// Set breakpoint in view's body
// In LLDB console:
(lldb) expression Self._printChanges()

var body: some View {
    let _ = Self._printChanges() // Debug only

    Text("Hello")
}

MyView: @self changed
  - Means the view value itself changed (parameters passed to view)

MyView: count changed
  - Means @State property "count" triggered the update

MyView: (no output)
  - Body not being called; view not updating at all

#Preview {
  YourView()
}

// ❌ WRONG: Direct mutation doesn't trigger update
@State var items: [String] = []

func addItem(_ item: String) {
    items.append(item)  // SwiftUI doesn't see this change
}

// ✅ RIGHT: Reassignment triggers update
@State var items: [String] = []

func addItem(_ item: String) {
    var newItems = items
    newItems.append(item)
    self.items = newItems  // Full reassignment
}

// ✅ ALSO RIGHT: Use a binding
@State var items: [String] = []

var itemsBinding: Binding<[String]> {
    Binding(
        get: { items },
        set: { items = $0 }
    )
}

// ❌ WRONG: Constant binding is read-only
@State var isOn = false

ToggleChild(value: .constant(isOn))  // Changes ignored

// ❌ WRONG: New binding created each render
@State var name = ""

TextField("Name", text: Binding(
    get: { name },
    set: { name = $0 }
))  // New binding object each time parent renders

// ✅ RIGHT: Pass the actual binding
@State var isOn = false

ToggleChild(value: $isOn)

// ✅ RIGHT (iOS 17+): Use @Bindable for @Observable objects
@Observable class Book {
    var title = "Sample"
    var isAvailable = true
}

struct EditView: View {
    @Bindable var book: Book  // Enables $book.title syntax

    var body: some View {
        TextField("Title", text: $book.title)
        Toggle("Available", isOn: $book.isAvailable)
    }
}

// ✅ ALSO RIGHT (iOS 17+): @Bindable as local variable
struct ListView: View {
    @State private var books = [Book(), Book()]

    var body: some View {
        List(books) { book in
            @Bindable var book = book  // Inline binding
            TextField("Title", text: $book.title)
        }
    }
}

// ✅ RIGHT (pre-iOS 17): Create binding once, not in body
@State var name = ""
@State var nameBinding: Binding<String>?

var body: some View {
    if nameBinding == nil {
        nameBinding = Binding(
            get: { name },
            set: { name = $0 }
        )
    }
    return TextField("Name", text: nameBinding!)
}

// ❌ WRONG: View identity changes when condition flips
@State var count = 0

var body: some View {
    VStack {
        if showCounter {
            Counter()  // Gets new identity each time showCounter changes
        }
        Button("Toggle") {
            showCounter.toggle()
        }
    }
}

// Counter gets recreated, @State count resets to 0

// ✅ RIGHT: Preserve identity with opacity or hidden
@State var count = 0

var body: some View {
    VStack {
        Counter()
            .opacity(showCounter ? 1 : 0)
        Button("Toggle") {
            showCounter.toggle()
        }
    }
}

// ✅ ALSO RIGHT: Use id() if you must conditionally show
@State var count = 0

var body: some View {
    VStack {
        if showCounter {
            Counter()
                .id("counter")  // Stable identity
        }
        Button("Toggle") {
            showCounter.toggle()
        }
    }
}

// ❌ WRONG: Property changes don't trigger update
class Model {
    var count = 0  // Not observable
}

struct ContentView: View {
    let model = Model()  // New instance each render, not observable

    var body: some View {
        Text("\(model.count)")
        Button("Increment") {
            model.count += 1  // View doesn't update
        }
    }
}

// ✅ RIGHT (iOS 17+): Use @Observable with @State
@Observable class Model {
    var count = 0  // No @Published needed
}

struct ContentView: View {
    @State private var model = Model()  // @State, not @StateObject

    var body: some View {
        Text("\(model.count)")
        Button("Increment") {
            model.count += 1  // View updates
        }
    }
}

// ✅ RIGHT (iOS 17+): Injected @Observable objects
struct ContentView: View {
    var model: Model  // Just a plain property

    var body: some View {
        Text("\(model.count)")  // View updates when count changes
    }
}

// ✅ RIGHT (iOS 17+): @Observable with environment
@Observable class AppModel {
    var count = 0
}

@main
struct MyApp: App {
    @State private var model = AppModel()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(model)  // Add to environment
        }
    }
}

struct ContentView: View {
    @Environment(AppModel.self) private var model  // Read from environment

    var body: some View {
        Text("\(model.count)")
    }
}

// ✅ RIGHT (pre-iOS 17): Use @StateObject/ObservableObject
class Model: ObservableObject {
    @Published var count = 0
}

struct ContentView: View {
    @StateObject var model = Model()  // For owned instances

    var body: some View {
        Text("\(model.count)")
        Button("Increment") {
            model.count += 1  // View updates
        }
    }
}

// ✅ RIGHT (pre-iOS 17): Use @ObservedObject for injected instances
struct ContentView: View {
    @ObservedObject var model: Model  // Passed in from parent

    var body: some View {
        Text("\(model.count)")
    }
}

View not updating?
├─ Can reproduce in preview?
│  ├─ YES: Problem is in code
│  │  ├─ Modified struct directly? → Struct Mutation
│  │  ├─ Passed binding to child? → Lost Binding Identity
│  │  ├─ View inside conditional? → Accidental Recreation
│  │  └─ Object changed but view didn't? → Missing Observer
│  └─ NO: Likely cache/Xcode state → See Preview Crashes

// ❌ WRONG: ContentView needs a model, preview doesn't provide it
struct ContentView: View {
    @EnvironmentObject var model: AppModel

    var body: some View {
        Text(model.title)
    }
}

#Preview {
    ContentView()  // Crashes: model not found
}

// ✅ RIGHT: Provide the dependency
#Preview {
    ContentView()
        .environmentObject(AppModel())
}

// ✅ ALSO RIGHT: Check for missing imports
// If using custom types, make sure they're imported in preview file

#Preview {
    MyCustomView()  // Make sure MyCustomView is defined or imported
}

// ❌ WRONG: Index out of bounds at runtime
struct ListView: View {
    @State var selectedIndex = 10
    let items = ["a", "b", "c"]

    var body: some View {
        Text(items[selectedIndex])  // Crashes: index 10 doesn't exist
    }
}

// ❌ WRONG: Optional forced unwrap fails
struct DetailView: View {
    @State var data: Data?

    var body: some View {
        Text(data!.title)  // Crashes if data is nil
    }
}

// ✅ RIGHT: Safe defaults
struct ListView: View {
    @State var selectedIndex = 0  // Valid index
    let items = ["a", "b", "c"]

    var body: some View {
        if selectedIndex < items.count {
            Text(items[selectedIndex])
        }
    }
}

// ✅ RIGHT: Handle optionals
struct DetailView: View {
    @State var data: Data?

    var body: some View {
        if let data = data {
            Text(data.title)
        } else {
            Text("No data")
        }
    }
}

Preview crashes?
├─ Error message visible?
│  ├─ "Cannot find in scope" → Missing Dependency
│  ├─ "Fatal error" or silent crash → State Init Failure
│  └─ No error → Likely Cache Corruption
└─ Try: Restart Preview → Restart Xcode → Nuke DerivedData

// ❌ WRONG: Can't see the blue view
ZStack {
    Rectangle().fill(.blue)
    Rectangle().fill(.red)
}

// ✅ RIGHT: Use zIndex to control layer order
ZStack {
    Rectangle().fill(.blue).zIndex(0)
    Rectangle().fill(.red).zIndex(1)
}

// ✅ ALSO RIGHT: Hide instead of removing from hierarchy
ZStack {
    Rectangle().fill(.blue)
    Rectangle().fill(.red).opacity(0.5)
}

// ❌ WRONG: GeometryReader expands to fill all available space
VStack {
    GeometryReader { geo in
        Text("Size: \(geo.size)")
    }
    Button("Next") { }
}
// Text takes entire remaining space

// ✅ RIGHT: Constrain the geometry reader
VStack {
    GeometryReader { geo in
        Text("Size: \(geo.size)")
    }
    .frame(height: 100)

    Button("Next") { }
}

// ❌ WRONG: Only the background ignores safe area
ZStack {
    Color.blue.ignoresSafeArea()
    VStack {
        Text("Still respects safe area")
    }
}

// ✅ RIGHT: Container ignores, children position themselves
ZStack {
    Color.blue
    VStack {
        Text("Can now use full space")
    }
}
.ignoresSafeArea()

// ✅ ALSO RIGHT: Be selective about which edges
ZStack {
    Color.blue
    VStack { ... }
}
.ignoresSafeArea(edges: .horizontal)  // Only horizontal

// ❌ WRONG: fixedSize() overrides frame()
Text("Long text here")
    .frame(width: 100)
    .fixedSize()  // Overrides the frame constraint

// ✅ RIGHT: Use frame() to constrain
Text("Long text here")
    .frame(width: 100, alignment: .leading)
    .lineLimit(1)

// ✅ RIGHT: Use fixedSize() only for natural sizing
VStack(spacing: 0) {
    Text("Small")
        .fixedSize()  // Sizes to text
    Text("Large")
        .fixedSize()
}

// ❌ WRONG: Corners applied after padding
Text("Hello")
    .padding()
    .cornerRadius(8)  // Corners are too large

// ✅ RIGHT: Corners first, then padding
Text("Hello")
    .cornerRadius(8)
    .padding()

// ❌ WRONG: Shadow after frame
Text("Hello")
    .frame(width: 100)
    .shadow(radius: 4)  // Shadow only on frame bounds

// ✅ RIGHT: Shadow includes all content
Text("Hello")
    .shadow(radius: 4)
    .frame(width: 100)

VStack {
    Text("First")   // Identity: VStack.child[0]
    Text("Second")  // Identity: VStack.child[1]
}

if showDetails {
    DetailView()  // Identity changes when condition changes
    SummaryView()
} else {
    SummaryView()  // Same type, different position = different identity
}

DetailView()
    .id(item.id)  // Explicit identity tied to item

// When item.id changes → SwiftUI treats as different view
// → @State resets
// → Animates transition

// ❌ PROBLEM: Identity changes when showDetails toggles
@State private var count = 0

var body: some View {
    VStack {
        if showDetails {
            CounterView(count: $count)  // Position changes
        }
        Button("Toggle") {
            showDetails.toggle()
        }
    }
}

// ✅ FIX: Stable identity with .opacity()
var body: some View {
    VStack {
        CounterView(count: $count)
            .opacity(showDetails ? 1 : 0)  // Same identity always
        Button("Toggle") {
            showDetails.toggle()
        }
    }
}

// ✅ ALSO FIX: Explicit stable ID
var body: some View {
    VStack {
        if showDetails {
            CounterView(count: $count)
                .id("counter")  // Stable ID
        }
        Button("Toggle") {
            showDetails.toggle()
        }
    }
}

// ❌ PROBLEM: Identity changes with selection
ForEach(items) { item in
    ItemView(item: item)
        .id(item.id + "-\(selectedID)")  // ID changes when selection changes
}

// ✅ FIX: Stable identity
ForEach(items) { item in
    ItemView(item: item, isSelected: item.id == selectedID)
        .id(item.id)  // Stable ID
}

// ❌ WRONG: Index-based ID changes when array changes
ForEach(Array(items.enumerated()), id: \.offset) { index, item in
    Text(item.name)
}

// ❌ WRONG: Non-unique IDs
ForEach(items, id: \.category) { item in  // Multiple items per category
    Text(item.name)
}

// ✅ RIGHT: Stable, unique IDs
ForEach(items, id: \.id) { item in
    Text(item.name)
}

// ✅ RIGHT: Make type Identifiable
struct Item: Identifiable {
    let id = UUID()
    var name: String
}

ForEach(items) { item in  // id: \.id implicit
    Text(item.name)
}

DetailView(item: item)
    .id(item.id)  // New item → new view → @State resets

var body: some View {
    let _ = Self._printChanges()
    // Check if "@self changed" appears when you don't expect
}