【iOS印象】GLPubSub 源码阅读笔记

@(iOS知行路)[源码阅读|iOS开发|NSNotification|iOS印象]

GLPubSub 是 NSNotificationCenter 的封装，目标是简化 iOS 开发中的发布订阅模式。该库通过添加 NSObject 的 Category，使得所有对象都可以方便地使用其订阅或发布通知。

源码阅读

• GLEvent 类，传递的事件对象，包含 事件名称 、 事件发布者 、 事件附带的额外数据

/**
 `GLEvent` is the event object passed into handler block.
 */
@interface GLEvent : NSObject

/**
 *  Name of the event
 */
@property (nonatomic, copy) NSString *name;

/**
 *  The object triggering the event
 */
@property (nonatomic, retain) id obj;

/**
 *  Additional data of the event
 */
@property (nonatomic, retain) id data;

/**
 *  Init `GLEvent` with `name`, `obj` and `data`.
 *
 *  @param name Name of the event
 *  @param obj  The object triggering the event
 *  @param data Additional data of the event
 *
 *  @return Initialized `GLEvent`
 */
- (id)initWithName:(NSString *)name obj:(id)obj data:(id)data;

@end
复制代码

• 通过为 NSObject 添加 Category，增加 订阅/取消订阅 、 发布 事件等方法，以及设置事件处理的处理队列

@interface NSObject (GLPubSub)

#pragma mark - Class Methods
/**
 *  Set the queue to handle events. If not set or set to `nil, events will be handled on the queue where the notification is posted. Otherwise, all the events will be handled on the set queue.
 *
 *  @param queue The queue on which to handle events.
 */
+ (void)setPubSubQueue:(NSOperationQueue *)queue;

#pragma mark - Publish Methods
/**
 *  Publish an event without additional data.
 *
 *  @param name Event name to publish.
 *
 *  @see -publish:data:
 */
- (void)publish:(NSString *)name;

/**
 *  Publish an event with additional data.
 *
 *  @param name Event name to publish.
 *  @param data Additional data.
 *
 *  @see -publish:
 */
- (void)publish:(NSString *)name data:(id)data;

#pragma mark - Subscribe Methods with Selector
/**
 *  Subscribe an event with selector.
 *
 *  @param eventName Event name to subscribe.
 *  @param selector  Selector to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:obj:selector:
 *  @see -subscribeOnce:selector:
 *  @see -subscribeOnce:obj:selector:
 */
- (id)subscribe:(NSString *)eventName selector:(SEL)selector;

/**
 *  Subscribe an event from a specified object with selector.
 *
 *  @param eventName Event name to subscribe.
 *  @param object    Publisher to subscribe from.
 *  @param selector  Selector to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:selector:
 *  @see -subscribeOnce:selector:
 *  @see -subscribeOnce:obj:selector:
 */
- (id)subscribe:(NSString *)eventName obj:(id)obj selector:(SEL)selector;

/**
 *  Subscribe an event only once with selector.
 *
 *  @param eventName Event name to subscribe.
 *  @param selector  Selector to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:selector:
 *  @see -subscribe:obj:selector:
 *  @see -subscribeOnce:obj:selector:
 */
- (id)subscribeOnce:(NSString *)eventName selector:(SEL)selector;

/**
 *  Subscribe an event from a specified object only once with selector.
 *
 *  @param eventName Event name to subscribe.
 *  @param object    Publisher to subscribe from.
 *  @param selector  Selector to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:selector:
 *  @see -subscribe:obj:selector:
 *  @see -subscribeOnce:selector:
 */
- (id)subscribeOnce:(NSString *)eventName obj:(id)obj selector:(SEL)selector;

#pragma mark - Subscribe Methods with Handler Block
/**
 *  Subscribe an event with handler block.
 *
 *  @param eventName Event name to subscribe.
 *  @param handler   Handler block to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:obj:handler:
 *  @see -subscribeOnce:handler:
 *  @see -subscribeOnce:obj:handler:
 */
- (id)subscribe:(NSString *)eventName handler:(GLEventHandler)handler;

/**
 *  Subscribe an event from a specified object with handler block.
 *
 *  @param eventName Event name to subscribe.
 *  @param obj       Publisher to subscribe from.
 *  @param handler   Handler block to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:handler:
 *  @see -subscribeOnce:handler:
 *  @see -subscribeOnce:obj:handler:
 */
- (id)subscribe:(NSString *)eventName obj:(id)obj handler:(GLEventHandler)handler;

/**
 *  Subscribe an event only once with handler block.
 *
 *  @param eventName Event name to subscribe.
 *  @param handler   Handler block to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:handler:
 *  @see -subscribe:obj:handler:
 *  @see -subscribeOnce:obj:handler:
 */
- (id)subscribeOnce:(NSString *)eventName handler:(GLEventHandler)handler;

/**
 *  Subscribe an event from a specified object only once with handler block.
 *
 *  @param eventName Event name to subscribe.
 *  @param obj       Publisher to subscribe from.
 *  @param handler   Handler block to handle the event.
 *
 *  @return An opaque object to act as the observer.
 *
 *  @see -subscribe:handler:
 *  @see -subscribe:obj:handler:
 *  @see -subscribeOnce:handler:
 */
- (id)subscribeOnce:(NSString *)eventName obj:(id)obj handler:(GLEventHandler)handler;

#pragma mark - Unsubscribe Methods
/**
 *  Unsubscribe speficied event for current instance.
 *
 *  @param eventName Name of event to unsubscribe.
 *
 *  @see -unsubscribeAll
 */
- (void)unsubscribe:(NSString *)eventName;

/**
 *  Unsubscribe all events for current instance.
 *
 *  @see -unsubscribe:
 */
- (void)unsubscribeAll;

@end
复制代码

• 发布事件的实现

- (void)publish:(NSString *)name data:(id)data {
    // 通知中心发出通知，并将事件对象放入`userInfo`
    NSDictionary *userInfo = nil;
    if (data != nil) {
        userInfo = @{kGLPubSubDataKey: data};
    }
    [[NSNotificationCenter defaultCenter] postNotificationName:name object:self userInfo:userInfo];
}
复制代码

• 订阅事件的实现

- (id)subscribe:(NSString *)eventName obj:(id)obj selector:(SEL)selector {
    // 用于判断`selector`的参数个数
    NSMethodSignature *sig = [self methodSignatureForSelector:selector];
    //  The hidden arguments self and _cmd (of type SEL) are at indices 0 and 1; method-specific arguments begin at index 2
    BOOL passEventObj = ([sig numberOfArguments] == 3);
    __weak __typeof__(self) weakSelf = self;
    return [self subscribe:eventName obj:obj handler:^(GLEvent *event) {
        __strong __typeof__(weakSelf) strongSelf = weakSelf;
        if (passEventObj) {
            [strongSelf performSelector:selector withObject:event];
        } else {
            [strongSelf performSelector:selector];
        }
    }];
}

- (id)subscribe:(NSString *)eventName obj:(id)obj handler:(GLEventHandler)handler {
    // 通知中心通过添加观察者，并在block中生成事件进行转发
    id observer =  [[NSNotificationCenter defaultCenter] addObserverForName:eventName object:obj queue:_pubSubQueue usingBlock:^(NSNotification *note) {
        GLEvent *event = [[GLEvent alloc] initWithName:eventName obj:note.object data:[note.userInfo objectForKey:kGLPubSubDataKey]];
        handler(event);
    }];
    // 添加观察者对象至该事件的观察者数组，用于取消订阅事件
    NSMutableDictionary *subscriptions = (NSMutableDictionary *)objc_getAssociatedObject(self, &kGLPubSubSubscriptionsKey);
    if (!subscriptions) {
        subscriptions = [[NSMutableDictionary alloc] init];
        objc_setAssociatedObject(self, &kGLPubSubSubscriptionsKey, subscriptions, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    NSMutableSet *observers = [subscriptions objectForKey:eventName];
    if (!observers) {
        observers = [[NSMutableSet alloc] init];
        [subscriptions setObject:observers forKey:eventName];
    }
    [observers addObject:observer];
    return observer;
}
复制代码

• 取消事件的实现

- (void)unsubscribe:(NSString *)eventName {
    NSMutableDictionary *subscriptions = (NSMutableDictionary *)objc_getAssociatedObject(self, &kGLPubSubSubscriptionsKey);
    if (!subscriptions)
        return;
    NSMutableSet *observers = [subscriptions objectForKey:eventName];
    if (observers) {
        // 通知中心移除所有该事件的观察者
        for (id observer in observers) {
            [[NSNotificationCenter defaultCenter] removeObserver:observer];
        }
        // 订阅数组中移除该事件观察者数组对象
        [subscriptions removeObjectForKey:eventName];
    }
}
复制代码

附：安装与使用

CocoaPods

• 在 Podfile 里添加以下依赖：

pod "GLPubSub", "~> 1.0"
复制代码

• 运行 pod install 来安装 GLPubSub

源文件

如果你的项目没有用 CocoaPods 来管理第三方依赖，你也可以直接导入源文件。

• 下载 最新代码 并解压
• 导入 NSObject+GLPubSub.h 和 NSObject+GLPubSub.m 到你的工程，记得在导入时勾选 "Copy items if needed"

因为 GLPubSub 是基于 NSNotificationCenter 并注册在 [NSNotificationCenter defaultCenter] 的，所以 GLPubSub 也支持大部分系统通知，例如 UIApplicationDidEnterBackgroundNotification ， UIApplicationDidBecomeActiveNotification ， UITextFieldTextDidChangeNotification 等等，但是转发的过程中会丢弃系统通知的 userInfo 字段。

设置 PubSub 的队列

GLPubSub 主要基于 NSNotificationCenter 的 -addObserverForName:object:queue:usingBlock: 方法。你可以调用 NSObject 的 +setPubSubQueue: 方法来设置传入该方法的 queue 。

默认传入的 queue 为 nil ，这意味着所有事件会在发布通知的线程中被执行。你可以手动设置为 [NSOperationQueue maniQueue] 使得所有事件在主线程被触发：

[NSObject setPubSubQueue:[NSOperationQueue mainQueue]];
复制代码

通过 Selector 订阅事件

大部分时候，我们用 self 作为订阅者：

[self subscribe:@"YourEventName" selector:@selector(yourEventHandler)];
复制代码

你也可以指定事件的发布者：

[self subscribe:@"YourEventName" obj:somePublisher selector:@selector(yourEventHandler)];
复制代码

如果你希望你的方法只触发一次，你可以用：

[self subscribeOnce:@"YourEventName" selector:@selector(yourEventHandler)];
复制代码

这样当该事件被触发后，就会自动取消订阅。

你的方法可以接受一个 GLEvent 参数，该参数包含了被触发事件的相关信息。

@interface GLEvent : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, retain) id obj;
@property (nonatomic, retain) id data;
复制代码

name 是事件名， obj 是发布者， data 是附加信息。

通过 Block 订阅事件

方法与上面通过 selector 订阅的方法类似：

GLEventHandler 定义如下：

typedef void (^GLEventHandler)(GLEvent *event);
复制代码

所以你可以如下用 block 订阅一个事件：

__weak __typeof__(self) weakSelf = self;
[self subscribe:UIApplicationDidEnterBackgroundNotification handler:^(GLEvent *event) {
    __strong __typeof__(weakSelf) strongSelf = weakSelf;
    [strongSelf appDidEnterBackground];
}];
复制代码

这里的 weak 化是为了避免循环引用。对应于前面 selector 的方法，用 block 也有 4 种调用方法：

- (id)subscribe:(NSString *)eventName handler:(GLEventHandler)handler;
- (id)subscribe:(NSString *)eventName obj:(id)obj handler:(GLEventHandler)handler;
- (id)subscribeOnce:(NSString *)eventName handler:(GLEventHandler)handler;
- (id)subscribeOnce:(NSString *)eventName obj:(id)obj handler:(GLEventHandler)handler;
复制代码

取消订阅

取消订阅某个事件：

- (void)unsubscribe:(NSString *)eventName;
复制代码

取消订阅所有事件：

- (void)unsubscribeAll;
复制代码

虽然当实例被销毁时，存在 associated object 中的观察者也都会被销毁，但还是建议手动取消订阅，如根据不同需求，在 -dealloc 或 -viewDidDisappear 方法中取消订阅。

- (void)dealloc
{
    [self unsubscribeAll];
}
复制代码

发布事件

你可以简单地发布一个事件：

[self publish:@"YourEventName"];
复制代码

也可以附带一些数据，很多时候我们会传入一个 NSDictionary 来附带更多结构化的数据：

[self publish:@"YourEventName" data:@{@"key": value}]
复制代码