Updating ListView from another Activity

Question

Activity 1: Contains ListView.

Acitivity 2: Contains SeekBar that changes a SharedPreferences variable which used by the ListView. After changing the SeekBar and returning to Activity 1, the ListView need to be updated and refreshed.

After I update the SeekBar at Activity 2 I use the following code to return to Activity 1:

        toolbar.setNavigationIcon(R.drawable.ic_arrow_forward_white_24dp);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
//I need to update the list here, There may be a new data!
                onBackPressed();

            }
        });

my Activity 1 remains the same ListView unupdated.

I can't find a good way to refresh my list in Activity 1 after changing the Seekbar, since the adapter available only at Activity 1. I was trying to pass it as intent, but I noticed it's impossible to pass adapters as extras.

Any help?

Code I use to update the ListView in Activity 1:

mainListView = (ListView) findViewById(R.id.main_listview);
                    // 5. Set this activity to react to list items being pressed
                    //mainListView.setOnItemClickListener(MainActivity.this);
                    // 10. Create a JSONAdapter for the ListView
                    mJSONAdapter = new JSONAdapter(MainActivity.this, getLayoutInflater());
                    // Set the ListView to use the ArrayAdapter
                    mainListView.setAdapter(mJSONAdapter);

Logcat error after trying Udi's solution:

08-27 12:58:29.400    1595-1595/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: il.co.test.test, PID: 1595
    java.lang.RuntimeException: Unable to resume activity {il.co.test.test/il.co.test.test.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2788)
            at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2817)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at il.co.test.test.MainActivity.onResume(MainActivity.java:334)
            at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1192)
            at android.app.Activity.performResume(Activity.java:5310)
            at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2778)
            at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2817)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

EDIT 2: FULL CODE HOW I UPDATE MY LIST - AFTER GPS TRACKING. Placed in MainActivity.java. Called in TabFragment1.java (code below)

public void getUpdates()
{
    // Access the device's key-value storage
    mSharedPreferences = getSharedPreferences(PREFS, MODE_PRIVATE);
    searchDistance = mSharedPreferences.getInt(PREF_DISTANCE, 0);
    if (searchDistance == 0)
        searchDistance = DEFAULT_SEARCH_DISTANCE;

        gpsDialog = new ProgressDialog(this);
        gpsDialog.setMessage("Checking your location..");
        gpsDialog.setCancelable(false);

        gpsDialog.show();

        MyLocation.LocationResult locationResult = new MyLocation.LocationResult() {
            @Override
            public void gotLocation(Location location) {
                gpsDialog.dismiss();
                //If we find a location, We will go to the list layout.
                //setContentView(R.layout.fb_list);
                // 4. Access the ListView
                mainListView = (ListView) findViewById(R.id.main_listview);
                // 5. Set this activity to react to list items being pressed
                //mainListView.setOnItemClickListener(MainActivity.this);
                // 10. Create a JSONAdapter for the ListView
                mJSONAdapter = new JSONAdapter(MainActivity.this, getLayoutInflater());
                // Set the ListView to use the ArrayAdapter
                mainListView.setAdapter(mJSONAdapter);
                double lat = location.getLatitude();
                double lon = location.getLongitude();
                query(lat, lon, searchDistance);
            }
        };
        MyLocation myLocation = new MyLocation(this);
        myLocation.getLocation(this, locationResult);
}

Calling this method from my Fragment:

    public View onCreateView(final LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    final View v = inflater.inflate(R.layout.fb_list, container, false);
    MainActivity king = (MainActivity)getActivity();
    king.getUpdates();
    return v;
}

When I add this:

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

    //if the data has changed
    mJSONAdapter.notifyDataSetChanged();
}

app crashes becuase it can't find the json adapter (since it's initalized only after gps location)

Answer 1

If the adapater's backing data is changed, you should notify the adapter. In your scenario what you might consider is adding a check in onResume method if the data has changed, and if it did change notify the adapter:

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

    //if the data has changed
    mJSONAdapter.notifyDataSetChanged(); 
}

Answer 2

This is just a Issue of Null Pinter Exception. But the basic steps are as follows

Create the Adapter and keep it. Keep a reference to the List of items that you pass into the Adapter also, When you come from the Activity 2 As Udi I you can clear the list of Items and Add the new items to it (Do not add the items as a new List, If you do that, you have to reinstantiate the Adapter again and reset the Adapter). And then call the notifyDataSetChanged() on the adapter.

EDIT1:

I suggest you to look into the Activity life-cycle more and communication between Activity and Fragment. I assume that you toggle fragments (TabFragment1) inside Activity 1.

I do not recommend this, But you can try this,

@Override
protected void onRestart() {
    super.onRestart();
    getUpdates();
    mainListView.setAdapter(mJSONAdapter);
}

But you have to make sure that,

// 10. Create a JSONAdapter for the ListView
                mJSONAdapter = new JSONAdapter(MainActivity.this, getLayoutInflater());

this mJsonAdapter is not null.

Answer 3

I solved it by replacing this:

        toolbar.setNavigationIcon(R.drawable.ic_arrow_forward_white_24dp);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
//I need to update the list here, There may be a new data!
                onBackPressed();

            }
        });

with this:

toolbar.setNavigationIcon(R.drawable.ic_arrow_forward_white_24dp);
            toolbar.setNavigationOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent refresh = new Intent(Settings.this, MainActivity.class);
                    Settings.this.startActivity(refresh);
                }
            });