Calling an AsyncTask twice behavior

Question

I am trying to implement a search bar that automatically searches as you type.

My idea is to have an AsyncTask which fetches the search data from the server, but I can't figure how exactly AsyncTask will behave with my use of it.

Let's say I have SearchAsyncTask.

Every time the text field is edited I will call

new SearchAsyncTask().execute(params);

So here's my question: what will the behavior of this be? Will I be starting many different threads which will all return and call onPostExecute()? Or will the first task called be stopped mid-task if another instance is called while it's still working? Or something totally different?

What if I write it this way?

SearchAsyncTask a = new SearchAsyncTask().execute(params);
...
a.execute(params2);
a.execute(params3);
...

score 5 · Accepted Answer · answered Jul 25 '13 at 17:04

I have implemented my app's search function in the same way. I use a TextWatcher to build search results as and when the user types. I keep a reference of my AsyncTask to achieve this. My AsyncTask declaration:

SearchTask mySearchTask = null;  // declared at the base level of my activity

Then, in the TextWatcher, on each character input, I do the following:

// s.toString() is the user input
if (s != null && !s.toString().equals("")) {

    // User has input some characters

    // check if the AsyncTask has not been initialised yet
    if (mySearchTask == null) {

        mySearchTask = new SearchTask(s.toString());

    } else {

        // AsyncTask is either running or has already finished, try cancel in any case
        mySearchTask.cancel(true);

        // prepare a new AsyncTask with the updated input            
        mySearchTask = new SearchTask(s.toString());

    }

    // execute the AsyncTask                
    mySearchTask.execute();

} else {

    // User has deleted the search string // Search box has the empty string "" now

    if (mySearchTask != null) {

        // cancel AsyncTask 
        mySearchTask.cancel(true);

    }

    // Clean up        

    // Clear the results list
    sResultsList.clear();

    // update the UI        
    sResultAdapter.notifyDataSetChanged();

}

score 0 · Answer 2 · edited May 23 '17 at 11:52

0

Regarding your second question, I believe you could have found your answer on this site, here for instance. Basically, you can only execute a single instance of an AsyncTask once. Trying to execute twice the same instance will raise an error.

edit : If you declare each time a new SearchAsyncTask(), you will receive numerous responses and call to onPostExecute(), but not necessarily in the right order. What would be best is to store the AsyncTask in a variable, and use the .cancel() method before starting a new one.

edited May 23 '17 at 11:52

Community

1
1

answered Jul 25 '13 at 16:41

Nerkatel

1,805
16
25

This doesn't really answer the question. In the first part of the question he is not executing the same instance. But it does answer the second question – codeMagic Jul 25 '13 at 16:47

Calling an AsyncTask twice behavior

2 Answers2