CFNotificationCenter

CFNotificationCenter 是 NSNotificationCenter 在 Core Foundation 中的 C 實作，一般來說，如果我們有比較高階的 API 可以使用的話，我們會盡量避免使用比較低階的 API，所以，只要有 NSNotificationCenter 可以使用的場合，我們應該不會用到 CFNotificationCenter。

比較有可能用到 CFNotificationCenter 的場合，大概是 iOS 8 之後，Hosting App 與 Extension 之間的溝通。iOS 8 之後推出了 Extension，可以允許開發者撰寫 Today Widget、Share Widget 以及模擬鍵盤等功能，Apple Watch 上的 Watch App 也屬於 Extension；每個 Extension都是額外可以讓作業系統載入的 Bundle，Extension 與我們原本的 App（便是 Hosting App）之間，可以用 Shared Data 共用資料，但是當 App 發生改變要通知 Extension，卻沒有什麼比較直接的辦法。在蘋果有新的 API 之前，我們就會倚賴透過 CFNotificationCenterGetDarwinNotifyCenter() 取得的 darwin notification center 發送通知。

而即使我們可以發送通知，CFNotificationCenter 用起來也不是很方便，主要原因是 CFNotificationCenter 不像 NSNotificationCenter，在傳遞通知的時候可以夾帶 user info。再來，就是像前面說的， CFNotificationCenter 是 C API，而我們會盡量希望使用比較高階的 API。

所以，KKBOX 在開發 Watch App 的時候，就在 CFNotificationCenter 上面又簡單架構了一層 Objective-C API，介面類似 NSNotificationCenter，叫做 KKWatchAppNotificationCenter。程式碼如下：

KKWatchAppNotificationCenter.h

@import Foundation;

@interface KKWatchAppNotificationCenter : NSObject
+ (instancetype)sharedCenter;
- (void)postNotification:(NSString *)key;
- (void)addTarget:(id)target selector:(SEL)selector name:(NSString *)notification;
- (void)removeObserver:(NSObject *)observer;
@end

KKWatchAppNotificationCenter.m

#import "KKWatchAppNotificationCenter.h"

#define LFSuppressPerformSelectorLeakWarning(doPerformSelector) \
do { \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
doPerformSelector; \
_Pragma("clang diagnostic pop") \
} while (0)

@interface KKWatchAppNotificationCenter ()
{
    NSMutableDictionary *notificationKeys;
}
@end

@implementation KKWatchAppNotificationCenter

+ (instancetype)sharedCenter
{
    static KKWatchAppNotificationCenter *sharedRegister = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedRegister = [[KKWatchAppNotificationCenter alloc] init];
    });
    return sharedRegister;
}

- (instancetype)init
{
    self = [super init];
    if (self) {
        notificationKeys = [[NSMutableDictionary alloc] init];
    }
    return self;
}

- (void)postNotification:(NSString *)key
{
    CFNotificationCenterRef const center = CFNotificationCenterGetDarwinNotifyCenter();
    CFDictionaryRef const userInfo = NULL;
    BOOL const deliverImmediately = YES;
    CFNotificationCenterPostNotification(center,
        (CFStringRef)key, NULL, userInfo, deliverImmediately);
}

- (void)addTarget:(id)target selector:(SEL)selector name:(NSString *)notification
{
    if (!target) {
        return;
    }

    if (!selector) {
        return;
    }

    if (!notification || ![notification length]) {
        return;
    }

    NSMutableArray *a = [notificationKeys objectForKey:notification];
    BOOL needRegisterNotification = NO;

    if (!a) {
        a = [NSMutableArray array];
        needRegisterNotification = YES;
    }

    for (NSDictionary *d in a) {
      if (d[@"target"] == target &&
        NSSelectorFromString(d[@"selector"]) == selector) {
            return;
        }
    }

    NSDictionary *d = @{@"target": target, @"selector": NSStringFromSelector(selector)};
    [a addObject:d];

    if (needRegisterNotification) {
        [self registerNotification:notification];
        [notificationKeys setObject:a forKey:notification];
    }
}

- (void)removeObserver:(NSObject *)observer
{
    NSMutableArray *notificationsToDelete = [[NSMutableArray alloc] init];
    for (NSString *notification in notificationKeys) {
        NSMutableArray *a = notificationKeys[notification];
        NSMutableArray *objectsToDelete = [NSMutableArray array];
        for (NSDictionary *d in a) {
            id target = d[@"target"];
            if (target == observer) {
                [objectsToDelete addObject:d];
            }
        }
        [a removeObjectsInArray:objectsToDelete];
        if (![a count]) {
            [notificationsToDelete addObject:notification];
        }
    }

    for (NSString *notification in notificationsToDelete) {
        [self unregisterNotification:notification];
        [notificationKeys removeObjectForKey:notificationKeys];
    }
}

- (void)registerNotification:(NSString *)key
{
    CFNotificationCenterRef const center = CFNotificationCenterGetDarwinNotifyCenter();
    CFNotificationSuspensionBehavior const suspensionBehavior = CFNotificationSuspensionBehaviorDeliverImmediately;
    CFNotificationCenterAddObserver(center,
        (__bridge const void *)(self),
        KKWatchAppNotificationRegisterCallback,
        (CFStringRef)key, NULL, suspensionBehavior);
}

- (void)unregisterNotification:(NSString *)key
{
    CFNotificationCenterRef const center = CFNotificationCenterGetDarwinNotifyCenter();
    CFNotificationCenterRemoveObserver(center,
      (__bridge const void *)(self),
      (CFStringRef)key, NULL);
}

void KKWatchAppNotificationRegisterCallback(CFNotificationCenterRef center,
  void * observer, CFStringRef name, void const * object, CFDictionaryRef userInfo)
{
    KKWatchAppNotificationCenter *self = (__bridge KKWatchAppNotificationCenter *)observer;
    NSString *notification = (__bridge NSString *)name;

    NSArray *a = [self->notificationKeys objectForKey:notification];
    for (NSDictionary *d in a) {
        id target = d[@"target"];
        SEL action = NSSelectorFromString(d[@"selector"]);
        LFSuppressPerformSelectorLeakWarning([target performSelector:action withObject:nil]);
    }
}

@end