Advent of Code Visualizer Redux

For the past couple of years, I’ve done my Advent of Code submissions in Swift, and used a custom pipeline of CoreGraphics, Metal, and AVFoundation to streamline the creation of visualizations. This worked great, but the solution to do this felt a little hacky. I’ve now rewritten this pipeline to follow modern practices and be more streamlined.

If you want to follow along, my new code is available on GitHub.

The Old Way

The basic process of generating the visualizations is:

  1. Run the Advent of Code solution until we’ve reached the point of creating a frame.
  2. Get a CVPixelBuffer from the AVFoundation API that’s appropriate for encoding.
  3. Create a CoreGraphics context pointing to the CVPixelBuffer memory.
  4. Draw the frame.
  5. Simultaneously:
    • Submit the CVPixelBuffer to the Metal renderer.
    • Submit the CVPixelBuffer to AVFoundation for encoding and mixing.

When I originally set up the code, SwiftUI was brand new, it was limited as an API, and my experience in it was next to none. A rough layout of the code was:

  1. A Metal view with a closure that does the “work”. This closure passed an “animator” object as its only parameter.
  2. During construction, the Metal view creates the “animator”, which builds all of the AVFoundation contexts needed for encoding and muxing the animation.
  3. Once the Metal view appears, it calls the “work” closure, which starts the Advent of Code solution.
  4. At the point of an animation frame, the “work” closure calls a draw method on the “animator”.
  5. This draw method takes a closure which passes a CGContext as its only parameter. The draw closure is where the frame drawing should occur.
    • Before the closure is called, a CVPixelBuffer is grabbed from the AVFoundation pixel buffer pool and a CGContext is created using the memory from the CVPixelBuffer.
    • After the closure is called, the CVPixelBuffer is submitted to the encoding and muxing parts of AVFoundation.
  6. The CVPixelBuffer is also stored in a @Published variable of the “animator”. The Metal view observes this variable and uses that as a means to render the pixel buffer on the next render pass.

Make sense? It shouldn’t. That’s way too many closures, a confusing ownership model, and a nearly incomprehensible code path.

The New Way

I’ve learned a lot since SwiftUI was released. SwiftUI has also changed. There has to be a better way!

The first step was to contain everything inside of one ObservableObject. At creation, this object builds the Metal rendering context and the AVFoundation contexts. To get new drawing contexts, a nextContext method returns both a new CVPixelBuffer and CGContext. When drawing is complete, both objects are passed back to a submit method, which then does the cleaning up and vending to Metal and AVFoundation.

All of this is done in a SolutionContext object. Any visualization just subclasses this object and overrides the run method, calling nextContext and submit as needed.

If I wanted a solution that just pulsed a color on the screen, I could write:

class VisualizationTestingContext: SolutionContext {
    override var name: String {
        "Visualization Testing"
    override func run() async throws {
        for t in stride(from: 0.0, through: 100.0, by: 0.01) {
            let (context, pixelBuffer) = try nextContext()
            let redColor = CGColor(red: 1.0 * alphaValue, green: 0.0, blue: 
            let backgroundRect = CGRect(
                x: 0, y: 0, 
                width: context.width, height: context.height

            submit(context: context, pixelBuffer: pixelBuffer)

The entire application code to run this becomes:

struct VisualizationTestingApp: App {
    @StateObject var context: SolutionContext = VisualizationTestingContext(width: 800, height: 800, frameRate: 60.0)
    var body: some Scene {
        WindowGroup {
A slightly more complex drawing example

With just that bit of code, you can have a fully rendering, encoding, and muxing system. No more closures, no more spaghetti, and no more rendering to JPEGs and then stitching them together with FFmpeg.

Bonus Round!

Since I’m already rewriting everything, let’s go a couple steps further.

Most visualizations boil down to filling in rectangles or drawing text. Instead of doing this by hand every time, I built a handful of functions to do the bounds measurements, origin coordinate conversions, and CoreGraphics object conversions for me.

// Draw a mushroom in box
let grayColor = CGColor(red: 0.5 * alphaValue, green: 0.5 * alphaValue, blue: 0.5 * alphaValue, alpha: 1.0)
let textColor = CGColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
let box = CGRect(x: 0.0, y: 0.0, width: 100.0, height: 100.0)
let font = NativeFont.boldSystemFont(ofSize: 12.0)

fill(rect: box, color: grayColor, in: context)
draw(text: "🍄", color: textColor, font: font, rect: box, in: context)

Some AppKit and UIKit APIs are nearly identical, so when I need universal access to fonts and colors, I can now just use my Native* versions of them:

#if os(macOS)
import AppKit

public typealias NativeColor = NSColor
public typealias NativeFont = NSFont

import UIKit

public typealias NativeColor = UIColor
public typealias NativeFont = UIFont


And with that said, all of the code is now universal, meaning it can be run on macOS, iOS, or iPadOS. There isn’t a huge benefit to this, but since the APIs are so close, and everything else is SwiftUI, why not?

Note that the iOS simulator is way slower than running natively on device. Any slow down in the code is typically from waiting for AVFoundation to be ready for writing the next frame, which the simulator is most likely not optimized for high speed streaming of data.