Select multiple items in GridView

I am developing a gallery-like Activity. Everything runs fine but there is a key functionality which is missing and I couldn't find a decent answer or explanation. What I need is to give the user the ability to select multiple items by long clicking on them.

Here is the desired result:

You can clearly see the selected pics and the options in the ActionBar.

My setup is this:

1.I have a GridView in my XML:

<GridView xmlns:android="http://schemas.android.com/apk/res/android"
          android:id="@+id/gridview"
          android:layout_width="fill_parent"
          android:layout_height="fill_parent"
          android:columnWidth="90dp"
          android:numColumns="auto_fit"
          android:verticalSpacing="5dp"
          android:horizontalSpacing="5dp"
          android:stretchMode="columnWidth"
          android:gravity="center"/>

2.It is attached to a class, which extends BaseAdapter and loads images using Picasso:

public class GalleryAdapter extends BaseAdapter {
    Context mContext;
    List<String> mDataset;

    public GalleryAdapter(Context context, List<String> dataset) {
        mContext = context;
        mDataset = dataset;
    }

    @Override
    public int getCount() {
        return mDataset.size();
    }

    @Override
    public Object getItem(int position) {
        return mDataset.get(position);
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ImageView imageView;
        if(convertView == null){
            imageView = new ImageView(mContext);
            imageView.setLayoutParams(new GridView.LayoutParams(200,200));
            imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
            imageView.setPadding(8,8,8,8);
        } else {
            imageView = (ImageView) convertView;
        }

        //TODO: REMOVE INTEGER.VALUEOFF. IT'S MADE FOR MOCK
        Picasso.with(mContext).load(Integer.valueOf(mDataset.get(position))).fit().into(imageView);

        return imageView;
    }


}

3.It is attached to the Activity:

//Get images paths
List<String> data = getImagesPath(this);
    List<String> sortedData = new ArrayList<>();
    for(String file : data){
        sortedData.add(0, file);
    }

    GalleryAdapter adapter = new GalleryAdapter(this, sortedData);
    mGallery.setAdapter(adapter);

From now on the struggle begins:

• I tried making my item in the GridView to implement Checkable:

  public class CheckableItem extends LinearLayout implements Checkable{
  private boolean mIsChecked = false;
  private static final int[] CHECKED_STATE_SET = { android.R.attr.state_checked };
  
  public CheckableItem(Context context) {
      super(context);
  }
  
  @Override
  protected int[] onCreateDrawableState(int extraSpace) {
      int[] drawableState = super.onCreateDrawableState(extraSpace + 1);
      if (isChecked())
          mergeDrawableStates(drawableState, CHECKED_STATE_SET);
      return drawableState;
  }
  
  @Override
  public void setChecked(boolean checked) {
      mIsChecked = checked;
      refreshDrawableState();
  }
  
  @Override
  public boolean isChecked() {
      return mIsChecked;
  }
  
  @Override
  public void toggle() {
      setChecked(!isChecked());
      refreshDrawableState();
  }
  

Then I load it in my XML and added .setChoiceMode(GridView.CHOICE_MODE_MULTIPLE) to my GridView in the Activity. Nothing Happened.

• Added MultiChoiceModeListener:

  class MultiChoiceListener implements GridView.MultiChoiceModeListener{ @Override public void onItemCheckedStateChanged(ActionMode mode, int position, long id, boolean checked) { int selectCount = mGallery.getCheckedItemCount(); switch (selectCount) { case 1: mode.setSubtitle("One item selected"); break; default: mode.setSubtitle("" + selectCount + " items selected"); break; } }

  @Override
  public boolean onCreateActionMode(ActionMode mode, Menu menu) {
      mode.setTitle("Select Items");
      mode.setSubtitle("One item selected");
      return true;
  }
  
  @Override
  public boolean onPrepareActionMode(ActionMode mode, Menu menu) {
      return false;
  }
  
  @Override
  public boolean onActionItemClicked(ActionMode mode, MenuItem item) {
      return false;
  }
  
  @Override
  public void onDestroyActionMode(ActionMode mode) {
  
  }
  

  }

Then added it to my gridView .setMultiChoiceModeListener(newMultiChoiceListener()); but still no results.

Can someone propose a way to achieve this item selecting behavior in GridView? I'd like to use the native Android API, no 3rd party libraries.