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How to pass a value from a runnable thread once it has finished in Android Studio?

I am using a thread to get an id from my database. Once the thread has finished running I need to use that id in another thread to get a list of items. Once that thread has finished I use the list of items to populate a spinner. The problem I am encountering is that it is populating the spinner before the thread has returned the list of items. How can I make it so the spinner doesn’t get populated until the thread has finished running?

        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

                new Thread(new Runnable() {
                    @Override
                    public void run() {

                        String bookName = existingBooks.get((int) spinner.getSelectedItemId());

                        Book myBook = AppDatabase.getInstance(getContext()).bookDAO().getBookByTitle(bookName);

                        long book_Id = myBook.getBook_id();

                        existingCountDates = AppDatabase.getInstance(getContext()).countDAO().getCountDatesInBook(book_Id);

                        List<Count> Counts = AppDatabase.getInstance(getContext()).countDAO().getAllCounts();

                        Log.e("Dates", existingCountDates.toString());
                        Log.e("All Counts", Counts.toString());

                    }
                }).start();

                //TODO: FIX THIS ISSUE
                Log.e("Problem:", "This gets called before the thread finishes. Not sure how to fix it");
                str_adapter.clear();
                for (int i = 0; i < existingCountDates.size(); i++) {
                    str_adapter.add(existingCountDates.get(i));
                }
                str_adapter.notifyDataSetChanged();

            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {
                //TODO: DISABLE OTHER DROPDOWN MENUS?
            }
        });

enter image description here

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3

Answers


  1. 0

    You can use another thread and wait previous thread till finish using thread.join(). bu better version is use ExecutorServices such as for instance the ThreadPoolExecutor.

    CODELAB

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  2. 0

    I am not sure whether this will help you or not. But you can use a bool variable to check whether the thread has finished its work.

    bool flag = false;
new Thread(new Runnable() {
    @Override
    public void run() {
        //statements
        flag = true;
    }
}).start();
while(flag != true);
//statements to be executed after completion of thread.

    NOTE: Make sure execution never goes to infinite loop.

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  3. 0

    How can I make it so the spinner doesn’t get populated until the thread has finished running?

    There are a couple of ways to do that. The easiest might be to get an ExecutorService which is usually recommended as opposed to creating a Thread "by hand". This also allows you to return a value from your job in which case you submit a Callable<?> instead of a Runnable.

    ExecutorService threadPool = Executors.newFixedThreadPool(1);
...
Future<Void> future = threadPool.submit(new Runnable() {
    public void run() {
       ...
    }
});
// wait for this job to finish
future.get();
...
// when you are done with the pool you need to shut it down
threadPool.shutdown();

    You can also start a thread and then join() with it. That waits for the thread to finish.

    Thread thread = new Thread(new Runnable() { ... });
thread.start();
// now we can do stuff while the thread runs in the background
...
// wait for the thread to finish before returning
thread.join();
...

    The get() method or the join() method also means that the existingCountDates or anything else that was modified inside of the job will be updated in the caller’s memory appropriately.

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