我正在编写一个 flickr 客户端，作为我正在阅读的书中项目的一部分。在其中我创建了一个 HandlerThread 的子类，它从 flickr 下载图像，然后将它们设置在 ImageView 中。我在程序的前面设置了一个占位符 .png 并且它绑定（但有延迟）但是当我尝试替换它时没有任何反应。占位符保留在那里并且不会被替换。似乎发生了一些事情，这阻止了我的 HandlerThread 正常运行。我尝试使用调试器逐步完成此操作并查看 logcat，但是自上次更新到 android studio 以来，logcat 一直特别无用。如果有的话，它可以部分工作。对了，我没有收到任何错误消息。早些时候我收到一个说我有一个空指针异常，但我无法看到它来自哪里，并且在再次运行应用程序时它就消失了。我正在发布我的代码示例，包括用作 UI 线程的主 Fragment，以及作为我的 HandlerThread 子类的 ThumbnailDownloader 类。
public class PhotoGalleryFragment extends Fragment {
private static final String TAG = "PhotoGalleryFragment";
private RecyclerView mRecyclerView;
private List<GalleryItem> mItems = new ArrayList<>();
private ThumbnailDownloader<PhotoHolder> mThumbnailDownloader;
public static PhotoGalleryFragment newInstance(){
return new PhotoGalleryFragment();
}
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setRetainInstance(true);
new FetchItemsTask().execute();
Handler responseHandler = new Handler();
mThumbnailDownloader = new ThumbnailDownloader<>(responseHandler);
mThumbnailDownloader.setThumbnailDownloadListener( new ThumbnailDownloader.ThumbnailDownloadListener<PhotoHolder>() {
@Override
public void onThumbnailDownloaded(PhotoHolder target, Bitmap bitmap) {
Log.i(TAG, "setThumbnailDownloadListener() called!");
Drawable drawable = new BitmapDrawable(getResources(), bitmap);
target.bindDrawable(drawable);
}
});
mThumbnailDownloader.start();
mThumbnailDownloader.getLooper();
Log.i(TAG, "background thread started!");
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View v = inflater.inflate(R.layout.fragment_photo_gallery, container, false);
mRecyclerView = (RecyclerView) v.findViewById(R.id.photo_recycler_view);
mRecyclerView.setLayoutManager(new GridLayoutManager(getActivity(), 3));
setupAdapter();
return v;
}
public void onDestroyView(){
super.onDestroyView();
mThumbnailDownloader.clearQueue();
}
@Override
public void onDestroy(){
super.onDestroy();
mThumbnailDownloader.quit();
Log.i(TAG, "background thread destroyed!");
}
private void setupAdapter(){
if (isAdded()){
mRecyclerView.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);
}
}//end PhotoHolder inner class
private class PhotoAdapter extends RecyclerView.Adapter<PhotoHolder>{
private List<GalleryItem> mGalleryItems;
public PhotoAdapter(List<GalleryItem> items){
mGalleryItems = items;
}
@Override
public PhotoHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
LayoutInflater layoutInflater = LayoutInflater.from(getActivity());
View view = layoutInflater.inflate(R.layout.list_item_gallery, viewGroup, false);
return new PhotoHolder(view);
}
@Override
public void onBindViewHolder(PhotoHolder photoHolder, int position) {
GalleryItem galleryItem = mGalleryItems.get(position);
Drawable placeholder = getResources().getDrawable(R.drawable.wait); <--this is the placeholder image
photoHolder.bindDrawable(placeholder);//this <--this sets the placeholder but seems to wait for the thumbnail download to complete for some reason
mThumbnailDownloader.queueThumbnail(photoHolder, galleryItem.getUrl());
}
@Override
public int getItemCount() {
return mGalleryItems.size();
}
}
private class FetchItemsTask extends AsyncTask<Void, Void, List<GalleryItem>>{
@Override
protected List<GalleryItem> doInBackground(Void... params) {
return new FlickrFetchr().fetchItems();
}
@Override
protected void onPostExecute(List<GalleryItem> items){
mItems = items;
setupAdapter();
}
}//end FetchItemsTask inner class
}//end class
这是我的 HandlerThread 实现：
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 = new Handler();
private Handler mResponseHandler = new Handler();
private ThumbnailDownloadListener<T> mThumbnailDownloadListener;
private ConcurrentMap<T, String> mRequestMap = new ConcurrentHashMap<>();
public interface ThumbnailDownloadListener<T>{
void onThumbnailDownloaded(T target, Bitmap bitmap);
}
public void setThumbnailDownloadListener(ThumbnailDownloadListener<T> listener){
mThumbnailDownloadListener = listener;
}
public ThumbnailDownloader(Handler responseHandler){
super(TAG);
mResponseHandler = responseHandler;
Log.i(TAG, "ThumbnailDownloader created!");
}
@Override
protected void onLooperPrepared(){
mRequestHandler = new Handler(){
@Override
public void handleMessage(Message msg){
if (msg.what == MESSAGE_DOWNLOAD){
T target = (T) msg.obj;
Log.i(TAG, "got a request for a url:" + mRequestMap.get(target));
handleRequest(target);
}
}
};
}
@Override
public boolean quit(){
mHasQuit = true;
return super.quit();
}
public void queueThumbnail(T target, String url){
Log.i(TAG, "got a url: " + url);
if(url == null){
mRequestMap.remove(target);
}else {
mRequestMap.put(target, url);
mRequestHandler.obtainMessage(MESSAGE_DOWNLOAD, target).sendToTarget();
}
}
public void clearQueue(){
mRequestHandler.removeMessages(MESSAGE_DOWNLOAD);
mRequestMap.clear();
}
private void handleRequest(final T target){
try{
final String url = mRequestMap.get(target);
if (url == null){
return;
}
byte[] bitmapBytes = new FlickrFetchr().getUrlBytes(url);
final Bitmap bitmap = BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length);
Log.i(TAG, "Bitmap created!"); //<---this is output in the logcat at the terminal but not the android monitor
mResponseHandler.post(new Runnable(){
@Override
public void run() {
if (mRequestMap.get(target) != null || mHasQuit){
return;
}
mRequestMap.remove(target);
mThumbnailDownloadListener.onThumbnailDownloaded(target, bitmap);
}
});
}catch (IOException ioe){
Log.e(TAG, "error downloading image: ", ioe);
}
}
}
老实说，在这一点上，我什至不确定问题出在哪里。queueThumbnail 似乎从未运行过。但是仅设置占位符 wait.png 需要很长时间，以至于在绑定到 ViewHolder 时，占位符和下载的图像之间似乎存在一些混淆。我只是不确定它可能在哪里。我添加了评论来指出这一点。