为什么 AsyncTask doInBackground 不运行但第二次运行

Question

So I am new with this and I need to call function that would run AsyncTask from separate file.

MainActivity.java code

public class MainActivity extends AppCompatActivity {
String res;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Asynctasktest newAsy = new Asynctasktest();



    res = newAsy.ValidateUser();

    Toast.makeText(getBaseContext(), "'"+res+"'", Toast.LENGTH_LONG).show();
}

}

and AsyncTaskTest.java code

public class Asynctasktest extends MainActivity {

String Res;

private class GetFWork extends AsyncTask<Void,Void,String> {

    @Override
    protected void onPreExecute() {
        Res = "onPreExecute";
    }

    @Override
    protected String doInBackground(Void... param) {

        return "am i here";
    }

    @Override
    protected void onPostExecute(String Result) {
        super.onPostExecute(Result);
        Res = Result;
    }
}

public String ValidateUser(){

    final GetFWork Fl = new GetFWork();

    Fl.execute();

    return Res;
}


}

So I need to get text "am i here", but I get "onPreExecute", which shows that on first call it doesnt do doInBackground.

Answer 1

This is how AsyncTask works.

res = newAsy.ValidateUser();
Toast.makeText(getBaseContext(), "'"+res+"'", Toast.LENGTH_LONG).show();

are both executed at the same time, and thus - since AsyncTask is asynchronous - the value of res is still "onPreExecute".

Answer 2

Good point made by Wukash. Just to elaborate. When ValidateUser() is returning Res , there is no guarantee that the AsyncTask has completed running, since, its asynchronous, and runs off the main thread. Hence, in your case, it returns null , since Res never got updated.

If you require returning the value result to update UI or something, send a handler function reference as callback, and make that do this work.

Please let me know if you need some further explanation. I recall having had a similar question when I was implementing an AysncTask for the first time.

Answer 3

Try calling the super in this method

@Override
protected void onPreExecute() {
    super.onPreExecute();
    Res = "onPreExecute";
}

Also remember that you cannot call the same AsyncTask object twice or you'll get this error:

Cannot execute task: the task has already been executed (a task can be executed only once)

So the approach you have in your ValidateUser() method creating a new GetFWork() is good.