updating UI thread while in asynctask

Question

as the title states, im trying to update something in my UI thread while running an asynctask.. i've read quite a bit on asynctask and it seems i should be able to change a variable from the onPostExecute() method. obviously this is not the case.

Here is my sample code:

TextView tv = (TextView) findViewById(R.id.thingsThatNeedToBeUpdated); 
Login login = new Login();
login.execute(userName, password);

and here the the login class

public class Login extends AsyncTask<String, void, String>{
    public String doInBackground(String... params){
       logMeIn(params[0], params[1]);
    }
    public void onPostExecute(String update){
       tv.setText(result); //this is not working!!
    }

whats actually happening is tv is underlined red and eclipse says i need to create a local variable.. but i thought the onPostExecute is ran from the UI thread? confused :?
im trying to do what i found at this website. I'm not entirly sure what i'm doing and i would love a point in the right direction! thanks in advance.

Where are you instantiating tv? Because findViewById() will return null in your activesync surely? — Rudi Kershaw, Mar 07 '14 at 18:39
To be honest to point you towards the right direction you should read a good Java Book, seems like you have no idea of variables scope and that's basic before moving forward in Android... — Martin Cazares, Mar 07 '14 at 18:39
It runs on the UI thread, but those UI elements you are trying to access are defined in another class or the references to them are local variables, — NameSpace, Mar 07 '14 at 18:40
ok, so i can't run this from a different .java class? my understanding of scope is fine, it's my lack of understanding of nested classes that seems to be the trouble. thank you all. — soricellia, Mar 07 '14 at 18:47
Ahhhh, you have it in a nested class. Could of helped us to know that in the question. :p — Rudi Kershaw, Mar 07 '14 at 18:52
Check this post for your answer. http://stackoverflow.com/questions/22234924/having-trouble-with-async-task-and-flow-of-program/22235712#22235712 — Sachin Shelke, Mar 07 '14 at 20:09
thank you, i resolved this and accepted the answer i used to fix this problem! — soricellia, Mar 19 '14 at 15:28

score 1 · Accepted Answer · edited May 23 '17 at 11:57

This line here

TextView tv = (TextView) findViewById(R.id.thingsThatNeedToBeUpdated);

is obviously declared either

Outside of a method -which would result in tv being null or
In a method that isn't part of the AsyncTask- which would mean the task doesn't have access to it

You should define it as a member variable (outside of a method)

TextView tv;

then initialize it inside of a method

tv = (TextView) findViewById(R.id.thingsThatNeedToBeUpdated);

This will give your task access to it as well as the rest of your class.

If your AsyncTask is a separate file than your Activity then you will want to see this answer on using an interface and create a callback to update the TextView in your Activity.

This helped a lot. My AsyncTask was in its own .java file, so my problem was a scope issue. I solved this by passing my textView into a constructor for my AsyncTask class. Thank you!! — soricellia, Mar 19 '14 at 15:27

Rudi Kershaw · Answer 2 · 2014-03-17T08:28:53.517

To bring your TextView instance into scope, pass your tv variable into your Login instance constructor and add the constructor and variable in as below.

public class Login extends AsyncTask<String, void, String>{

    private TextView tv;

    public Login(TextView tv){
       this.tv = tv;
    }
    public String doInBackground(String... params){
       logMeIn(params[0], params[1]);
    }
    public void onPostExecute(String update){
       tv.setText(result); //this should work
    }
}

You have to pass the variable to this instance, otherwise it is out of scope. You won't be able to access it unless you declare and instantiate a new variable within the Login class.

score 0 · Answer 3 · answered Mar 07 '14 at 18:46

0

Place TextView tv; into the scope of the class and outside of the function. Leave tv = (TextView) findViewById(R.id.thingsThatNeedToBeUpdated); where it is. You should then be able to access tv from onPostExecute().

answered Mar 07 '14 at 18:46

BVB

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score 0 · Answer 4 · answered Mar 07 '14 at 18:49

This is because the TextView variable tv is defined in another class that does not have Login as an inner class. If Login is an inner class of the class containing the variable tv, it should be able to see the variable tv.

An easy way to solve this would be to make your AsyncTask an inner class of your main class.

updating UI thread while in asynctask

4 Answers4