Whether you’re new to streaming or an iOS dev curious about how video gets from server to screen — this post is for you. Lets dive into!

🧱 1. Begins with HLS

Apple's default streaming protocol is HLS — HTTP Live Streaming. It breaks video into chunks (called segments) of a few seconds each.

When you hit play:

The app fetches a .m3u8 manifest file
This manifest lists available quality levels (bitrate ladders)
Based on your network, a suitable stream is picked

➡️ URL ➝ master.m3u8
    ➝ variant_720p.m3u8
        ➝ segment1.ts
        ➝ segment2.ts

🎬 2. AVPlayer Takes Over

Apple’s AVPlayer is the workhorse. Once you provide it the streaming URL:

let player = AVPlayer(url: hlsURL)
player.play()

AVPlayer automatically:

Parses the playlist
Handles adaptive bitrate (ABR) switching
Manages playback buffer
Talks to decoders behind the scenes

You don’t have to handle this manually — but understanding it helps you debug and optimize better.

📦 3. CDN Fetching and Buffering

Segments are fetched from CDNs (like Akamai, Cloudflare, or Fastly). Downloads 2–3 segments (~6–10 seconds) ahead of what’s being shown. This forms the buffer.

If the network drops:

AVPlayer plays from buffer
If recovery takes too long → stall occurs
Player switches to lower bitrate to avoid rebuffering

🧠 4. From Byte to Frame: Decoding

Once the segment arrives:

Frames are rendered onto the screen — usually at 30 or 60 FPS

If the CPU or GPU is overwhelmed, or decoding lags, you might see:

Dropped frames
Janky playback
Lag behind live

⚠️ 5. ABR, Stall Recovery & Playback Events

AVPlayer constantly listens to:

Network conditions
Buffer status
Player timebase

It may:

Switch to a lower bitrate if bandwidth drops
Retry failed segments
Notify you via AVPlayerItemPlaybackStalled or AVPlayerItemDidPlayToEndTime

🧰 Bonus: Minimal AVPlayer Code Example

import AVKit

let url = URL(string: "https://yourstream.com/live.m3u8")!
let player = AVPlayer(url: url)

let playerVC = AVPlayerViewController()
playerVC.player = player

present(playerVC, animated: true) {
    player.play()
}

🧭 What’s Next?

This was just the start. In the next post, we’ll go deeper into:

🎯 “What is HLS and why does Apple love it?”

You’ll learn:

The anatomy of .m3u8 files
Master vs Variant playlists
Bitrate ladders
How ABR really works

🧵 Part of the Streaming 101 Series

Stay tuned on Streaming Demystified for more insights into AVFoundation, video streaming, and iOS engineering at scale.

💬 Have questions, thoughts, or feedback?

Reply below or connect with me on LinkedIn if you're building or thinking about video, media, mobile tech !

What Happens When You Hit Play?