What happens if you update UI from a background thread?

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

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

Copy code// 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."
        );
    }
}

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

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

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

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

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

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