CODEWITHSUNDEEP
androidandroid-asynctask
Kevin Bradshaw
Kevin Bradshaw

Reputation: 6427

How to check if Async Task is already running

I have an app that needs to do an intensive database operation on start up. The app holds a local copy of the contacts on the phone and synchronizes with the android contact database on startup.

If a user starts the app, an Async Task is started that does the database synch in the background. If the user closes the app, the operation continues running which is fine. However if the user opens the app again, the Async Task is started and an error is produced.

Is there anyway of checking if the Task is already running from a different instance of the app?

Upvotes: 34

Views: 55247

Answers (3)

Jeremy Heitz
Jeremy Heitz

Reputation: 71

I've managed to handle this problem with some sort of Singleton pattern. Hope it helps.

// fill the places database from a JSON object
public class myAsyncTask extends AsyncTask<Void,Integer,Integer> {

    Activity mContext = null;
    static AsyncTask<Void,Integer,Integer> myAsyncTaskInstance = null; 

    // Private Constructor: can't be called from outside this class
    private myAsyncTask(Activity iContext) {
        mContext = iContext; 
    }

    public static AsyncTask<Void, Integer, Integer> getInstance(Activity iContext) {
        // if the current async task is already running, return null: no new async task 
        // shall be created if an instance is already running
        if (myAsyncTaskInstance != null && myAsyncTaskInstance.getStatus() == Status.RUNNING) {
            // it can be running but cancelled, in that case, return a new instance
            if (myAsyncTaskInstance.isCancelled()) {
                myAsyncTaskInstance = new myAsyncTask(iContext);
            } else {
                // display a toast to say "try later"
                Toast.makeText(iContext, "A task is already running, try later", Toast.LENGTH_SHORT).show();    

                return null;
            }
        }

        //if the current async task is pending, it can be executed return this instance
        if (myAsyncTaskInstance != null && myAsyncTaskInstance.getStatus() == Status.PENDING) {
            return myAsyncTaskInstance;
        }

        //if the current async task is finished, it can't be executed another time, so return a new instance
        if (myAsyncTaskInstance != null && myAsyncTaskInstance.getStatus() == Status.FINISHED) {
            myAsyncTaskInstance = new myAsyncTask(iContext);
        }


        // if the current async task is null, create a new instance
        if (myAsyncTaskInstance == null) {
            myAsyncTaskInstance = new myAsyncTask(iContext);
        }
        // return the current instance
        return myAsyncTaskInstance;
    }

    @Override
    protected Integer doInBackground(Void... iUnUsed) {
        // ...
    }
}

Upvotes: 7

AlexN
AlexN

Reputation: 2554

I think you should check the concept of Application in Android. http://developer.android.com/reference/android/app/Application.html

In fact there is no such thing as

different instance of the app

. The Application is always the same for all your Activities/Services. That means that you'd left the Activity and opened it again, 2 cases are possible:

  1. The system already killed your application. In this case AsyncTask is dead already and it's safe to start a new one
  2. The Application was still alive, so AsyncTask possibly still running.

In 2nd case I will recommend to use some static variables, pointing to this AsyncTask or it's state. If your app was still alive when 2nd time opened - all static references will be still valid, so you can successfully operate.

PS: By the way, in current approach be aware that your application can be terminated by the system at any time. So AsyncTask can be interrupted in any moment. It it's not ok for you - please check IntentServices - components, specially designed for background-operation purpose. http://developer.android.com/reference/android/app/IntentService.html

Good luck!

Upvotes: 3

Ovidiu Latcu
Ovidiu Latcu

Reputation: 72331

Use getStatus() to get the status of your AsyncTask. If status is AsyncTask.Status.RUNNING then your task is running.

EDIT: you should reconsider your implementation and hold the AsyncTask probably in a Service or IntentService to fetch your data from the web.

Upvotes: 97

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