学习从Parse获取图像并填充到gridview，我按照http://www.androidbegin.com/tutorial/android-parse-com-gridview-tutorial/的示例来实现从Parse数据库加载图像的Gridview。
public class Public_download_preview extends Activity
{
GridView gridview;
int gridsize = 256 / 3;
List<ParseObject> ob;
ProgressDialog mProgressDialog;
GridViewAdapter adapter;
private List<Photos> photoarraylist = null;
Button btn_back, btn_refresh;
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.gridview_main);
btn_refresh = (Button) findViewById(R.id.btn_refresh);
btn_back = (Button) findViewById(R.id.btn_back);
new RemoteDataTask().execute();
btn_back.setOnClickListener(new OnClickListener()
{
public void onClick(View arg0)
{
onBackPressed();
}
});
btn_refresh.setOnClickListener(new OnClickListener()
{
public void onClick(View arg0)
{
Intent intent = new Intent(Public_download_preview.this, Public_download_preview.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
startActivityForResult(intent, 0);
overridePendingTransition(0, 0); // 0 for no animation
Public_download_preview.this.finish();
}
});
}
private class RemoteDataTask extends AsyncTask<Void, Void, Void>
{
@Override
protected void onPreExecute()
{
super.onPreExecute();
mProgressDialog = new ProgressDialog(Public_download_preview.this);
mProgressDialog.setTitle("Fetching Gags");
mProgressDialog.setMessage("Please wait...");
mProgressDialog.setIndeterminate(false);
mProgressDialog.show();
}
@Override
protected Void doInBackground(Void... params)
{
// Create the array
photoarraylist = new ArrayList<Photos>();
try
{
ParseQuery<ParseObject> query = new ParseQuery<ParseObject>("photo_database");
query.orderByAscending("photo_id");
ob = query.find();
for (ParseObject photo_data : ob)
{
ParseFile image = (ParseFile) photo_data.get("photo_file");
Photos map = new Photos();
map.set_photo_ref(image.getUrl());
map.set_user_name((String) photo_data.get("user_name"));
map.set_photo_title((String) photo_data.get("photo_title"));
map.set_photo_content((String) photo_data.get("photo_content"));
map.set_photo_category((String) photo_data.get("category"));
map.set_photo_status((String) photo_data.get("status"));
photoarraylist.add(map);
}
}
catch (ParseException e)
{
Log.e("Error", e.getMessage());
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Void result)
{
gridview = (GridView) findViewById(R.id.gridview);
// Pass the results into ListViewAdapter.java
adapter = new GridViewAdapter(Public_download_preview.this, photoarraylist, gridsize);
gridview.setAdapter(adapter);
mProgressDialog.dismiss();
}
}
@Override
protected void onResume()
{
super.onResume();
DisplayMetrics metrics = getResources().getDisplayMetrics();
int screen_width = metrics.widthPixels;
int screen_height = metrics.heightPixels;
if(screen_height<screen_width)
{
int temp = screen_height;
screen_height = screen_width;
screen_width = temp;
}
Constant.SCREEN_H = screen_height;
Constant.SCREEN_W = screen_width;
gridsize = screen_width / 3;
}
public class GridViewAdapter extends BaseAdapter
{
Context context;
LayoutInflater inflater;
ImageLoader imageLoader;
private List<Photos> photoarraylist = null;
private ArrayList<Photos> arraylist;
public GridViewAdapter(Context context, List<Photos> photoarraylist, int size)
{
this.context = context;
this.photoarraylist = photoarraylist;
inflater = LayoutInflater.from(context);
this.arraylist = new ArrayList<Photos>();
this.arraylist.addAll(photoarraylist);
imageLoader = new ImageLoader(context, size);
}
public class ViewHolder
{
ImageView photo_file;
}
@Override
public int getCount()
{
return photoarraylist.size();
}
@Override
public Object getItem(int position)
{
return photoarraylist.get(position);
}
@Override
public long getItemId(int position)
{
return position;
}
public View getView(final int position, View view, ViewGroup parent)
{
final ViewHolder holder;
if (view == null)
{
holder = new ViewHolder();
view = inflater.inflate(R.layout.gridview_item, null);
holder.photo_file = (ImageView) view.findViewById(R.id.photo_file);
view.setTag(holder);
}
else
{
holder = (ViewHolder) view.getTag();
}
imageLoader.DisplayImage(photoarraylist.get(position).get_photo_ref(), holder.photo_file);
view.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View arg0)
{
Intent intent = new Intent(context, Public_download_detail.class);
intent.putExtra("photo", photoarraylist.get(position).get_photo_ref());
intent.putExtra("user_name", (photoarraylist.get(position).get_user_name()));
intent.putExtra("photo_title", (photoarraylist.get(position).get_photo_title()));
intent.putExtra("photo_content", (photoarraylist.get(position).get_photo_content()));
intent.putExtra("category", (photoarraylist.get(position).get_photo_category()));
intent.putExtra("status", (photoarraylist.get(position).get_photo_status()));
((Activity) context).startActivity (intent);
}
});
return view;
}
}
public class ImageLoader
{
MemoryCache memoryCache = new MemoryCache();
FileCache fileCache;
int sizee =100;
private Map<ImageView, String> imageViews = Collections.synchronizedMap(new WeakHashMap<ImageView, String>());
ExecutorService executorService;
Handler handler = new Handler();
public ImageLoader(Context context, int size)
{
fileCache = new FileCache(context);
sizee = size;
executorService = Executors.newFixedThreadPool(5);
}
final int stub_id = R.drawable.temp_img;
public void DisplayImage(String url, ImageView imageView)
{
imageViews.put(imageView, url);
Bitmap bitmap = memoryCache.get(url);
if (bitmap != null)
{
imageView.setImageBitmap(bitmap);
}
else
{
queuePhoto(url, imageView);
imageView.setImageResource(stub_id);
}
}
private void queuePhoto(String url, ImageView imageView)
{
PhotoToLoad p = new PhotoToLoad(url, imageView);
executorService.submit(new PhotosLoader(p));
}
private Bitmap getBitmap(String url, int size)
{
File f = fileCache.getFile(url);
//from SD cache
Bitmap b = decodeFile(f, size);
if (b != null)
return b;
// Download Images from the Internet
try
{
Bitmap bitmap = null;
URL imageUrl = new URL(url);
HttpURLConnection conn = (HttpURLConnection) imageUrl.openConnection();
conn.setConnectTimeout(30000);
conn.setReadTimeout(30000);
conn.setInstanceFollowRedirects(true);
InputStream is = conn.getInputStream();
OutputStream os = new FileOutputStream(f);
Utils.CopyStream(is, os);
os.close();
conn.disconnect();
bitmap = decodeFile(f, size);
return bitmap;
}
catch (Throwable ex)
{
ex.printStackTrace();
if (ex instanceof OutOfMemoryError)
memoryCache.clear();
return null;
}
}
// Decodes image and scales it to reduce memory consumption
private Bitmap decodeFile(File f, int size)
{
try {
// Decode image size
BitmapFactory.Options o = new BitmapFactory.Options();
o.inJustDecodeBounds = true;
FileInputStream stream1 = new FileInputStream(f);
BitmapFactory.decodeStream(stream1, null, o);
stream1.close();
// Find the correct scale value. It should be the power of 2.
// Required size means the min requried dimension
final int REQUIRED_SIZE = size;
int width_tmp = o.outWidth, height_tmp = o.outHeight;
int scale = 1;
while (true)
{
if (width_tmp / 2 < REQUIRED_SIZE || height_tmp / 2 < REQUIRED_SIZE)
break;
width_tmp /= 2;
height_tmp /= 2;
scale *= 2;
}
// Decode with inSampleSize
BitmapFactory.Options o2 = new BitmapFactory.Options();
o2.inSampleSize = scale;
FileInputStream stream2 = new FileInputStream(f);
Bitmap bitmap = BitmapFactory.decodeStream(stream2, null, o2);
stream2.close();
return bitmap;
}
catch (FileNotFoundException e)
{
}
catch (IOException e)
{
e.printStackTrace();
}
return null;
}
// Task for the queue
private class PhotoToLoad
{
public String url;
public ImageView imageView;
public PhotoToLoad(String u, ImageView i)
{
url = u;
imageView = i;
}
}
class PhotosLoader implements Runnable
{
PhotoToLoad photoToLoad;
PhotosLoader(PhotoToLoad photoToLoad)
{
this.photoToLoad = photoToLoad;
}
@Override
public void run()
{
try
{
if (imageViewReused(photoToLoad))
return;
Bitmap bmp = getBitmap(photoToLoad.url, sizee);
memoryCache.put(photoToLoad.url, bmp);
if (imageViewReused(photoToLoad))
return;
BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);
handler.post(bd);
}
catch (Throwable th)
{
th.printStackTrace();
}
}
}
boolean imageViewReused(PhotoToLoad photoToLoad)
{
String tag = imageViews.get(photoToLoad.imageView);
if (tag == null || !tag.equals(photoToLoad.url))
return true;
return false;
}
// Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable
{
Bitmap bitmap;
PhotoToLoad photoToLoad;
public BitmapDisplayer(Bitmap b, PhotoToLoad p)
{
bitmap = b;
photoToLoad = p;
}
public void run()
{
if (imageViewReused(photoToLoad))
return;
if (bitmap != null)
{
photoToLoad.imageView.setImageBitmap(bitmap);
}
else
{
photoToLoad.imageView.setImageResource(stub_id);
}
}
}
public void clearCache()
{
memoryCache.clear();
fileCache.clear();
}
每次返回public_download_preview类时，都会执行RemoteDataTask而不考虑是否存在缓存文件。我想以一种方式实现，如果有缓存文件，那么就没有必要获取在线图像。如果那么如何在代码中进行修改？
非常感谢您的建议！