在异步任务或异步任务上执行多个任务，并从执行后返回数据

Question

I have an API that I use to retrieve daily schedules on the live cable-tv for various channels. I have a scenario in which I need a guidance as to which approach should work here.

Lets say I need schedules for 10 different channels from the API.

• Should I execute 10 different async tasks for the retrieval of the required data?

Problem: How would I collect the data in an arraylist and return it once all execution is completed? How will I access the arraylist in my main function once onpostexecute returns the result?

• Or I should just provide the list of channels to my single async task and make it build a single output of arraylist for my main function invoking it?

Problem: Since I will be accessing a webservice for this purpose, will it make it run slow as compared to my 1st approach? Second problem with this approach is the same as I am having with my 1st one, I need to know when and how to get the complete resultset once the execution of the task is completed?

Here is some code to explain the problem:

//going with the first approach
//invoking my asynctask from an activity or another class

//I need a global arraylist which I can use after postexecute returns its result
ArrayList<String> channels = channelManager.getAllChannelsByRegion("xyz");
        final ArrayList<ChannelSchedule> schedules = new ArrayList<ChannelSchedule>();
        final ObjectMapper mapper = new ObjectMapper(); // can reuse, share globally
        for (int i = 0; i < channels.size(); ++i){
            AsyncInvokeURLTask task = null;
            try {
                task = new AsyncInvokeURLTask(
                    channels.get(i), context, new AsyncInvokeURLTask.OnPostExecuteListener() {
                        @Override
                        public void onPostExecute(String result) {
                            // TODO Auto-generated method stub                                                          
                            try {
//Need to add results to arraylist here...But cannot know when it ends completely
                                ChannelSchedule schedule = mapper.readValue(result, ChannelSchedule.class);
                                Log.v("channel name", schedule.getChannelName());
                                Log.v("channel date", schedule.getDate());
                                Log.v("channel thumb", schedule.getListOfShows().get(0).getShowThumb());
                                Log.v("channel time", schedule.getListOfShows().get(0).getShowTime());
                            } catch (JsonParseException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            } catch (JsonMappingException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            } catch (IOException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }
                        }
                    });
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            task.execute();
        }

Please let me know if something is not clear or missing.

Answer 1

Launching 10 AsyncTask is perfectly fine.

You can keep a count of the number of pending requests. As OnPostExecute is run on the UI thread there are no risks of race condition.

private int numberOfPendingRequests;

public void MyFunc() {
  ArrayList<String> channels = channelManager.getAllChannelsByRegion("xyz");
  final ArrayList<ChannelSchedule> schedules = new ArrayList<ChannelSchedule>();
  final ObjectMapper mapper = new ObjectMapper(); // can reuse, share globally
  numberOfPendingRequests = channels.size();
  for (int i = 0; i < channels.size(); ++i) {
    schedules.add(null);
  }
  for (int i = 0; i < channels.size(); ++i) {
    AsyncInvokeURLTask task = null;
    final int index = i;  // final so it can be used in the onPostExecute.
    try {
      task = new AsyncInvokeURLTask(
        channels.get(i), context, new AsyncInvokeURLTask.OnPostExecuteListener() { 
          @Override public void onPostExecute(String result) {
            try {
              ChannelSchedule schedule = mapper.readValue(result, ChannelSchedule.class);
              Log.v("channel name", schedule.getChannelName());
              Log.v("channel date", schedule.getDate());
              Log.v("channel thumb", schedule.getListOfShows().get(0).getShowThumb());
              Log.v("channel time", schedule.getListOfShows().get(0).getShowTime());
              schedules.set(index, schedule);
              numberOfPendingRequests--;
              if (numberOfPendingRequests == 0) {
                // Everything is received, do stuff here.
              }
            } catch (JsonParseException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (JsonMappingException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
          }
       });
    } catch (Exception e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
    task.execute();
  }
}