didSet not called by Array.append() in Swift - swift

Question

I am following the 100 Days of SwiftUI and have reached Day 37. While doing Making changes permanent with UserDefaults, I encounter a problem with didSet.
(I am using Swift 5 with iOS 13.4)
In the example code, it writes
.navigationBarItems(trailing: Button("Save") {
if let actualAmount = Int(self.amount) {
let item = ExpenseItem(name: self.name, type: self.type, amount: actualAmount)
self.expenses.items.append(item)
}
})
where didSet should be called by .append().
However, in practice, the didSet is not called unless I change the above code to
.navigationBarItems(trailing: Button("Save") {
if let actualAmount = Int(self.amount) {
let item = ExpenseItem(name: self.name, type: self.type, amount: actualAmount)
let newItems = self.expenses.items + [item]
self.expenses.items = newItems
}
})
I also write a small test (see below) in Playground which shows that .append() works pretty well with didSet
struct Count {
var array: [Int] {
didSet {
print("struct Count - didSet() called")
}
}
}
class CountClass {
var array: [Int] {
didSet {
print("class CountClass - didSet() called")
}
}
init() {
array = [1, 2, 3]
}
}
struct Test {
var countA = Count(array: [1, 2, 3])
var countB = CountClass()
mutating func testDidSet() {
countA.array.append(4)
countB.array.append(4)
}
}
var t = Test()
t.testDidSet()
This strange behaviour really makes me wonder how didSet works. Or is this problem related to the use of #ObservedObject (which is the case of the example project)?
PS: I have downloaded the finished version from Project7 and it also has the problem.

Answer 1

This is a known Swift 5.2 bug: observers of wrapped properties are not called upon modification (https://bugs.swift.org/browse/SR-12089). They had known about this since January and all the same released an update breaking a bunch of production code ¯\_(ツ)_/¯
A temporary workaround is already presented in the question - property reassignment instead of modification.

Answer 2

I went through this project as well. Make sure your items property in the Expenses class is marked as #Published. As below;
import SwiftUI
struct ExpenseItem: Identifiable, Codable {
let id = UUID()
let name: String
let type: String
let amount: Int
}
class Expenses: ObservableObject {
init() {
if let items = UserDefaults.standard.data(forKey: "Items") {
let decoder = JSONDecoder()
if let decoded = try? decoder.decode([ExpenseItem].self, from: items) {
self.items = decoded
return
}
}
self.items = []
}
#Published var items: [ExpenseItem] {
didSet {
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(items) {
UserDefaults.standard.set(encoded, forKey: "Items")
}
}
}
}
and add self.presentationMode.wrappedValue.dismiss() in your navigation bar Save button.
.navigationBarItems(trailing: Button("Save") {
if let actualAmount = Int(self.amount) {
let item = ExpenseItem(name: self.name, type: self.type, amount: actualAmount)
self.expenses.items.append(item)
self.presentationMode.wrappedValue.dismiss()
} else {
self.isShowingAlert = true
}
})

Answer 3

I finished this project last week I think, but checking the Swift GitHub page it seems there was an update on the 24th.
When I did this project everything worked fine, but now I'm on day 47 and I'm having this very problem. Maybe it's related to the Swift 5.2 update.

Answer 4

What's happening is that Swift is not recognizing the .append() as setting the variable. You can get around this by copying your array to a new temporary array, append the new value and then set your class array to this temporary array.
var newActivityList = [Activity]() //new temporary array
for activity in self.activityList.activities {
newActivityList.append(activity) //copy class array to temp array
}
newActivityList.append(newActivity) //append new value
self.activityList.activities = newActivityList //set class array to temp array
PS: the example above is for day 47, but the logic is the same.