Yesterday, I wrote about streaks and motivation. To let me experiment with streaks and streak freezes, I’ve started work on a simple habit-tracking app. One of my personal goals is to become proficient in SwiftUI, so I used SwiftUI for this project.
The good news: My project, Captain’s Log, is done “enough” for me to use it. It’s also pleasingly compact (1200 lines of code). However, I’m now wrestling with performance. This is the slowest app I’ve written in a long time, and in spite of working as an iOS performance engineer at Facebook for years, I have no idea how to make this simple program faster. The time profiler tool in Instruments shows me deep, incomprehensible stacks that have nothing obvious to do with my SwiftUI code. The new View Body and View Properties tools are a little more helpful. For instance, one performance problem I have is the app takes too long to process keystrokes. Using the new tools, it looks like my central document property updates on each keystroke, and this causes most of the app to redraw. However, I can’t figure out why this property is updating on each keystroke, nor can I tell if I’ve broken some intelligent View diffing that’s supposed to be happening. I feel stuck.
When Apple introduced SwiftUI, they explained the difference between imperative and declarative programming with a sandwich shop metaphor. If you walk into a sandwich shop and say, “I’d like an avocado toast,” that’s like declarative programming. You’re describing what you want and you let the server figure out how to make it. To get an avocado toast imperative-style, you’d need to tell the server individual steps instead. (“First, I want you to get a slice of bread. Next, toast it for 2 minutes. Then, get a properly ripe avocado. Mush some avocado and spread it on the toast…“)
I love this metaphor! It shows the promise of declarative frameworks — and also hints at why performance problems might be inherently harder to solve with them. Suppose I order an avocado toast at brunch, and the server disappears. 20 minutes pass. 30 minutes. Where’s my food? Since I don’t know the steps that the server takes to fulfill my order, there’s no way to figure out why things are taking so long. This seems to be the state of performance tuning in SwiftUI: You, sitting at a table, alone & hungry, wondering where your food is.
Clearly, if I’m going to become proficient with SwiftUI, I’m going to need to learn some new performance skills. Paul Hudson has the best performance tuning guide I’ve found so far, and my next project is to see if I can use this to make Captain’s Log pleasantly snappy.
Always new skills to learn!