DocumentGroup is sometimes creating multiple views - swiftui

Question

In my application, I'm creating an object and adding data to it with an .onAppear. I've noticed sometimes (1 in 4 to 6 times), my object is being created multiple times, and the .onAppear is not being called for each.
After much trial and error, I reduced the code to the following.
I added an init to the ContentView to print when it is being called and in the read for the TextDocument. Most of the time when I open a text file, there is one "Read data" and one "Create view". Every 3 to 6 times, there is one "Read data" and three "Create view. The size of the text file does not change the frequency of the error.
I tried the same experiment with a WindowView and never saw multiple "Create view" messages.
import SwiftUI
import UniformTypeIdentifiers
#main
struct OneMoreTimeApp: App {
var body: some Scene {
DocumentGroup(newDocument: TextDocument()) { file in
ContentView(document: file.$document)
}
}
}
struct ContentView: View {
#Binding var document: TextDocument
init(document: Binding<TextDocument>) {
print("Create view")
self._document = document
}
var body: some View {
VStack {
Text("Hello World")
Text(document.data)
}
}
}
struct TextDocument: FileDocument {
static var readableContentTypes : [UTType] {[.text]}
var data: String
init(text: String = "This is some text") {
print("init data")
self.data = text
}
init(configuration: ReadConfiguration) throws {
print("read data")
if let data = configuration.file.regularFileContents {
self.data = String(decoding: data, as: UTF8.self)
} else {
throw CocoaError(.fileReadCorruptFile)
}
}
func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
print("write data")
let outData = Data(data.utf8)
return FileWrapper(regularFileWithContents: outData)
}
}
I'm thinking this is an Apple bug, but want it to run it by here first incase I'm doing something wrong.
Test Case
Launch the application
Open a text file
Wait ten seconds
Verify that the create view message is only printed once
Close the text file
Repeat steps 1 through 5 ten time.
It usually fails within 5 try.
I can also reproduce the issue by skipping step 5 and repeating steps 2 through 4, but it takes long to reproduce, up to 30 times.
I reproduced the problem on an iPad Pro (12.9-inch) )3rd gen) running 16.2
I reproduced the problem on an iPhone 14 Pro Max running 16.2
I reproduced the problem on an iPhone 14 simulator
I built the app with a target of 16.1, 16.0, 15.6 and 14.7. All shows the same problem.
Running Xcode 14.2
Here is a less trivial version that creates an object, which gets created multiple times.
import SwiftUI
import UniformTypeIdentifiers
#main
struct OneMoreTimeApp: App {
var body: some Scene {
DocumentGroup(newDocument: TextDocument()) { file in
ContentView(document: file.$document)
}
}
}
struct ContentView: View {
#Binding var document: TextDocument
#State var myObject = MyObject()
init(document: Binding<TextDocument>) {
print("Create view")
self._document = document
}
var body: some View {
VStack {
Text("Hello World")
Text("\(myObject.text)")
Text(document.data)
.onAppear{
print("onAppear - myObject is \(myObject.text)")
myObject.text += " - onAppear"
}
}
}
}
struct TextDocument: FileDocument {
static var readableContentTypes : [UTType] {[.text]}
var data: String
init(text: String = "This is some text") {
print("init data")
self.data = text
}
init(configuration: ReadConfiguration) throws {
print("read data")
if let data = configuration.file.regularFileContents {
self.data = String(decoding: data, as: UTF8.self)
} else {
throw CocoaError(.fileReadCorruptFile)
}
}
func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
print("write data")
let outData = Data(data.utf8)
return FileWrapper(regularFileWithContents: outData)
}
}
class MyObject : ObservableObject {
static var numberOfObjects = 0
#Published var text = "none"
init() {
MyObject.numberOfObjects += 1
text = "Object number \(MyObject.numberOfObjects)"
print("created \(text)")
}
deinit {
print("killed \(text)")
}
}

Answer 1

Whatever work you're doing in ContentView.init that you don't want repeated, you should move into an ObservableObject and store in a #StateObject-wrapped property instead.

Answer 2

Rob pointed me down a path that led to this slightly more complicated code. In it I create three copies of the object, one as a #state, one a StateObject and one as a #ObservedObjects.
The code compiles and generally works with all three. In the case where the View is created multiple times all three act differently.
The #State gets destroyed and recreated, but behind the scenes the old data is copied into the new object.
The #StateObject does not get destroyed.
The #ObservableObject is destroyed and left in its initialized state (the old data does not get copied into it.)
I the failing case I get the following prints:
read data
created Object0 number 1
created Object2 number 2
Create view
created Object1 number 3
onAppear
onChange - size is (1366.0, 878.0)
created Object0 number 4
created Object2 number 5
Create view
killed Object2 number 2 - onAppear
created Object0 number 6
created Object2 number 7
Create view
killed Object0 number 4
killed Object2 number 5
On the screen it shows:
Hello World
object 0 1,366.000000 text: Object0 number 1 - onAppear
object 1 1,366.000000 text: Object1 number 3 - onAppear
object 2 1,366.000000 text: Object7 number 7
CGFloat 1,366.000000
So, I had a bug in my code, I was using #ObservedObject when I should have been using #StateObject.
However, it still seems strange that view is getting created extra times and data is being copied from the old one to the new one (#State vars).
What do you guys think? should I report this to Apple or figure since there is code that does the copying when it happens it must be right?
import SwiftUI
import UniformTypeIdentifiers
#main
struct OneMoreTimeApp: App {
var body: some Scene {
DocumentGroup(newDocument: TextDocument()) { file in
ContentView(document: file.$document)
}
}
}
struct ContentView: View {
#Binding var document: TextDocument
#State var myObject0 = MyObject(0)
#StateObject var myObject1 = MyObject(1)
#ObservedObject var myObject2 = MyObject(2) // does not work
#State var theWidth : CGFloat = 0
init(document: Binding<TextDocument>) {
print("Create view")
self._document = document
}
var body: some View {
VStack {
GeometryReader { geomerty in
VStack {
Rectangle()
.frame(height: 50)
.frame(maxWidth: .infinity)
Text("Hello World")
Text("object 0 \(myObject0.width) text:\(myObject0.text)")
Text("object 1 \(myObject1.width) text:\(myObject1.text)")
Text("object 2 \(myObject2.width) text:\(myObject2.text)")
Text("CGFloat \(theWidth)")
Text(document.data)
.onAppear{
print("onAppear")
myObject0.text += " - onAppear"
myObject1.text += " - onAppear"
myObject2.text += " - onAppear"
}
.onChange(of: geomerty.size) { size in
print("onChange - size is \(geomerty.size)")
myObject0.width = size.width
myObject1.width = size.width
myObject2.width = size.width
theWidth = size.width
}
Button("set to 42") {
myObject0.width = 42.0
myObject1.width = 42.0
myObject2.width = 42.0
theWidth = 42.0
}
}
}
}
}
}
struct TextDocument: FileDocument {
static var readableContentTypes : [UTType] {[.text]}
var data: String
init(text: String = "This is some text") {
print("init data")
self.data = text
}
init(configuration: ReadConfiguration) throws {
print("read data")
if let data = configuration.file.regularFileContents {
self.data = String(decoding: data, as: UTF8.self)
} else {
throw CocoaError(.fileReadCorruptFile)
}
}
func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
print("write data")
let outData = Data(data.utf8)
return FileWrapper(regularFileWithContents: outData)
}
}
class MyObject : ObservableObject {
static var numberOfObjects = 0
#Published var text = "none"
#Published var width : CGFloat = 0.0
init(_ i:Int) {
MyObject.numberOfObjects += 1
text = "Object\(i) number \(MyObject.numberOfObjects)"
print("created \(text)")
}
deinit {
print("killed \(text)")
}
}