This solution is build upon my previous code for any part of the code that is not included in this post please refer to Solution to Challenge: Paging.I also took some ideas from other people’s example found in this forum.
Notes to changes that I made:
- ThumbnailDownloader.queueThumbnail() was moved from PhotoAdapter.onBindViewHolder() to RecyclerView.OnScrollListener()
- RecyclerView.OnScrollListener() expanded to handle extra thumbnail massage queueing for preloading
- ThumbnailDownloader < PhotoHolder > changed to ThumbnailDownloader < Intger > turns out it doesn’t really need to know which PhotoHolder request the thumbnail
- PhotoCache of class LruCache<String,Bitmap> was add to ThumbnailDownloader
- ThumbnailDownloader.ThumbnailDownloadListener now only notify the adapter that item has changed
- Minor change to variable names and additional logs
ThumbnailDownloader.java
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Message;
import java.io.IOException;
import android.util.Log;
import android.util.LruCache;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
public class ThumbnailDownloader<T> extends HandlerThread {
private static final String TAG = "ThumbnailDownloader";
private static final int MESSAGE_DOWNLOAD = 0;
private boolean mHasQuit = false;
private Handler mRequestHandler;
private Handler mResponseHandler;
private ConcurrentMap<T,String> mRequestMap = new ConcurrentHashMap<>();
private ThumbnailDownloadListener<T> mThumbnailDownloadListener;
public LruCache<String,Bitmap> mPhotoCache;
public interface ThumbnailDownloadListener<T>{
void onThumbnailDownloaded(T target, Bitmap thumbnail);
}
public void setThumbnailDownloadListener(ThumbnailDownloadListener<T> listener){
mThumbnailDownloadListener = listener;
}
public ThumbnailDownloader(Handler responseHandler){
super(TAG);
mResponseHandler = responseHandler;
mPhotoCache = new LruCache<>(1330*1024);
}
@Override
protected void onLooperPrepared(){
mRequestHandler = new Handler() {
@Override
public void handleMessage(Message msg){
if (msg.what == MESSAGE_DOWNLOAD){
T position = (T) msg.obj;
Log.i(TAG,"Got a request for URL:"+ mRequestMap.get(position));
handleRequest(position);
}
}
};
}
@Override
public boolean quit(){
mHasQuit = true;
return super.quit();
}
public void queueThumbnail(T position,String url){
Log.i(TAG,"Got a URL: " + url);
if(url == null){
mRequestMap.remove(position);
}else{
mRequestMap.put(position,url);
mRequestHandler.obtainMessage(MESSAGE_DOWNLOAD,position).sendToTarget();
}
}
public void clearQueue(){
mRequestHandler.removeMessages(MESSAGE_DOWNLOAD);
mRequestMap.clear();
}
public void clearCache() {
mPhotoCache.evictAll();
}
private void handleRequest(final T position){
try {
final String url = mRequestMap.get(position);
if (url == null) {
return;
}
byte[] bitmapBytes = new FlickrFetchr().getUrlBytes(url);
final Bitmap bitmap = BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length);
mPhotoCache.put(url,bitmap);
Log.i(TAG, "Bitmap of size "+ bitmapBytes.length/1024+"KB created");
mResponseHandler.post(new Runnable() {
@Override
public void run() {
if (mRequestMap.get(position)!= url|| mHasQuit)return;
mRequestMap.remove(position);
mThumbnailDownloadListener.onThumbnailDownloaded(position,bitmap);
}
});
}catch (IOException ioe){
Log.e(TAG,"Error downloading image",ioe);
}
}
}
PhotoGalleryFragment.java
import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewTreeObserver;
import android.widget.ImageView;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.List;
public class PhotoGalleryFragment extends Fragment {
private static String TAG = "PhotoGallery";
private RecyclerView mPhotoRecyclerView;
private TextView mCurrentPageText;
private GridLayoutManager mGridManager;
private List<GalleryItem> mItems = new ArrayList<>();
private ThumbnailDownloader<Integer> mThumbnailDownloader;
boolean asyncFetching = false;
int mCurrentPage = 1;
int mMaxPage = 1;
int mItemsPerPage = 1;
int mFirstItemPosition, mLastItemPosition;
public static PhotoGalleryFragment newInstance(){
return new PhotoGalleryFragment();
}
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setRetainInstance(true);
new FetchItemTask().execute();
Handler responseHandler = new Handler();
mThumbnailDownloader = new ThumbnailDownloader<>(responseHandler);
mThumbnailDownloader.setThumbnailDownloadListener(new ThumbnailDownloader.ThumbnailDownloadListener<Integer>() {
@Override
public void onThumbnailDownloaded(Integer position, Bitmap thumbnail) {
mPhotoRecyclerView.getAdapter().notifyItemChanged(position);
}
});
mThumbnailDownloader.start();
mThumbnailDownloader.getLooper();
Log.i(TAG,"Background thread started");
}
@Override
public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View v =inflater.inflate(R.layout.fragment_photo_gallery,container,false);
mCurrentPageText = (TextView) v.findViewById(R.id.currentPageText);
mPhotoRecyclerView = (RecyclerView) v.findViewById(R.id.photo_recycler_view);
mPhotoRecyclerView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
float columnWidthInPixels = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 140, getActivity().getResources().getDisplayMetrics());
int width = mPhotoRecyclerView.getWidth();
int columnNumber = Math.round(width / columnWidthInPixels);
mGridManager= new GridLayoutManager(getActivity(), columnNumber);
mPhotoRecyclerView.setLayoutManager(mGridManager);
mPhotoRecyclerView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
});
mPhotoRecyclerView.addOnScrollListener( new RecyclerView.OnScrollListener() {
@Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
int lastVisibleItem = mGridManager.findLastVisibleItemPosition();
int firstVisibleItem = mGridManager.findFirstVisibleItemPosition();
if (mLastItemPosition != lastVisibleItem || mFirstItemPosition != firstVisibleItem) {
Log.d(TAG,"Showing item " + firstVisibleItem +" to " + lastVisibleItem);
updatePageText(firstVisibleItem);
mLastItemPosition = lastVisibleItem;
mFirstItemPosition = firstVisibleItem;
int begin = Math.max(firstVisibleItem-10,0 );
int end = Math.min(lastVisibleItem +10,mItems.size()-1);
for (int position = begin; position <= end; position++){
String url=mItems.get(position).getUrl();
if(mThumbnailDownloader.mPhotoCache.get(url)== null) {
Log.d(TAG,"Requesting Download at position: "+ position);
mThumbnailDownloader.queueThumbnail(position,url);
}
}
}
if (!(asyncFetching) && (dy > 0) && (mCurrentPage < mMaxPage) && (lastVisibleItem >= (mItems.size() - 1))) {
Log.d(TAG, "Fetching more items");
new FetchItemTask().execute();
}
}
});
if(mPhotoRecyclerView.getAdapter()==null)setupAdapter();
return v;
}
@Override
public void onDestroyView(){
super.onDestroyView();
mThumbnailDownloader.clearQueue();
mThumbnailDownloader.clearCache();
}
@Override
public void onDestroy(){
super.onDestroy();
mThumbnailDownloader.quit();
Log.i(TAG,"Background thread destroyed");
}
private void updatePageText(int pos){
mCurrentPage = pos / mItemsPerPage+1;
String pageText = getString(R.string.page_text,mCurrentPage, mMaxPage);
mCurrentPageText.setText(pageText);
}
private void setupAdapter(){
if(isAdded()){
Log.d(TAG,"Setup Adapter");
mPhotoRecyclerView.setAdapter(new PhotoAdapter(mItems));
}
}
private class PhotoHolder extends RecyclerView.ViewHolder{
private ImageView mItemImageView;
public PhotoHolder(View itemView){
super(itemView);
mItemImageView = (ImageView) itemView.findViewById(R.id.item_image_view);
}
public void bindDrawable(Drawable drawable){
mItemImageView.setImageDrawable(drawable);
}
}
private class PhotoAdapter extends RecyclerView.Adapter<PhotoHolder>{
private List<GalleryItem> mGalleryItems;
public PhotoAdapter(List<GalleryItem> galleryItems){
mGalleryItems = galleryItems;
}
@NonNull
@Override
public PhotoHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
LayoutInflater inflater = LayoutInflater.from(getActivity());
View view = inflater.inflate(R.layout.list_item_gallery,parent,false);
return new PhotoHolder(view);
}
@Override
public void onBindViewHolder(@NonNull PhotoHolder photoHolder, int position) {
Log.d(TAG,"Binding item "+ position + " to " + photoHolder.hashCode());
GalleryItem galleryItem = mGalleryItems.get(position);
String url = galleryItem.getUrl();
Bitmap bitmap = mThumbnailDownloader.mPhotoCache.get(url);
if(bitmap == null) {
Drawable placeholder = getResources().getDrawable(R.drawable.bill_up_close);
photoHolder.bindDrawable(placeholder);
}else {
Drawable drawable = new BitmapDrawable(getResources(), bitmap);
photoHolder.bindDrawable(drawable);
}
}
@Override
public int getItemCount() {
return mGalleryItems.size();
}
}
private class FetchItemTask extends AsyncTask<Void,Void,List<GalleryItem>>{
@Override
protected List<GalleryItem> doInBackground(Void... params){
asyncFetching =true;
return new FlickrFetchr().fetchItems(mCurrentPage+1);
}
@Override
protected void onPostExecute(List<GalleryItem> items){
asyncFetching =false;
mItems.addAll(items);
GalleryPage pge=GalleryPage.getGalleryPage();
mMaxPage = pge.getTotalPages();
mItemsPerPage= pge.getItemPerPage();
if(mPhotoRecyclerView.getAdapter()==null)setupAdapter();
mPhotoRecyclerView.getAdapter().notifyDataSetChanged();
updatePageText(mGridManager.findFirstVisibleItemPosition());
}
}
}
strings.xml
<resources>
<string name="app_name">PhotoGallery</string>
<string name="page_text">Page %1d of %2d</string>
</resources>
The cache here is fixed size and sould hold around 60-100 thunmbnails(they are not very big).I tested and It works like a charm.Anybody see any hidden error or any improvement I should made please let me know.