Answer

The Short Answer

Your app crashes with a CalledFromWrongThreadException:

android.view.ViewRootImpl$CalledFromWrongThreadException: 
Only the original thread that created a view hierarchy can touch its views.

Why This Happens

Android's View system is not thread-safe. If multiple threads modify views simultaneously:

• UI could be in inconsistent state
• Drawing could be corrupted
• Race conditions everywhere

So Android enforces that only the main thread touches views.

Code That Crashes

Thread {
    val data = fetchDataFromNetwork()
    textView.text = data  // 💥 CRASH!
}.start()

The Check Mechanism

// Inside View's internal code
void checkThread() {
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
            "Only the original thread that created a view hierarchy can touch its views."
        );
    }
}

How to Fix

1. runOnUiThread()

Thread {
    val data = fetchData()
    runOnUiThread {
        textView.text = data  // ✅ Safe
    }
}.start()

2. Handler

val handler = Handler(Looper.getMainLooper())
Thread {
    val data = fetchData()
    handler.post {
        textView.text = data  // ✅ Safe
    }
}.start()

3. View.post()

Thread {
    val data = fetchData()
    textView.post {
        textView.text = data  // ✅ Safe
    }
}.start()

4. Coroutines (Best)

lifecycleScope.launch {
    val data = withContext(Dispatchers.IO) {
        fetchData()
    }
    textView.text = data  // ✅ Safe, already on Main
}

5. LiveData

// In ViewModel
_liveData.postValue(data)  // Thread-safe

// In Activity (observes on main thread)
liveData.observe(this) { textView.text = it }

Exception: Some Methods Are Thread-Safe

A few View methods can be called from any thread:

• View.postInvalidate()
• View.postInvalidateDelayed()
• View.post()
• View.postDelayed()

These internally post to the main thread's Handler.

Key Takeaway

Always update UI from the main thread. Use the modern tools (Coroutines, LiveData) that handle this automatically.