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