Why does my AsyncTask freeze my UI Thread?

Issue

I have this problem: I want, when I insert and submit my data in a form, that an AsyncTask starts checking that everything is ok. In the specific case I am facing now, I want to verify that the username entered has not already been used, so I would like it to go to the server and via php to query the user's database. I wish that when the whole thing starts, the user screen is not blocked, that is, I would like to allow the user to move inside the screen anyway.

I enclose the code:

public class ControlloUsername extends AsyncTask<String, Void, String> {

AlertDialog a;
Context context;

public ControlloUsername(Context ct) {
    // TODO Auto-generated constructor stub
    context = ct;
}

@Override
protected void onPreExecute() {
    // TODO Auto-generated method stub
}

@Override
protected void onPostExecute(String result) { //sync
    // TODO Auto-generated method stub
}

@Override
protected void onProgressUpdate(Void... values) {
    // TODO Auto-generated method stub
}

@Override
protected String doInBackground(String... params) { //sync
    // TODO Auto-generated method stub

    String username = params[0];
    String login_url = Connector.db + "script/usercontrol.php";
    String resultado = "";

    URL u = null;
    try {
        u = new URL(login_url);
    } catch (MalformedURLException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = writer.toString();
    }
    HttpURLConnection http = null;
    try {
        http = (HttpURLConnection) u.openConnection();
    } catch (IOException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = writer.toString();
    }
    try {
        http.setRequestMethod("POST");
    } catch (ProtocolException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = writer.toString();
    }
        http.setDoInput(true);
        http.setDoOutput(true);


        http.setConnectTimeout(7000);


    try {
        OutputStream out = http.getOutputStream();
        BufferedWriter bf = new BufferedWriter(new OutputStreamWriter(out, "UTF-8"));
        String post_data = URLEncoder.encode("username", "UTF-8") + "=" + URLEncoder.encode(username, "UTF-8");
        bf.write(post_data);
        bf.flush();
        bf.close();
        out.close();
        InputStream in = http.getInputStream();
        BufferedReader br = new BufferedReader(new InputStreamReader(in, "iso-8859-1"));
        String result = "";
        String line = "";
        while ((line = br.readLine()) != null) {
            result += line;
        }
        bf.close();
        in.close();
        http.disconnect();
        return result;
    } catch (ConnectException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = "CONNECT" + writer.toString();
        Log.d("PRINT", "CONNECT");
    } catch (UnsupportedEncodingException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = "UNSUPPORTEDENCODING" + writer.toString();
        Log.d("PRINT", "UNSUPPORTEDENCODING");
    } catch (ProtocolException e) {
       Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = "PROTOCOL" + writer.toString();
        Log.d("PRINT", "PROTOCOL");
    } catch (MalformedURLException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = "MALFORMEDURL" + writer.toString();
        Log.d("PRINT", "MALFORMEDURL");
    } catch (IOException e) {
        Writer writer = new StringWriter();
        e.printStackTrace(new PrintWriter(writer));
        resultado = "IO" + writer.toString();
        Log.d("PRINT", "IO");
    }
    return resultado;
}
}

RegisterActivity

String u = user.getText().toString();
            ControlloUsername bg = new ControlloUsername(this);
            try {
                String res = bg.execute(u).get();
                Log.d("PRINT", res);
                if (res.contains("...

I've read that it's a problem of execute().get () and the AsyncTask's own methods.

Solution

You can use an interface to get the result back after your AsyncTask finishes. Inside your AsyncTask:

onFinishListener mListener; // Create a variable for your interface

// Define the interface & callbacks
public interface onFinishListener{
    // Replace 'variableType' with the appropriate type for your result
    void onFinish(variableType myResult);
}

public void setOnFinishListener(onFinishListener l){
    mListener = l;
}

@Override
protected void onPostExecute(variableType result){
    if(mListener != null){
        mListener.onFinish(result);
    }
}

Then inside of your Activity, make sure to set the interface:

MyAsyncTask task = new MyAsyncTask();
task.execute();
task.setOnFinishListener(new MyAsyncTask.onFinishListener(){
    @Override
    public void onFinish(variableType myResult){
        // The task has finished and you can now use the result
    }
});

Answered By - Sammy T