Difficult issue when implementing multithreading for a function which returns a value

Question

TokenCache class is passed to Facebook class to do save() and load() tokens. TokenCache has these functions:

public abstract void save(Bundle bundle);
public abstract Bundle load();

Now I want to do that saving and loading in multiple threads, so I run a AsyncTask in load(). The problem is it was already designed to return a Bundle! How to implement it so that the value only returned when the AsyncTask finishes?

If it only returned a value when it finished, it wouldn't really be an AsyncTask, now, would it? What you need to do is refactor it such that it takes in an anonymous class with a single method that accepts that return value as an argument, or some other such mechanism for handling the result asynchronously. — jonvuri
– jonvuri, Commented Nov 30, 2012 at 3:23
yes, it's function which returns value. It's designed as abstract for developer to give implementation for it. The problem is the loading could be a long-time operation. — emeraldhieu
– emeraldhieu, Commented Nov 30, 2012 at 3:26

LuxuryMode · Accepted Answer · 2012-11-30 03:42:15Z

1

Simply have a field of Bundle in your TokenCache implementation that your load() method returns. Just make sure you don't yet pass a reference to the TokenCache to FB until that Bundle is initialized by some init(bundle) method, e.g.

  static class BundleRetrievalTask extends AsyncTask<String, Void, Bundle> {

        @Override
        protected Bundle doInBackground(String... strings) {
            return someLongOperationThatFetchesBundle(strings[0]);
        }

        @Override
        protected void onPostExecute(Bundle bundle) {
            super.onPostExecute(bundle);
            MyFacebookTokenCache tokenCache = MyFacebookTokenCache.init(bundle);
            doSomethingWithCache(tokenCache);

        }
    }

    static class MyFacebookTokenCache extends TokenCache {

        private Bundle mBundle;

        public MyFacebookTokenCache(Bundle bundle) {
            mBundle = bundle;
        }

        public static MyFacebookTokenCache init(Bundle bundle) {
            return new MyFacebookTokenCache(bundle);
        }

        @Override
        public Bundle load() {
            return mBundle;
        }

        @Override
        public void save(Bundle bundle) {
            //todo
        }

        @Override
        public void clear() {
            //todo
        }
    }

edited Nov 30, 2012 at 3:42

answered Nov 30, 2012 at 3:33

LuxuryMode

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5 Comments

emeraldhieu Over a year ago

Thank you, but Facebook uses Bundle bundle = tokenCache.load(); everywhere in its code. Do I have to replace all of them by calling your Task everywhere? It works but not really what I want.

LuxuryMode Over a year ago

Who cares that FB uses it everywhere. The only instance of a TokenCache that FB has is yours where the bundle is initialized already.

LuxuryMode Over a year ago

Also, can you explain to us the mysterious reason why the bundle needs to be retrieved asynchronously?

emeraldhieu Over a year ago

^ Because I want to get it from server, it's a long-time operation. BTW Where could I call BundleRetrievalTask?

LuxuryMode Over a year ago

@Emerald214 wherever it's relevant ;) Create that class and call new BundleRetrievalTask.execute(someString)//or nothing if you don't need a string..

TieDad · Accepted Answer · 2012-11-30 03:35:53Z

1

You might use a callback mechanism. In the AsyncTask, after load() finished, call a callback method to do operation based on the returned bundle.

edited Nov 30, 2012 at 3:35

answered Nov 30, 2012 at 3:25

TieDad

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1 Comment

emeraldhieu Over a year ago

If I implement like that, the load() function will be meaningless. Because the work TokenCache does is loading, so I can't give its task for an outside AsyncTask as you said.

Ponyets · Accepted Answer · 2012-11-30 03:48:43Z

0

public static BundleAsyncTask extends AsyncTask<Void, Void, Void>{
    public Bundle result;
    private final CountDownLatch latch;
    public BundleAsyncTask(CountDownLatch latch){
        this.latch = latch;
    }

    public void doInBackground(){
        //get the bundle
        latch.countDown();
    }
}

public Bundle load(){
    final CountDownLatch latch = new CountDownLatch(1);
    BundleAsyncTask task = new BundleAsyncTask(latch);
    task.execute();
    try{
        latch.await();
        return task.result;
    }catch(InterruptedException e){
        return null;
    }
}

btw: I don't actually understand why you want to use AsyncTask here.

answered Nov 30, 2012 at 3:48

Ponyets

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1 Comment

Ponyets Over a year ago

Not a joke. I just answer the question though I don't know why use AsyncTask here. If you want to use AsyncTask(I mean the mechanism), I think this is the only way.

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