换为 url编码. delegate: 首先声明一个如下.h文件:

@protocol PhilosopherDelegate <nsobject>
@required
-(void) sleep;
-(void) eat;
-(void) work;
@end

然后声明两个 相互依赖的.h.m

#import "PhilosopherDelegate.h"
@interface Philosopher : NSObject
{
    NSTimer *timer;
    int count;
}
@property (nonatomic,weak) id<philosopherDelegate> delegate;
-(void) start;
-(void) handle;
@end

#import "Philosopher.h"
@implementation Philosopher
@synthesize delegate;
-(void) start{
    count =0;
    timer = [NSTimer scheduledTimerWithTimeInterval:3 target:self selector:@selector(handle) userInfo:nil repeats:YES];
}
-(void)handle{
    switch (count) {
        case 0:
            [self.delegate sleep];
            count++;
            break;
        case 1:
            [self.delegate eat];
            count++;
            break;
        case 2:
            [self.delegate work];
            [timer invalidate];
            break;
    }
}
@end

然后再ViewController 中import,初始化上面相互依赖的.h文件 代码如下:

#import "ViewController.h"
#import "Philosopher.h"
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    Philosopher *obj = [[Philosopher alloc]init];
    obj.delegate = self;
    [obj start];
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
-(void) sleep{
    NSLog(@"sleep");
}
-(void) eat{
    NSLog(@"eat");
}
-(void) work{
    NSLog(@"work");
}
@end