在开发IOS应用的时候，一个类与另一个类之间的通信方式通常有四种模式：

1、委托delegation
2、通知中心Notification Center
3、block
4、键值观察key value observing，KVO
但是，有时候我们会出现一个类同时和多个类进行通信的情况，这时候大家通常会使用Notification，它可以一对多进行通信；但是Notification有一定的局限性，例如：1.不能从观察者获得任何的反馈信息（相比于delegate）；2.Notification不关心接受者的态度,只管把通告放出来;如果使用delegate就可以避免这些不足之处；

delegation 实现一对多通信：

MessageSingleton.h (我为了方便演示使用了单例)

import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN
@class MessageSingleton;
@protocol MessageSingletonDelegate <NSObject>

• (void)messageSingleton:(MessageSingleton) messageSingleton logNowDate:(NSDate)nowDate;

• (void)messageSingleton:(MessageSingleton) messageSingleton updateName:(NSString)nameStr;

@end

@interface MessageSingleton : NSObject

• (instancetype)sharedInstance;

/**

• 添加单个 delegate 到MessageSingleton.
  */

• (void)addDelegate:(id<MessageSingletonDelegate>) messageSingletonDelegate;

/**

• 从MessageSingleton里移除一个delegate
  */

• (void)removeDelegate:(id<MessageSingletonDelegate>) messageSingletonDelegate;

/**

• 移除所有添加过的delegate.
  */

• (void)removeAllDelegates;
  @end

NS_ASSUME_NONNULL_END

MessageSingleton.m

import "MessageSingleton.h"

@interface MessageSingleton()
/**

• 用来存储所有添加j过的delegate
• NSHashTable 与 NSMutableSet相似，但NSHashTable可以持有元素的弱引用，而且在对象被销毁后能正确地将其移除。
  */
  @property (strong, nonatomic, readonly) NSHashTable *delegates;

/**

• delegateLock 用于给delegate的操作加锁，防止多线程同时调用
  */
  @property (strong, nonatomic) NSLock *delegateLock;
  @end

@implementation MessageSingleton

• (instancetype)sharedInstance {
  static MessageSingleton *messageSingleton = nil;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  messageSingleton = [[self alloc] init];
  });
  return messageSingleton;
  }

pragma mark - get & set

• (instancetype)init {
  self = [super init];
  if (self) {
  _delegates = [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory];
  //使用timer模式调用delegate
  NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
  [self callMessageSingletonLogNowDateDelegate];
  [self callMessageSingletonUpdateNameDelegate];
  }];

    [timer fire];
  

  }
  return self;
  }

• (NSLock *)delegateLock {
  if (_delegateLock == nil) {
  _delegateLock = [[NSLock alloc] init];
  }
  return _delegateLock;
  }

pragma mark - delegate

• (void)addDelegate:(id<MessageSingletonDelegate>) messageSingletonDelegate {
  [self.delegateLock lock];//防止多线程同时调用
  [self.delegates addObject:messageSingletonDelegate];
  [self.delegateLock unlock];
  }

• (void)removeDelegate:(id<MessageSingletonDelegate>) messageSingletonDelegate {
  [self.delegateLock lock];//防止多线程同时调用
  [self.delegates removeObject:messageSingletonDelegate];
  [self.delegateLock unlock];
  }

• (void)removeAllDelegates {
  [self.delegateLock lock];//防止多线程同时调用
  [self.delegates removeAllObjects];
  [self.delegateLock unlock];
  }

pragma mark - call delegates

• (void)callMessageSingletonLogNowDateDelegate {
  [self.delegateLock lock];
  for (id delegate in self.delegates) {//遍历delegates ，call delegate
  if ([delegate respondsToSelector:@selector(messageSingleton:logNowDate:)]) {
  [delegate messageSingleton:self logNowDate:[NSDate date]];
  }
  }
  [self.delegateLock unlock];
  }

• (void)callMessageSingletonUpdateNameDelegate {
  [self.delegateLock lock];
  for (id delegate in self.delegates) {
  if ([delegate respondsToSelector:@selector(messageSingleton:updateName:)]) {
  [delegate messageSingleton:self updateName:@"奋斗的小鸟"];
  }
  }
  [self.delegateLock unlock];
  }

@end

在A类中实现MessageSingletonDelegate：

import "ViewController.h"

import "MessageSingleton.h"

@interface ViewController ()<MessageSingletonDelegate>

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  [[MessageSingleton sharedInstance] addDelegate:self];
  }

• (void)messageSingleton:(MessageSingleton) messageSingleton logNowDate:(NSDate)nowDate {
  NSLog(@"logNowDate Delegate :%@",nowDate);
  }

• (void)messageSingleton:(MessageSingleton) messageSingleton updateName:(NSString)nameStr {
  NSLog(@"updateName Delegate :%@",nameStr);
  }

@end

同时在B类中也实现MessageSingletonDelegate：

import "NextViewController.h"

import "MessageSingleton.h"

@interface NextViewController ()<MessageSingletonDelegate>

@end

@implementation NextViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  [[MessageSingleton sharedInstance] addDelegate:self];
  }

• (void)dealloc {
  [[MessageSingleton sharedInstance] removeDelegate:self];
  }

• (void)messageSingleton:(MessageSingleton) messageSingleton logNowDate:(NSDate)nowDate {
  NSLog(@"NextVC logNowDate Delegate :%@",nowDate);
  }

• (void)messageSingleton:(MessageSingleton) messageSingleton updateName:(NSString)nameStr {
  NSLog(@"NextVC updateName Delegate :%@",nameStr);
  }

运行可以看到，MessageSingleton发出的delegate可以在A类和B类中都能收到.