In Android Activities and Services, most callbacks are run on the main thread. This makes it simple to update the UI, but running processor- or I/O-heavy tasks on the main thread can cause your UI to pause and become unresponsive (official documentation on what then happens).

You can remedy this by putting these heavier tasks on a background thread.

One way to do this is using an AsyncTask, which provides a framework to facilitate easy usage of a background Thread, and also perform UI Thread tasks before, during, and after the background Thread has completed its work.

Methods that can be overridden when extending AsyncTask:

• onPreExecute() : invoked on the UI thread before the task is executed
• doInBackground(): invoked on the background thread immediately after onPreExecute() finishes executing.
• onProgressUpdate(): invoked on the UI thread after a call to publishProgress(Progress...).
• onPostExecute(): invoked on the UI thread after the background computation finishes

Example

public class MyCustomAsyncTask extends AsyncTask<File, Void, String> {

    
    @Override
    protected void onPreExecute(){
        // This runs on the UI thread before the background thread executes.
        super.onPreExecute();
        // Do pre-thread tasks such as initializing variables. 
        Log.v("myBackgroundTask", "Starting Background Task");  
    }

    @Override
    protected String doInBackground(File... params) {
        // Disk-intensive work. This runs on a background thread.
        // Search through a file for the first line that contains "Hello", and return
        // that line.
        try (Scanner scanner = new Scanner(params[0])) {
            while (scanner.hasNextLine()) {
                final String line = scanner.nextLine();
                publishProgress(); // tell the UI thread we made progress

                if (line.contains("Hello")) {
                    return line;
                }
            }
            return null;
        }
    }

    @Override
    protected void onProgressUpdate(Void...p) {
        // Runs on the UI thread after publishProgress is invoked
        Log.v("Read another line!")
    }        

    @Override
    protected void onPostExecute(String s) {
        // This runs on the UI thread after complete execution of the doInBackground() method
        // This function receives result(String s) returned from the doInBackground() method.
        // Update UI with the found string.
        TextView view = (TextView) findViewById(R.id.found_string);
        if (s != null) {
            view.setText(s);
        } else {
            view.setText("Match not found.");
        }
    }

}

Usage:

MyCustomAsyncTask asyncTask = new MyCustomAsyncTask<File, Void, String>();
// Run the task with a user supplied filename.
asyncTask.execute(userSuppliedFilename);

or simply:

new MyCustomAsyncTask().execute(userSuppliedFilename);

Note

When defining an AsyncTask we can pass three types between < > brackets.

Defined as <Params, Progress, Result> (see Parameters section)

In the previous example we’ve used types <File, Void, String>:

AsyncTask<File, Void, String>
// Params has type File
// Progress has unused type
// Result has type String

Void is used when you want to mark a type as unused.

Note that you can’t pass primitive types (i.e. int, float and 6 others) as parameters. In such cases, you should pass their wrapper classes, e.g. Integer instead of int, or Float instead of float.

The AsyncTask and Activity life cycle

AsyncTasks don’t follow Activity instances’ life cycle. If you start an AsyncTask inside an Activity and you rotate the device, the Activity will be destroyed and a new instance will be created. But the AsyncTask will not die. It will go on living until it completes.

Solution: AsyncTaskLoader

One subclass of Loaders is the AsyncTaskLoader. This class performs the same function as the AsyncTask, but much better. It can handle Activity configuration changes more easily, and it behaves within the life cycles of Fragments and Activities. The nice thing is that the AsyncTaskLoader can be used in any situation that the AsyncTask is being used. Anytime data needs to be loaded into memory for the Activity/Fragment to handle, The AsyncTaskLoader can do the job better.