在ViewPager Android中从URL设置图像

我正在按照本教程ViewPager在我的项目中实施。我已经成功地使用了静态图像。现在，我想对其进行更改，以便从url中检索图像并在中显示ViewPager。下面是我的代码。

我应该在哪里添加下载图像的方法以及如何将其设置为我的方法ViewPager？

任何帮助将不胜感激。

主要活动：

public class MainActivity extends AppCompatActivity {

    private ArrayList<Integer> images;
    private BitmapFactory.Options options;
    private ViewPager viewPager;
    private View btnNext, btnPrev;
    private FragmentStatePagerAdapter adapter;
    private LinearLayout thumbnailsContainer;
    private final static int[] resourceIDs = new int[]{R.mipmap.a, R.mipmap.b,
            R.mipmap.c, R.mipmap.d, R.mipmap.e, R.mipmap.f, R.mipmap.g};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        images = new ArrayList<>();

        //find view by id
        viewPager = (ViewPager) findViewById(R.id.view_pager);
        thumbnailsContainer = (LinearLayout) findViewById(R.id.container);
        btnNext = findViewById(R.id.next);
        btnPrev = findViewById(R.id.prev);

        btnPrev.setOnClickListener(onClickListener(0));
        btnNext.setOnClickListener(onClickListener(1));

        setImagesData();

        // init viewpager adapter and attach
        adapter = new ViewPagerAdapter(getSupportFragmentManager(), images);
        viewPager.setAdapter(adapter);

        inflateThumbnails();
    }

    private View.OnClickListener onClickListener(final int i) {
        return new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (i > 0) {
                    //next page
                    if (viewPager.getCurrentItem() < viewPager.getAdapter().getCount() - 1) {
                        viewPager.setCurrentItem(viewPager.getCurrentItem() + 1);
                    }
                } else {
                    //previous page
                    if (viewPager.getCurrentItem() > 0) {
                        viewPager.setCurrentItem(viewPager.getCurrentItem() - 1);
                    }
                }
            }
        };
    }

    private void setImagesData() {
        for (int i = 0; i < resourceIDs.length; i++) {
            images.add(resourceIDs[i]);
        }
    }

    private void inflateThumbnails() {
        for (int i = 0; i < images.size(); i++) {
            View imageLayout = getLayoutInflater().inflate(R.layout.item_image, null);
            ImageView imageView = (ImageView) imageLayout.findViewById(R.id.img_thumb);
            imageView.setOnClickListener(onChagePageClickListener(i));
            options = new BitmapFactory.Options();
            options.inSampleSize = 3;
            options.inDither = false;
            Bitmap bitmap = BitmapFactory.decodeResource(getResources(), images.get(i), options );
            imageView.setImageBitmap(bitmap);
            //set to image view
            imageView.setImageBitmap(bitmap);
            //add imageview
            thumbnailsContainer.addView(imageLayout);
        }
    }

    private View.OnClickListener onChagePageClickListener(final int i) {
        return new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                viewPager.setCurrentItem(i);
            }
        };
    }
}

PageFragment类：

   public class PageFragment extends Fragment {

        private int imageResource;
        private Bitmap bitmap;

        public static PageFragment getInstance(int resourceID) {
            PageFragment f = new PageFragment();
            Bundle args = new Bundle();
            args.putInt("image_source", resourceID);
            f.setArguments(args);
            return f;
        }

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            imageResource = getArguments().getInt("image_source");
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_page, container, false);
        }

        @Override
        public void onViewCreated(View view, Bundle savedInstanceState) {
            super.onViewCreated(view, savedInstanceState);
            ImageView imageView = (ImageView) view.findViewById(R.id.image);

            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inSampleSize = 4;
            o.inDither = false;
            bitmap = BitmapFactory.decodeResource(getResources(), imageResource, o);
            imageView.setImageBitmap(bitmap);
        }

        @Override
        public void onDestroy() {
            super.onDestroy();
            bitmap.recycle();
            bitmap = null;
        }
    }

ViewPager Adapter class:

public class ViewPagerAdapter extends FragmentStatePagerAdapter {

    private List<Integer> images;

    public ViewPagerAdapter(FragmentManager fm, List<Integer> imagesList) {
        super(fm);
        this.images = imagesList;
    }

    @Override
    public Fragment getItem(int position) {
        return PageFragment.getInstance(images.get(position));
    }

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