The Android tutorial "Making ListView Scrolling Smooth" show how to improve getView() by loading images in AsyncTask.
The page advise to check (v.position == position) to varify if this item hasn't been recycled already. But it not work in this case. (actually, I don't understand how is the logic) In my implementation, roll-out images will be shown on incorrect cell, but the final images will replace and correct it finally.
To load images in AsyncTask will not speed up your image loading, but improve UI response.
MainActivity.java
package com.example.androidgridview;
import java.io.File;
import java.util.ArrayList;
import java.util.Arrays;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.GridView;
import android.widget.ImageView;
import android.widget.Toast;
public class MainActivity extends Activity {
AsyncTaskLoadFiles myAsyncTaskLoadFiles;
public class AsyncTaskLoadFiles extends AsyncTask<Void, String, Void> {
File targetDirector;
ImageAdapter myTaskAdapter;
public AsyncTaskLoadFiles(ImageAdapter adapter) {
myTaskAdapter = adapter;
}
@Override
protected void onPreExecute() {
String ExternalStorageDirectoryPath = Environment
.getExternalStorageDirectory().getAbsolutePath();
String targetPath = ExternalStorageDirectoryPath + "/test/";
targetDirector = new File(targetPath);
myTaskAdapter.clear();
super.onPreExecute();
}
@Override
protected Void doInBackground(Void... params) {
File[] files = targetDirector.listFiles();
Arrays.sort(files);
for (File file : files) {
publishProgress(file.getAbsolutePath());
if (isCancelled()) break;
}
return null;
}
@Override
protected void onProgressUpdate(String... values) {
myTaskAdapter.add(values[0]);
super.onProgressUpdate(values);
}
@Override
protected void onPostExecute(Void result) {
myTaskAdapter.notifyDataSetChanged();
super.onPostExecute(result);
}
}
public class ImageAdapter extends BaseAdapter {
private Context mContext;
ArrayList<String> itemList = new ArrayList<String>();
public ImageAdapter(Context c) {
mContext = c;
}
void add(String path) {
itemList.add(path);
}
void clear() {
itemList.clear();
}
void remove(int index){
itemList.remove(index);
}
@Override
public int getCount() {
return itemList.size();
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return itemList.get(position);
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return 0;
}
//getView load bitmap ui thread
/*
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ImageView imageView;
if (convertView == null) { // if it's not recycled, initialize some
// attributes
imageView = new ImageView(mContext);
imageView.setLayoutParams(new GridView.LayoutParams(220, 220));
imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
imageView.setPadding(8, 8, 8, 8);
} else {
imageView = (ImageView) convertView;
}
Bitmap bm = decodeSampledBitmapFromUri(itemList.get(position), 220,
220);
imageView.setImageBitmap(bm);
return imageView;
}
*/
//getView load bitmap in AsyncTask
@Override
public View getView(final int position, View convertView, ViewGroup parent) {
ViewHolder holder;
ImageView imageView;
if (convertView == null) { // if it's not recycled, initialize some
// attributes
imageView = new ImageView(mContext);
imageView.setLayoutParams(new GridView.LayoutParams(220, 220));
imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
imageView.setPadding(8, 8, 8, 8);
convertView = imageView;
holder = new ViewHolder();
holder.image = imageView;
holder.position = position;
convertView.setTag(holder);
} else {
//imageView = (ImageView) convertView;
holder = (ViewHolder) convertView.getTag();
((ImageView)convertView).setImageBitmap(null);
}
//Bitmap bm = decodeSampledBitmapFromUri(itemList.get(position), 220, 220);
// Using an AsyncTask to load the slow images in a background thread
new AsyncTask<ViewHolder, Void, Bitmap>() {
private ViewHolder v;
@Override
protected Bitmap doInBackground(ViewHolder... params) {
v = params[0];
Bitmap bm = decodeSampledBitmapFromUri(itemList.get(position), 220, 220);
return bm;
}
@Override
protected void onPostExecute(Bitmap result) {
super.onPostExecute(result);
//Not work for me!
/*
if (v.position == position) {
// If this item hasn't been recycled already,
// show the image
v.image.setImageBitmap(result);
}
*/
v.image.setImageBitmap(result);
}
}.execute(holder);
//imageView.setImageBitmap(bm);
//return imageView;
return convertView;
}
public Bitmap decodeSampledBitmapFromUri(String path, int reqWidth,
int reqHeight) {
Bitmap bm = null;
// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeFile(path, options);
// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth,
reqHeight);
// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
bm = BitmapFactory.decodeFile(path, options);
return bm;
}
public int calculateInSampleSize(
BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;
if (height > reqHeight || width > reqWidth) {
if (width > height) {
inSampleSize = Math.round((float) height
/ (float) reqHeight);
} else {
inSampleSize = Math.round((float) width / (float) reqWidth);
}
}
return inSampleSize;
}
class ViewHolder {
ImageView image;
int position;
}
}
ImageAdapter myImageAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final GridView gridview = (GridView) findViewById(R.id.gridview);
myImageAdapter = new ImageAdapter(this);
gridview.setAdapter(myImageAdapter);
/*
* Move to asyncTaskLoadFiles String ExternalStorageDirectoryPath =
* Environment .getExternalStorageDirectory() .getAbsolutePath();
*
* String targetPath = ExternalStorageDirectoryPath + "/test/";
*
* Toast.makeText(getApplicationContext(), targetPath,
* Toast.LENGTH_LONG).show(); File targetDirector = new
* File(targetPath);
*
* File[] files = targetDirector.listFiles(); for (File file : files){
* myImageAdapter.add(file.getAbsolutePath()); }
*/
myAsyncTaskLoadFiles = new AsyncTaskLoadFiles(myImageAdapter);
myAsyncTaskLoadFiles.execute();
gridview.setOnItemClickListener(myOnItemClickListener);
Button buttonReload = (Button)findViewById(R.id.reload);
buttonReload.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View arg0) {
//Cancel the previous running task, if exist.
myAsyncTaskLoadFiles.cancel(true);
//new another ImageAdapter, to prevent the adapter have
//mixed files
myImageAdapter = new ImageAdapter(MainActivity.this);
gridview.setAdapter(myImageAdapter);
myAsyncTaskLoadFiles = new AsyncTaskLoadFiles(myImageAdapter);
myAsyncTaskLoadFiles.execute();
}});
}
OnItemClickListener myOnItemClickListener = new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id) {
String prompt = "remove " + (String) parent.getItemAtPosition(position);
Toast.makeText(getApplicationContext(), prompt, Toast.LENGTH_SHORT)
.show();
myImageAdapter.remove(position);
myImageAdapter.notifyDataSetChanged();
}
};
}
/res/layout/activity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<Button
android:id="@+id/reload"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Reload"/>
<GridView
android:id="@+id/gridview"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:columnWidth="220dp"
android:numColumns="auto_fit"
android:verticalSpacing="10dp"
android:horizontalSpacing="10dp"
android:stretchMode="columnWidth"
android:gravity="center"/>
</LinearLayout>
Permission of "android.permission.READ_EXTERNAL_STORAGE" is needed to read storage.
Download the files.Next:
- Improve JPG loading with ExifInterface.getThumbnail()
