APNSClient is an APNS client.

AlertDictionary is a more complex notification payload. From the APN docs: "Use the ... alert dictionary in general only if you absolutely need to."

Client contains the fields necessary to communicate with Apple, such as the gateway to use and your certificate contents. You'll need to provide your

FeedbackResponse represents a device token that Apple has indicated should not be sent to in the future.

Payload contains the notification data for your request. Alert is an interface here because it supports either a string or a dictionary, represented

PushNotification is the wrapper for the Payload. The length fields are computed in ToBytes() and aren't represented here.

PushNotificationResponse details what Apple had to say, if anything.