iOS-NSNotificationCenter通知原理解析

一、基本概念

NSNotification和NSNotificationCenter是使用观察者模式来实现的用于跨层传递消息。

NSNotificationCenter采用单例模式。

二、基本实现

通知实现由三个类组成：NSNotificationCenter、NSNotification、NSObserverModel,对应关系和接口设计：

NSNotificationCenter是注册中心，发通知时，发送的是一个NSNotification对象。

NSNotification里面包含发通知需要传递的内容：

name（通知的字符串），

object（发送通知的对象），

userInfo（需要传递的参数），

NSObserverModel是注册通知时需要保持的参数，包括：

observer（观察者对象），

selector（执行的方法），

notificationName（通知名字），

object（携带参数），

operationQueue（队列），

block（回调），

三：原理图：

四：注意点：

1、每次调用addObserver时，都会在通知中心重新注册一次，即使是同一对象，监听同一个消息，而不是去覆盖原来的监听。这样，当通知中心转发某一消息时，如果同一对象多次注册了这个通知的观察者，则会收到多个通知。
2、observer 观察者（不能为nil，通知中心会弱引用，ARC是iOS9之前是unsafe_unretained，iOS9及以后是weak，MRC是assign，所以这也是MRC不移除会crash，ARC不移除不会crash的原因）

五：仿照写一个通知中心：WYNotificationCenter，WYNotification，WYObserverModel

#import <Foundation/Foundation.h>

@class WYNotification;
NS_ASSUME_NONNULL_BEGIN

typedef void(^OperationBlock)(WYNotification *);

@interface WYObserverModel : NSObject
@property (nonatomic, weak) id observer;  //观察者对象
@property (nonatomic, assign) SEL selector;  //执行的方法
@property (nonatomic, copy) NSString *notificationName; //通知名字
@property (nonatomic, strong) id object;  //携带参数
@property (nonatomic, strong) NSOperationQueue *operationQueue;//队列
@property (nonatomic, copy) OperationBlock block;  //回调
@end

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface WYNotification : NSObject

@property (readonly, copy) NSNotificationName name;
@property (nullable, readonly, retain) id object;
@property (nullable, readonly, copy) NSDictionary *userInfo;

- (instancetype)initWithName:(NSNotificationName)name object:(nullable id)object userInfo:(nullable NSDictionary *)userInfo;

@end

NS_ASSUME_NONNULL_END

#import "WYNotification.h"

@implementation WYNotification

- (instancetype)initWithName:(NSString *)name object:(nullable id)object userInfo:(nullable NSDictionary *)userInfo {
    self = [super init];
    if (self) {
        _name = name;
        _object = object;
        _userInfo = userInfo;
    }
    return self;
}

@end

#import "WYNotificationCenter.h"
#import "WYObserverModel.h"
#import "WYNotification.h"

@interface WYNotificationCenter()
@property(nonatomic,strong)NSMutableDictionary *obsetvers;
@end

@implementation WYNotificationCenter

+ (WYNotificationCenter *)defaultCenter {
    static WYNotificationCenter *instance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc] init];
        instance.obsetvers = [NSMutableDictionary dictionary];
    });
    return instance;
}

- (void)addObserver:(id)observer selector:(SEL)aSelector name:(nullable NSString*)aName object:(nullable id)anObject
{

    // 创建数据模型
    WYObserverModel *observerModel = [[WYObserverModel alloc] init];
    observerModel.observer = observer;
    observerModel.selector = aSelector;
    observerModel.notificationName = aName;
    observerModel.object = anObject;

    // 如果不存在,才创建
    if (![self.obsetvers objectForKey:aName]) {
        NSMutableArray *arrays = [NSMutableArray array];

        [arrays addObject:observerModel];

        // 添加进 json 中
        [self.obsetvers setObject:arrays forKey:aName];
    } else {
        // 如果存在,取出来,继续添加进对应数组即可
        NSMutableArray *arrays = (NSMutableArray *)[self.obsetvers objectForKey:aName];

        [arrays addObject:observerModel];
    }

}

- (id <NSObject>)addObserverForName:(nullable NSString *)name object:(nullable id)obj queue:(nullable NSOperationQueue *)queue usingBlock:(void (^)(WYNotification *note))block
{
    WYObserverModel *observerModel = [[WYObserverModel alloc] init];
    observerModel.block = block;
    observerModel.notificationName = name;
    observerModel.object = obj;
    observerModel.operationQueue = queue;

    //如果不存在，那么即创建
    if (![self.obsetvers objectForKey:name]) {

        NSMutableArray *arrays = [[NSMutableArray alloc]init];
        [arrays addObject:observerModel];
        //填充进入数组
        [self.obsetvers setObject:arrays forKey:name];

    }else{
        //如果存在，取出来，继续添加即可
        NSMutableArray *arrays = (NSMutableArray*)[self.obsetvers objectForKey:name];
        [arrays addObject:observerModel];
    }
    return nil;
}
- (void)postNotificationName:(nonnull NSString *)name object:(nullable id)objec {
    [self postNotificationName:name object:objec userInfo:nil];
};

- (void)postNotificationName:(nonnull NSString *)name object:(nullable id)object userInfo:(nullable NSDictionary *)userInfo {
    WYNotification *notification = [[WYNotification alloc] initWithName:name object:object userInfo:userInfo];
    [self postNotification:notification];
}
- (void)postNotification:(WYNotification *)notification
{
    //name 取出来对应观察者数组，执行任务
    NSMutableArray *arrays = (NSMutableArray*)[self.obsetvers objectForKey:notification.name];
    NSLog(@"%@",arrays);
    [arrays enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {

        //取出数据模型
        WYObserverModel *observerModel = obj;
        id observer = observerModel.observer;
        SEL secector = observerModel.selector;

        if (!observerModel.operationQueue) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
            [observer performSelector:secector withObject:notification];
#pragma clang diagnostic pop
        }else{

            //创建任务
            NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{

                //这里用block回调出去
                observerModel.block(notification);

            }];

            // 如果添加观察者 传入 队列，那么就任务放在队列中执行(子线程异步执行)
            NSOperationQueue *operationQueue = observerModel.operationQueue;
            [operationQueue addOperation:operation];

        }

    }];

}

#pragma mark - 移除通知

- (void)removeObserver:(nonnull id)observer {
    [self removeObserver:observer name:nil object:nil];
}

- (void)removeObserver:(nonnull id)observer name:(nullable NSString *)name object:(nullable id)object {
    // 移除观察者 - 当有 name 参数时
    if (name.length > 0 && [self.obsetvers objectForKey:name]) {
        NSMutableArray *arrays = (NSMutableArray *)[self.obsetvers objectForKey:name];
        [arrays removeObject:observer];
    } else {
        // 移除观察者 - 当没有 name 参数时
        if (self.obsetvers.allKeys.count > 0 && self.obsetvers.allValues.count > 0) {
            NSArray *allKeys = self.obsetvers.allKeys;

            for (int i = 0; i < allKeys.count; i++) {
                NSMutableArray *keyOfAllObservers = [self.obsetvers objectForKey:allKeys[i]];

                BOOL isStop = NO;   // 如果找到后就不再遍历后面的数据了

                for (int j = 0; j < keyOfAllObservers.count; j++) {
                    // 取出数据模型
                    WYObserverModel *observerModel = keyOfAllObservers[j];

                    if (observerModel.observer == observer) {
                        [keyOfAllObservers removeObject:observerModel];
                        isStop = YES;
                        break;
                    }
                }

                if (isStop) {   // 找到了,退出循环
                    break;
                }
            }
        } else {
            NSAssert(false, @"当前通知中心没有观察者");
        }
    }
}
@end

六、参考：

https://www.jianshu.com/p/2c9fef32a383

https://blog.csdn.net/m0_37182854/article/details/78782724