AsyncTask not updating UI after doInBackground finishes

Question

I'm trying to make a simple app that's looks for WiFi networks, and connects to them. I'm currently having a problem with updating the UI.

A few pointers would be great. Thank you for your time.

class UiUpdater extends AsyncTask<Void, Void, List<ScanResult>> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();

        TextView searching = (TextView) findViewById(R.id.searching);
        searching.setText("Currently searching...");
    }

    @Override
    protected List<ScanResult> doInBackground(Void... params) {
        WifiManager manager = (WifiManager) Client.this.getSystemService(Context.WIFI_SERVICE);

        if (!manager.isWifiEnabled())
            manager.setWifiEnabled(true);

        return manager.getScanResults();
    }

    @Override
    protected void onPostExecute(List<ScanResult> items) {
        super.onPostExecute(items);

        ArrayList<Items> wifi = new ArrayList<>();

        for (ScanResult s : items)
            wifi.add(new Items(s.SSID, s.capabilities));

        ///TextView searching = (TextView) findViewById(R.id.searching);
        ///searching.setText("");

        ListView list = (ListView) findViewById(R.id.list);
        Explorer adapter = new Explorer(Client.this, R.layout.listview_item_row, wifi);

        list.setAdapter(adapter);

        clickListener(list, wifi);
    }

}

What is the problem you are having exactly? Are you receiving an error? Is there an error log? — CzarMatt, Jan 06 '16 at 01:08
I don't get any errors, it just won't update the UI, no matter how long I wait. — Edward Alexander, Jan 06 '16 at 01:09
Have you verified that you are getting any `ScanResult`s being returned from your doInBackground ? — CzarMatt, Jan 06 '16 at 01:10
Is onPostExecute being called? If not, then it seems like getScanResults never returns. If it is, then the problem is likely in your adapter. — Gabe Sechan, Jan 06 '16 at 01:12
Yep, I tried 'if(wifi!=null)', apparently it returns something. — Edward Alexander, Jan 06 '16 at 01:12
Have you made a call to `startScan()` ? See: http://developer.android.com/reference/android/net/wifi/WifiManager.html#startScan() — CzarMatt, Jan 06 '16 at 01:15
I also tried that, still nothing. My fear is that somehow doInBackground finishes earlier, and some other thread holds the getScanResults, but since doInBackground finishes earlier it also calls onPostExecute earlier. — Edward Alexander, Jan 06 '16 at 01:18
Try this: http://stackoverflow.com/questions/34433282/how-to-create-toast-in-android-inside-asyntask/34433575#34433575 — Quang Doan, Jan 06 '16 at 03:01

score 1 · Answer 1 · answered Jan 06 '16 at 01:14

1

please use Toast message or logcat or breakpoint to check onPost execute is being called or not

answered Jan 06 '16 at 01:14

MinFu

353
1
13

I just checked onPostExecute is called and it can't be my adapter because if I enter some dummy text, it will show it with ease. – Edward Alexander Jan 06 '16 at 01:16
your view should update i suspect your arraylist of wifi size is zero mean your getScanResult return nothing that why you can not see your view being update – MinFu Jan 06 '16 at 01:26

score 0 · Accepted Answer · answered Jan 06 '16 at 02:31

I managed to get it working, but by taking a different approach.

I used the BroadcastReceiver and registerReceiver to get the onReceive event to update the list.

Here's the code:

    mWifiManager = (WifiManager)getSystemService(Context.WIFI_SERVICE);

    if(!mWifiManager.isWifiEnabled())
        mWifiManager.setWifiEnabled(true);
    mWifiManager.startScan();

    wifiReceiver = new BroadcastReceiver(){
        @Override
        public void onReceive(Context c, Intent intent)
        {
            if(mWifiManager != null) {
                List<ScanResult> networks = mWifiManager.getScanResults();
                showWifi(networks);
            }
        }
    };

    registerReceiver(wifiReceiver, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));

Although I haven't managed to understand how AsyncTask works, I did get some nice results with loading a simple progress bar.

I think the problem I encountered was as such:

I started an AsyncTask on another thread.
In the doInBackground, getScanResults started another background thread, leading doInBackground to think the job's done.
onPostExecute was called because doInBackground finished its job.

Bottom line, it wasn't the AsyncTask's fault, it was mine for not knowing that getScanResults starts another background thread.

AsyncTask not updating UI after doInBackground finishes

2 Answers2