How does AsyncTask work and why is it deprecated?

Copy codeclass DownloadTask : AsyncTask<URL, Int, String>() {
    
    override fun onPreExecute() {
        // Runs on UI thread - show loading
        progressBar.visibility = View.VISIBLE
    }
    
    override fun doInBackground(vararg urls: URL): String {
        // Runs on background thread
        var progress = 0
        urls.forEach { url ->
            downloadFile(url)
            progress += 25
            publishProgress(progress)  // Update progress
        }
        return "Complete"
    }
    
    override fun onProgressUpdate(vararg values: Int) {
        // Runs on UI thread
        progressBar.progress = values[0]
    }
    
    override fun onPostExecute(result: String) {
        // Runs on UI thread
        progressBar.visibility = View.GONE
        textView.text = result
    }
}

// Usage
DownloadTask().execute(url1, url2, url3)

Copy codeUI Thread:    onPreExecute() ──────────────────────── onPostExecute()
                     ↓                                       ↑
Background:         doInBackground() ─ publishProgress() ───┼
                                              ↓              │
UI Thread:                            onProgressUpdate() ────┘

Copy codeclass MyActivity : Activity() {
    inner class MyTask : AsyncTask<...>() {
        // Holds implicit reference to Activity
        // Activity can't be garbage collected during task
    }
}

Copy code// Activity rotates while task is running
// Task completes, tries to update destroyed Activity → Crash!

Copy codetask.cancel(true)
// But doInBackground keeps running unless you check isCancelled()

Copy code// Doesn't know about Activity/Fragment lifecycle
// Can cause crashes if UI is gone

Copy code// Tasks run one at a time on single background thread
// Can cause unexpected delays

Copy codelifecycleScope.launch {
    progressBar.visibility = View.VISIBLE
    val result = withContext(Dispatchers.IO) {
        downloadFile(url)
    }
    progressBar.visibility = View.GONE
    textView.text = result
}

Copy codeObservable.fromCallable { downloadFile(url) }
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe { result -> textView.text = result }

Copy code// Execute on background, post to LiveData