APNS机制的简单分析

1. 苹果在WWDC 2012上宣布每天发送70亿条消息推送
2. 有分析称在苹果的2019财年，iPhone活跃用户数量将突破10亿

1. App开启消息推送服务，用户配置是否想要收到消息推送
2. App向APNS注册，APNS向App返回设备码(device token)
3. App将收到的设备码提供给应用服务器
4. 当应用服务器想要给App发送消息时，根据设备码将需要发送的内容push到APNS
5. APNS将消息推送到设备

//应用发起注册
[[UIApplication sharedApplication] registerForRemoteNotificationTypes (UIRemoteNotificationTypeAlert | UIRemoteNotificationTypeBadge | UIRemoteNotificationTypeSound)];

//收到注册后的成功返回
- (void)application:(UIApplication *)application didRegisterForRemoteNotificationsWithDeviceToken:(NSData *)deviceToken;

//收到注册后的失败返回
- (void)application:(UIApplication *)app didFailToRegisterForRemoteNotificationsWithError:(NSError *)error;

//收到消息推送
- (void) application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(nonnull void (^)(UIBackgroundFetchResult))completionHandler;