簡單說單例模式就是創(chuàng)建一個(gè)對(duì)象的全局唯一實(shí)體。方便管理全局?jǐn)?shù)據(jù)。具體單例模式的講解可以看設(shè)計(jì)模式相關(guān)書籍，這邊主要是介紹下現(xiàn)階段可以如何在Objective-C語言中實(shí)現(xiàn)單例模式。

如何實(shí)現(xiàn)

蘋果加入 GCD 之后我們可以利用 GCD實(shí)現(xiàn)。
MyManager.h
#import <foundation/Foundation.h>

@interface MyManager : NSObject {
    NSString *someProperty;
}

@property (nonatomic, retain) NSString *someProperty;

+ (id)sharedManager;

@end

MyManager.m

#import "MyManager.h"

@implementation MyManager

@synthesize someProperty;

#pragma mark Singleton Methods

+ (id)sharedManager {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (id)init {
  if (self = [super init]) {
      someProperty = [[NSString alloc] initWithString:@"Default Property Value"];
  }
  return self;
}

- (void)dealloc {
  // Should never be called, but just here for clarity really.
}

@end

GCD 的 dispatch_once 能保證在多線程調(diào)用的時(shí)候 sharedMyManager 初始化一次。
如果不想使用 GCD來實(shí)現(xiàn)，我們可以使用如下 :

+ (id)sharedManager {
    static MyManager *sharedMyManager = nil;
    @synchronized(self) {
        if (sharedMyManager == nil)
            sharedMyManager = [[self alloc] init];
    }
    return sharedMyManager;
}

Non-ARC （非 ARC 模式）

推薦如下實(shí)現(xiàn)方式

#import "MyManager.h"

static MyManager *sharedMyManager = nil;

@implementation MyManager

@synthesize someProperty;

#pragma mark Singleton Methods
+ (id)sharedManager {
  @synchronized(self) {
      if(sharedMyManager == nil)
          sharedMyManager = [[super allocWithZone:NULL] init];
  }
  return sharedMyManager;
}
+ (id)allocWithZone:(NSZone *)zone {
  return [[self sharedManager] retain];
}
- (id)copyWithZone:(NSZone *)zone {
  return self;
}
- (id)retain {
  return self;
}
- (unsigned)retainCount {
  return UINT_MAX; //denotes an object that cannot be released
}
- (oneway void)release {
  // never release
}
- (id)autorelease {
  return self;
}
- (id)init {
  if (self = [super init]) {
      someProperty = [[NSString alloc] initWithString:@"Default Property Value"];
  }
  return self;
}
- (void)dealloc {
  // Should never be called, but just here for clarity really.
  [someProperty release];
  [super dealloc];
}

@end