Question

I want to update the UI from a timer. That’s not a problem at all but when it comes to Gallery/ListViews it gets difficult. I have a Gallery with custom BaseAdapter. I need a counter for every (gallery) item (every item counts different depending on the items data). That counter should run outside of the main thread. In addition I don’t want to run 10 threads for 10 items of the gallery when just one item is visible. It’s not a problem to define a Handler and start a thread (the counting) in the adapters getView()-method when a item/view gets visible. I can think of something like the following code:


public class MyAdapter extends BaseAdapter {

    static class ViewHolder {
         //...
         public Handler myHandler;
     }

     public View getView(int position, View convertView, ViewGroup parent) {

            //...
            // getView() gets called indefinite so first remove callback because it may be added already        
            holder.myHandler.removeCallbacks(myRunnable);
            holder.myHandler.postDelayed(myRunnable, 0);

            //...
     }

}


The problem is to remove the callback for a view thats not visible anymore because in getView() I get noticed when a view becomes visible but I have no clue how to get the view (and thereby the holder and it’s handler) that became invisible to remove the callback.

Is there a(nother) approach to solve that?

Answer 1

Found a clean solution (if someone needs something like that). I needed to update a TextView to set the new counter value (every second).

In BaseAdapters getView I add the TextViews(s) to a WeakHashMap. The TextView is the key of the map. A key/value mapping will be removed if the key is no longer referenced. So I do not cause memory leaks. The GarbageCollector does the work.

A thread counts “counter objects” down and refreshes the TextViews with the corresponding values (GarbageCollector runs all the time so in my case the map consists mostly of 3 items because the gallery shows just one item. Due to GarbageCollection the map gets immediately cleared by “unused” TextViews specially when scrolling fast through the list/gallery)

BaseAdapter’s getView():

private WeakHashMap<TextView, Integer> counterMap = new WeakHashMap<TextView,Integer>();
private Handler counterHandler = new Handler();

public View getView(...) {
    //...
    counterMap.put(holder.tvCounter, position);
    //...
}

The counter:

// Thread decrementing the time of the counter objects and updating the UI
private final Runnable counterTimeTask = new Runnable() {
    public void run() {
        //...
        // decrement the remaining time of all objects
        for (CounterData data : counterDataList) 
            data.decrementTimeLeftInSeconds();

        // iterate through the map and update the containing TextViews
        for (Map.Entry<TextView, Integer> entry : counterMap.entrySet()) {
            TextView tvCounter = entry.getKey();
            Integer position = entry.getValue();
            CounterData data = counterDataList.get(position);
            long timeLeftInSeconds = data.getTimeLeftInSeconds();
            tvCounter.setText(" " + timeLeftInSeconds);
        }

        if (yourCondition)
            counterHandler.postDelayed(this, 1000);
    }
};

Start/stop the counter:

public void startCounter() { counterHandler.post(counterTimeTask); }
public void stopCounter() { counterHandler.removeCallbacks(counterTimeTask); }