iOS Debug Guide

Table of Contents

• 1. Compile Errors ............. L15-L120
• 2. SwiftUI Errors ............. L122-L200
• 3. Runtime Errors ............. L202-L280
• 4. Build Errors ................. L282-L330
• 5. Performance Issues ..... L332-L400
• 6. Debug Workflow ............. L402-L450
• 7. Debug Tools ................... L452-L500
• 8. Checklist ....................... L502-L550

---

1. Compile Errors

1.1 Syntax Errors

Missing Brace/Bracket

Error: Expected '}' at end of closure

Fix:

• Count { and } pairs
• Use Xcode auto-indent to detect
• Check nested closures

Missing Comma

Error: Expected ',' separator

Fix:

• Check array/dictionary literals
• Check function parameters

1.2 Type Errors

Type Mismatch

Error: Cannot convert value of type 'String' to expected argument type 'Int'

Fix:

// Bad
let age: Int = "25"

// Good
let age: Int = Int("25") ?? 0

Optional Unwrapping

Error: Value of optional type 'String?' must be unwrapped

Fix:

// Option 1: Optional binding
if let name = getName() {
    print(name.uppercased())
}

// Option 2: Nil coalescing
let name = getName() ?? "Unknown"

// Option 3: Optional chaining
print(getName()?.uppercased() ?? "")

Cannot Infer Type

Error: Type of expression is ambiguous without more context

Fix:

// Bad
let items = []

// Good
let items: [String] = []
let items = [String]()

1.3 Access Control

Error: 'init' is inaccessible due to 'private' protection level

Fix:

• Change private to internal or public
• Or access from within the same file

1.4 Protocol Conformance

Error: Type 'MyClass' does not conform to protocol 'Codable'

Fix:

• Implement required methods/properties
• Ensure all properties are Codable for auto-synthesis

---

2. SwiftUI Errors

2.1 View Body Type

Multiple Views Without Container

Error: Function declares an opaque return type, but has no return statements

Fix:

// Bad
var body: some View {
    Text("Hello")
    Text("World")
}

// Good
var body: some View {
    VStack {
        Text("Hello")
        Text("World")
    }
}

Too Many Views (ViewBuilder Limit)

Error: Extra argument in call

Fix:

// Use Group to organize
VStack {
    Group {
        Text("1")
        Text("2")
        // ... up to 10
    }
    Group {
        Text("11")
        // ...
    }
}

2.2 State Management

@State Outside View

Error: @State can only be applied to properties of structs

Fix: Use @State only in View structs, use @Published in ObservableObject classes

@Published Outside ObservableObject

Error: @Published can only be applied to classes

Fix: Use @Published only in classes conforming to ObservableObject

2.3 Preview Errors

Error: Cannot preview in this file

Fix:

• Ensure PreviewProvider exists
• Provide mock data if needed
• Check for runtime errors in view

---

3. Runtime Errors

3.1 Force Unwrap Crash

Fatal error: Unexpectedly found nil while unwrapping an Optional value

Fix:

// Bad
let name = user.name!

// Good
guard let name = user.name else { return }
// or
let name = user.name ?? "Unknown"

3.2 Array Index Out of Bounds

Fatal error: Index out of range

Fix:

// Bad
let item = items[5]

// Good
guard items.indices.contains(5) else { return }
let item = items[5]

// Or use safe subscript extension
extension Array {
    subscript(safe index: Int) -> Element? {
        indices.contains(index) ? self[index] : nil
    }
}

3.3 Main Thread Violation

Warning: Publishing changes from background threads is not allowed

Fix:

// Bad
DispatchQueue.global().async {
    self.items = newItems // UI update on background
}

// Good
DispatchQueue.global().async {
    let newItems = fetchItems()
    DispatchQueue.main.async {
        self.items = newItems
    }
}

// Or with async/await
Task {
    let newItems = await fetchItems()
    await MainActor.run {
        self.items = newItems
    }
}

3.4 Memory Leaks (Retain Cycles)

Fix:

// Bad - Strong reference cycle
onComplete = {
    self.doSomething() // Strong self
}

// Good - Weak self
onComplete = { [weak self] in
    self?.doSomething()
}

---

4. Build Errors

4.1 Missing File

Error: No such file or directory

Fix:

• Check file exists in project
• Check file added to target
• Clean build folder

4.2 Duplicate Symbols

Error: Duplicate symbol '_OBJC_CLASS_$_MyClass'

Fix:

• Check file added to target multiple times
• Remove duplicate

4.3 Framework Not Found

Error: Framework not found 'SomeFramework'

Fix:

• Check SPM dependencies resolved
• Check CocoaPods installed
• Clean + rebuild

4.4 Code Signing

Error: Code signing failed

Fix:

• Check provisioning profile
• Check bundle identifier
• Check team selection

---

5. Performance Issues

5.1 Slow View Rendering

Symptoms: UI lag, slow scrolling

Debug:

.onAppear {
    print("⏱️ View appeared: \(Date())")
}

Fix:

• Use lazy loading for lists
• Reduce view complexity
• Cache computed values
• Use @StateObject instead of @ObservedObject for owned objects

5.2 Memory Warning

Symptoms: App crash on low memory

Fix:

• Release unused objects
• Reduce image sizes
• Implement pagination
• Use weak references appropriately

5.3 Network Timeout

Symptoms: Requests fail, long loading

Fix:

• Increase timeout
• Optimize API
• Add retry logic
• Show appropriate loading states

---

6. Debug Workflow

Step 1: Read Error Message

• File path
• Line number
• Error description

Step 2: Locate Issue

• Go to file and line
• Read surrounding code
• Check recent changes

Step 3: Understand Context

• What is the code trying to do?
• What are the inputs?
• What are the expected outputs?

Step 4: Apply Fix

• Fix based on patterns above
• Test the fix
• Rebuild

Step 5: Verify

• Build success
• Tests pass
• No new warnings

---

7. Debug Tools

7.1 Print Debugging

print("🔍 Debug: \(variable)")
print("🔍 Type: \(type(of: variable))")
dump(complexObject)

7.2 Breakpoints

• Set breakpoint at issue line
• Inspect variables
• Step through code

7.3 LLDB Commands

po variable          // Print object
p variable           // Print value
bt                   // Backtrace
frame variable       // All variables in frame

7.4 View Hierarchy

• Xcode → Debug → View Debugging
• Inspect view layout
• Check constraints

---

8. Checklist

When Encountering Compile Error

• Read full error message
• Locate file and line number
• Check syntax (braces, commas)
• Check types
• Check imports

When Encountering Runtime Error

• Check crash log
• Identify crash line
• Check force unwraps
• Check array access
• Check thread safety

When Encountering Performance Issue

• Profile with Instruments
• Check memory usage
• Check CPU usage
• Optimize hot paths
• Add caching

Before Commit

• No compile errors
• No runtime crashes
• No memory leaks
• Tests pass
• Performance acceptable