Crash Safety

Crash & Error Resilience Audit

Find risky unwraps, missing guards, and unhandled errors. Add safe fallbacks and user-friendly error states.

When to use this skill

• Before App Store submission
• After adding new async/error-prone code
• When reviewing PRs with optional handling
• After crash reports from TestFlight

Quick Reference: Risk Levels

| Pattern | Risk | Fix | |---------|------|-----| | fatalError() | 🔴 Critical | Remove or guard with #if DEBUG | | ! force unwrap | 🔴 High | Use guard let, if let, or ?? | | try! | 🔴 High | Use do/catch or try? with fallback | | preconditionFailure() | 🔴 High | Remove from production paths | | Implicit unwrap (Type!) | 🟡 Medium | Convert to optional with safe access | | Missing @MainActor | 🟡 Medium | Add for UI-updating code |

---

Step 1: Fatal Crash Patterns

1.1 fatalError / preconditionFailure / assertionFailure

# These crash in production!
rg --type swift -g '!*Tests*' -g '!*UITests*' 'fatalError\(|preconditionFailure\(|assertionFailure\(' YourApp/

Fix: Remove or wrap in #if DEBUG:

#if DEBUG
fatalError("Unimplemented")
#else
return defaultValue
#endif

1.2 Force Unwraps (!)

# Force unwraps - tighter pattern to reduce false positives
rg --type swift -g '!*Tests*' -g '!*UITests*' '[a-zA-Z_][a-zA-Z0-9_]*\s*!' YourApp/ | grep -v '!=' | grep -v '//' | head -40

Exceptions (OK to skip):

• @IBOutlet (UIKit pattern)
• NSImage(named:)! with known system images
• URL(string: "https://...")! with hardcoded valid URLs
• Code inside #Preview { } blocks
• Code inside *Tests*/ directories

1.3 Force Try (try!)

rg --type swift -g '!*Tests*' -g '!*UITests*' 'try!' YourApp/

> Note: try! is acceptable in tests and previews where failure should crash during development.

Fix: Replace with do/catch or try? with default:

// ❌ Crashes on failure
let data = try! encoder.encode(object)

// ✅ Safe with error handling
let data = try? encoder.encode(object) ?? Data()

1.4 Implicitly Unwrapped Optionals (Type!)

rg --type swift -g '!*Tests*' ': [A-Z][a-zA-Z0-9_]*!' YourApp/ | grep -v '@IBOutlet' | head -20

---

Step 2: Optional Safety Audit

2.1 Chained Optional Access Without Guards

# Deep optional chains that may be fragile
rg --type swift -g '!*Tests*' '\?\.\w+\?\.\w+' YourApp/ | head -20

2.2 Array/Dictionary Subscript Without Bounds Check

# Direct array indexing (may crash on out-of-bounds)
rg --type swift -g '!*Tests*' '\[\d+\]|\[index\]|\[i\]' YourApp/ | head -20

Fix: Use safe subscript or bounds check:

let item = array.indices.contains(index) ? array[index] : nil

---

Step 3: Error Handling Audit

3.1 Empty Catch Blocks

# Swallowed errors (should at least log)
rg --type swift -g '!*Tests*' -A2 'catch\s*\{' YourApp/ | grep -B1 '^\s*\}' | head -20

3.2 Result Types Without Failure Handling

rg --type swift -g '!*Tests*' 'Result<' YourApp/ | head -15
rg --type swift -g '!*Tests*' '\.failure\(' YourApp/ | head -15

---

Step 4: Async/Concurrency Safety

4.1 Missing @MainActor for UI Updates

# Task blocks that may update UI
rg --type swift -g '!*Tests*' 'Task\s*\{' YourApp/Views/ | head -20

# Check for MainActor annotations
rg --type swift -g '!*Tests*' '@MainActor' YourApp/

Fix: UI-updating async code needs @MainActor:

Task { @MainActor in
    self.isLoading = false
}

4.2 Task.detached & Continuation Misuse

# Task.detached (often misused, can cause actor isolation issues)
rg --type swift -g '!*Tests*' 'Task\.detached' YourApp/

# Continuations (must resume exactly once - missing resume = deadlock)
rg --type swift -g '!*Tests*' 'withCheckedContinuation|withUnsafeContinuation|withCheckedThrowingContinuation' YourApp/

Verify continuations:

• Every code path must call continuation.resume() exactly once
• Missing resume = deadlock
• Double resume = crash

4.3 Unstructured Task Cancellation

# Tasks that should be cancelled when view disappears
rg --type swift -g '!*Tests*' '\.task\s*\{|Task\s*\{' YourApp/Views/ | head -20

Verify: Long-running tasks should use .task { } modifier (auto-cancels) or manual cancellation.

---

Step 5: User-Facing Error States

5.1 ContentUnavailableView for Empty/Error States

rg --type swift -g '!*Tests*' 'ContentUnavailableView' YourApp/Views/

5.2 Alert for User-Facing Errors

rg --type swift -g '!*Tests*' '\.alert\(' YourApp/Views/ | head -15

---

Step 6: Watch App Audit

WATCH_DIR="YourApp Watch App"

# Fatal patterns in Watch app
rg --type swift 'fatalError\(|preconditionFailure\(' "$WATCH_DIR/"

# Force unwraps in Watch app
rg --type swift '[a-zA-Z_][a-zA-Z0-9_]*!' "$WATCH_DIR/" | grep -v '!=' | grep -v '//' | head -20

---

Skip Patterns

• *Tests*/, *UITests*/ — expected to crash on failure
• #Preview { } blocks — development only
• @IBOutlet force unwraps — UIKit pattern
• Hardcoded valid URLs: URL(string: "https://...")!

---

Output Format

| Category | Issues | Status | |----------|--------|--------| | fatalError/precondition | X | ✅/❌ | | Force unwraps (!) | X | ✅/❌ | | Force try (try!) | X | ✅/❌ | | Empty catch blocks | X | ✅/❌ | | Task.detached/continuations | X | ✅/❌ | | Missing @MainActor | X | ✅/❌ | | User error states | X | ✅/❌ | | Watch app | X | ✅/❌ |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: mwd1234
• Source: mwd1234/ios-agentic-skills
• License: MIT

Install and usage instructions live in the source repository linked above.