Updating stored JSON due to a struct change

Tue, 25 Oct 2022 00:00:00 +0000

I mentioned yesterday “I could use a renamed old version of my struct to load the existing data, and convert it across to the new model.”. Since I’ve been testing the app on my phone, and using plausible data, it was going to be painful enough to lose it that I thought I should go through those steps.

First, I make a copy of the old struct, and renamed it with the app version number that used it. No need to bring all the computed properties into this struct, just the bits that get encoded into the JSON.

struct V01HabitItem: Identifiable, Codable, Equatable {
 var id = UUID()
 let name: String
 var started = Date()
 var timesDone = 0
 var lastDone: Date
 var daysBetweenCompletions = 1.0
}

Then I could go ahead and change the official struct to add the new properties it needs. Now when the JSON is attempted to be decoded into the new struct, it will fail, so we need to detect that and try with the old version of the struct.

class Habits: ObservableObject {

 @Published var items = [HabitItem]() {
 didSet {
 if let encoded = try? JSONEncoder().encode(items) {
 UserDefaults.standard.set(encoded, forKey: "Habits")
 } else {
 print("JSON encoding fail")
 }
 }
 }

 init() {
 if let savedItems = UserDefaults.standard.data(forKey: "Habits") {
 if let decodedItems = try? JSONDecoder().decode([HabitItem].self, from: savedItems) {
 items = decodedItems
 return
 } else {
 print("JSON decoding fail - trying v0.1")
 var v01Items = [V01HabitItem]()
 if let decodedItems = try? JSONDecoder().decode([V01HabitItem].self, from: savedItems) {
 v01Items = decodedItems
 v01Items.forEach { oldHabit in
 items.append(HabitItem(
 id: oldHabit.id,
 name: oldHabit.name,
 started: oldHabit.started,
 timesDone: oldHabit.timesDone,
 lastDone: oldHabit.lastDone,
 daysBetweenCompletions: oldHabit.daysBetweenCompletions
 ))
 }
 return
 } else {
 print("JSON decoding fail")
 }
 }
 }
 items = []
 }

Once it’s decoded, we need to ForEach through and create instances of the new struct from the old ones.

It’s made my init() a bit messy, normally I’d move that out - maybe have a separate file with the old struct and a function for decoding it and copying into the new struct, but this is not a production app, it’s only on my phone and on two simulations on my macBook, so this code won’t stay in once I’ve updated each instance by running it once.

If it works correctly, I should get the debug message and the data will still be there on the first run, then on a second run the message should not appear (since the new struct version will have been written to the file.

JSON encode/decode

Mon, 24 Oct 2022 00:00:00 +0000

As usual, I’m spending way more time on the apps written from scratch in the 100Days series. The Habit tracking app I’m working on has been good practice, especially of the architecture of the simple list based app.

My version has a couple of refinements I quite like. I’m using a checkmark in a rectangle as the button to mark that activity as done, and I’ve added a nice fade to the checkmark as time goes on to represent the percentage of time from when it is done until it becomes due again.

Until this app I’ve purely been using the preview and simulator for looking and and checking the operation of the app. But now I’ve started using my own iPhone for testing and actually trying to use the app from day to day. There’s a noticeable difference between thinking up use-cases in your head or dogfooding it. This has led to several improvements, but now I’m at a crossroads with one to do with the data persistence.

I’m using the Swift built in JSON encode/decode which has been quite painless. It uses the struct property names (so not really in line with the JSON snakecase convention) and just magics everthing up for you. Here’s the structs, and the JSON produced after a week of using the app.

struct HabitItem: Identifiable, Codable, Equatable {
 var id = UUID()
 let name: String
 var started = Date()
 var timesDone = 0
 var lastDone: Date
 var daysBetweenCompletions = 1.0
}

class Habits: ObservableObject {

 @Published var items = [HabitItem]() {
 didSet {
 if let encoded = try? JSONEncoder().encode(items) {
 UserDefaults.standard.set(encoded, forKey: "Habits")
 }
 }
 }

 init() {
 if let savedItems = UserDefaults.standard.data(forKey: "Habits") {
 if let decodedItems = try? JSONDecoder().decode([HabitItem].self, from: savedItems) {
 items = decodedItems
 return
 }
 }

 items = []
 }
}

[
 {
 "id":"5D52B636-030A-4571-8B6E-2117AFD69304",
 "timesDone":2,
 "daysBetweenCompletions":1,
 "name":"Dishes",
 "lastDone":687916240.08548605,
 "started":687741044.36479199
 },
 {
 "id":"26D89BA0-D3FA-49CF-BB01-249A19E90871",
 "timesDone":4,
 "daysBetweenCompletions":1,
 "name":"Bed",
 "lastDone":688060851.81663799,
 "started":687702031.59694302
 },
 {
 "id":"C73234ED-3FA9-4B91-AC24-42E7391E3BE1",
 "timesDone":10,
 "daysBetweenCompletions":0.5,
 "name":"Eating ",
 "lastDone":688108866.69676399,
 "started":687702494.12051105
 },
 {
 "id":"0940450A-2A5E-4B11-B049-9B33F9C01F7D",
 "timesDone":6,
 "daysBetweenCompletions":0.5,
 "name":"Brush teeth",
 "lastDone":688110048.34211302,
 "started":687702045.01338601
 },
 {
 "id":"FA6556A8-17A8-4556-8690-9522A5B22856",
 "timesDone":4,
 "daysBetweenCompletions":1,
 "name":"Write code",
 "lastDone":688108275.33836806,
 "started":687702594.956689
 },
 {
 "id":"DA2A1F85-1274-4C58-B411-D6FF9B2A2AAF",
 "timesDone":0,
 "daysBetweenCompletions":7,
 "name":"Walk exercise",
 "lastDone":687702620.82404804,
 "started":687702620.82411301
 },
 {
 "id":"155D50A7-1403-47C6-AFCA-12DB13270A55",
 "timesDone":1,
 "daysBetweenCompletions":7,
 "name":"Bins out",
 "lastDone":687879744.60854602,
 "started":687702107.33206701
 },
 {
 "id":"86FE01B9-4716-4C2C-B768-1AC8E3B03DDB",
 "timesDone":1,
 "daysBetweenCompletions":7,
 "name":"Roomba kitchen",
 "lastDone":687916244.61723006,
 "started":687702077.16412401
 },
 {
 "id":"EEA3D2CD-3717-49DD-B5E2-924956F41189",
 "timesDone":2,
 "daysBetweenCompletions":7,
 "name":"Washing",
 "lastDone":688115581.30661595,
 "started":687702096.33233297
 }
]

The problem I’ve got now I’d like to change the struct to add some different functionality. As soon as I change the struct, on the next run the JSON decode will fail and my data model will be empty. This can be caught, and I could use a renamed old version of my struct to load the existing data, and convert it across to the new model. What I’d really like is an option in the JSON decoder that if a property is missing, and I’ve provided a default in the struct, that it just uses that.

I’m not the first developer to have this wish, and there’s a number of solid solutions, but they all seem to require a bit more work than my simple dream above.

Swift on blog.iankulin.com

Updating stored JSON due to a struct change

JSON encode/decode