App Hangs in iOS: Causes, Code Fixes, and How to Spot Them

Ever tapped a button in your app and waited... and waited... until you started questioning your life choices?

Yeah, that's an app hang.

It's not a crash. It's worse. Your app doesn't explode, it just freezes. Quietly. Awkwardly. Like someone forgot their lines on stage and now the whole audience is staring.

App hangs are sneaky. They don't always show up in crash reports. But your users feel them. In the lags, the unresponsive screens, the moments when they swipe but nothing moves. And if it happens too often? That uninstall button starts looking real attractive.

But it doesn't have to be that way.

Let's fix the freeze before the curtain falls.

1. Heavy Work on the Main Thread

The main thread in iOS has one core responsibility: keeping the user interface responsive. You know — reacting to taps, updating views, making the app feel alive.

So when you throw a massive task onto the main thread — like downloading a huge image, parsing a 10,000-row JSON, or calculating the meaning of life — it chokes.

Suddenly your app stops responding. And for the user, it's like texting someone who left you on read.

The Fix? Offload that weight.

Here's how to do it using Grand Central Dispatch (GCD):

DispatchQueue.global(qos: .background).async {
    // Do the heavy lifting here — maybe a network call or data parsing
    let result = performHeavyTask()
    
    DispatchQueue.main.async {
        // Now update the UI
        self.resultLabel.text = result
    }
}

Don't: Perform expensive tasks on the main thread.
Do: Offload them to a background queue, then return to the main thread when updating the UI.

---

2. CPU or Memory Overload

Think of your app like a stage actor again. Give it too many lines, a costume change every second, and zero rehearsal time. It's going to collapse.

The same thing happens when your app consumes way too much memory or CPU. This can come from memory leaks, retain cycles, or just plain inefficient code.

And once it runs out of breath? Hangs. Crashes. The curtain falls.

The Fix? Time to get nosy with Xcode Instruments.

Steps:

1. Open your project in Xcode.
2. Run Instruments → choose Time Profiler or Leaks.
3. Analyze CPU spikes or retained objects that shouldn't still be there.

Tools like Appxiom and Instruments will highlight areas of your code that are hogging resources so you can clean up inefficient code and optimize performance.

---

3. UI-Blocking Operations

Ever seen someone freeze mid-sentence while loading a YouTube video? That's what your app does when it waits synchronously for something, like a network request or file read — on the main thread.

It blocks everything. The UI can't animate. Taps aren't registered. It's like your app is giving you the silent treatment while it figures its life out.

Fix it with async goodness:

URLSession.shared.dataTask(with: url) { data, response, error in
    DispatchQueue.main.async {
        // Update your UI here
        self.imageView.image = UIImage(data: data!)
    }
}.resume()

Or go modern with async/await:

func loadImage() async {
    do {
        let (data, _) = try await URLSession.shared.data(from: url)
        imageView.image = UIImage(data: data)
    } catch {
        print("Failed to load image: \(error)")
    }
}

Your users will never even know something heavy happened in the background. Smooth UX = happy users.

---

4. Deadlocks & Race Conditions

Concurrency issues are like love triangles. You've got threads waiting on each other, no one backing down, and the whole thing ends in a deadlock.

• A deadlock happens when two or more threads wait on each other forever.
• A race condition is when threads compete to change data at the same time, causing unpredictable results.

Both can cause hangs that are tough to trace, like mysterious silences in a group chat that no one admits to causing.

The Fix? Defensive coding + tools.

• Use serial queues for shared resources.
• Apply locks carefully. Don't overuse them.
• Try DispatchSemaphore, NSLock, or DispatchBarrier for control.
• And let Thread Sanitizer in Xcode spot the hidden chaos.

let lock = NSLock()

func safeIncrement() {
    lock.lock()
    counter += 1
    lock.unlock()
}

Don't let threads play dirty. Keep the peace with proper synchronization.

---

5. Infinite Loops

Ever listened to someone rant for hours without pause? That's what an infinite loop feels like in your app.

If your code enters a loop without a proper exit condition, it just keeps going. The app hangs. The processor cries. Your user force-quits.

Example:

while isRunning {
    // doing something
}

If isRunning is never set to false, your app's trapped in its own thoughts — forever.

The Fix?

• Double-check your loop conditions.
• Add breakpoints and logging.
• Always ensure your loop has a valid exit path.

---

Bonus: Use APM Tools to Catch the Freeze

Even with all your care, hangs can sneak in. That's why tools like Appxiom and Firebase Performance Monitoring are your backstage crew.

• Firebase gives you metrics like slow frames, frozen frames, and detailed session data.
• Appxiom tracks hangs in real time, highlights offending code paths, and shows the business impact, so you know exactly what to fix and why.

These tools don't just monitor — they help you debug, optimize, and understand the hang's root cause.

---

Curtain Call: Final Thoughts

The bad news? App hangs are more than technical hiccups. They're silent moments that break trust with your users. And in a world of instant swipes and short attention spans, you don't get many second chances.

The good news? You can fix them.

Offload heavy work, tame your threads, profile religiously, and keep your loops in check.

And let smart tools like Appxiom help you spot trouble before users even notice.

So the next time your app forgets its lines, you'll know exactly what to rewrite. Give your users the smooth performance they deserve.

No awkward silences. No hangs. Just flow.