IOS has a four-tier architecture,

From top to bottom for touchable layer, media layer, core service layer, core system layer;

Cocoa Touch Layer

It mainly provides services related to user interaction such as interface controls, event management, notification center, notification center and map, including the following frameworks:

UIKit

eventKit

NotifacationCenter

MapKit

Address Book

iAd

Message UI

PushKit (iOS8 new push mechanism)

Media Layer Media Layer

Mainly provides image engine, audio and video engine frame

Image engine (Core Graphics, Core Image, Core Animation, OpenGL ES)

Audio Engine (Core Audio, AVFoundation, OpenAL)

Video engine (AV Foundation, CoreMedia)

Core Services Layer

The core service layer provides basic system services for programs, such as network access, browser engine, location, file access, database access, etc., mainly including architecture

CFNetwork Network access

Core Data Storage

Core Location Location function

Core Motion, gyroscope

Foundation Basic functions such as NSString

Webkit browser engine

JavaScript JS engine

Core OS Layer

The core system layer provides the most basic services for the superstructure, such as operating system kernel services, local authentication, security, acceleration, etc

Operating system kernel services (BSD Sockets, I/O access, memory request, file system, math, etc.)

Local authentication (fingerprint authentication, etc.)

Security (provides an interface to manage certificates, public keys, keys, etc.)

Acceleration (performing math, large numbers, and DSP calculations)