Develop Reference

How does AsyncTask work one process to another one?

I'm currently studying android on my own and pretty new to java. I'm wondering how AsyncTask works like this: onPreExecute() -> doInBackground() -> onPostExecute(). When I look at others define their AsynTask, it seems like only method is declared in their code with no calls upon the method. I can't figure out how doInBackground() comes after onPreExecute() with no code that links both like:
onPreExecute(){ ~~~~~ call doInBackground()}
My point is that when AsyncTask.execute() is called, onPreExecute() is called, then doInBackground(), finally onPostExecute(). I couldn't find any code in library that actually connects these together. All I could find is this:
#MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
#MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
Here when AsyncTask.execute() is called, onPreExecute() is called. But without any connection to doInBackground the task works just fine. I feel like I'm missing some fundamental logic or process of java or android. Plz, help me with this unsolved question in mind. Sample code is shown below. Thank you in advance.
#Override
protected void onPreExecute() {
super.onPreExecute();
mLoadingIndicator.setVisibility(View.VISIBLE);
}
#Override
protected String[] doInBackground(String... params) {
/* If there's no zip code, there's nothing to look up. */
if (params.length == 0) {
return null;
}
String location = params[0];
URL weatherRequestUrl = NetworkUtils.buildUrl(location);
try {
String jsonWeatherResponse = NetworkUtils
.getResponseFromHttpUrl(weatherRequestUrl);
String[] simpleJsonWeatherData = OpenWeatherJsonUtils
.getSimpleWeatherStringsFromJson(MainActivity.this, jsonWeatherResponse);
return simpleJsonWeatherData;
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
#Override
protected void onPostExecute(String[] weatherData) {
// COMPLETED (19) As soon as the data is finished loading, hide the loading indicator
mLoadingIndicator.setVisibility(View.INVISIBLE);
if (weatherData != null) {
// COMPLETED (11) If the weather data was not null, make sure the data view is visible
showWeatherDataView();
/*
* Iterate through the array and append the Strings to the TextView. The reason why we add
* the "\n\n\n" after the String is to give visual separation between each String in the
* TextView. Later, we'll learn about a better way to display lists of data.
*/
for (String weatherString : weatherData) {
mWeatherTextView.append((weatherString) + "\n\n\n");
}
} else {
// COMPLETED (10) If the weather data was null, show the error message
showErrorMessage();
}

I guess you shouldn't waste time on AsyncTask since it is deprecated.
Instead you should focus on coroutines, recommended by google here , or some other state of the art framework to achive what you want (e.g. rx java)

Yes, you are correct. The logic is onPreExecute() -> doInBackground() -> onPostExecute()
Synchronous VS asynchronous
You can read this article for a better understanding even though it's using Javascript to explain it.

Efficiently download and use images in recyclerview

Currently I'm using this to download and display the images in my recyclerview:
private class RetrieveImageTask extends AsyncTask<URL,Void,Drawable>{
URL url;
InputStream inputStream;
protected Drawable doInBackground(URL... urls){
url = urls[0];
try {
inputStream = (InputStream)url.getContent();
} catch (IOException e) {
e.printStackTrace();
}
return Drawable.createFromStream(inputStream, null);
}
protected void onPostExecute(Drawable drawable) {
}
}
I then use this class in onBindViewHolder in my RecyclerViewAdapter:
#Override
public void onBindViewHolder(ItemViewHolder item, int i) {
try {
item.title.setText(aushangdata.getJSONObject(i).getString("title"));
item.desc.setText(aushangdata.getJSONObject(i).getString("text"));
item.tag.setText(aushangdata.getJSONObject(i).getString("category"));
item.link.setText(aushangdata.getJSONObject(i).getString("link"));
URL myUrl = new URL(aushangdata.getJSONObject(i).getString("image"));
RetrieveImageTask task = new RetrieveImageTask();
Drawable drawable = task.execute(myUrl).get();
item.thumb.setImageDrawable(drawable);
} catch (JSONException | IOException | InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
This is obviously an extremely shitty solution, and it is very noticeable: when I open the fragment containing the recyclerview, I get a lag of up to 0.5 seconds, and when I scroll down fast it's also lagging because the images take up a lot of space on the cards.
My quesiton is: What is the most optimal way to download and display images in a RecyclerView?

You can use Glide: https://github.com/bumptech/glide
Also is so simple to use:
Glide.with(CONTEXT).load(URL or DRAWABLE).asBitmap().into(IMAGEVIEW);
Check the wiki for more options: https://github.com/bumptech/glide/wiki
In your code:
Glide.with(context).load(myUrl).asBitmap().into(item.thumb);
Hope it helps.

I had a similar issues with Picasso. The answer above me is right. Additional things you might consider: What i did to improve load time was use the methods resize and centerCrop.
resize(xxx,xxx).
centerCrop().
into(IMAGEVIEW);
Glide probably has similar methods.

showing image in listview with json [duplicate]

I am using a ListView to display some images and captions associated with those images. I am getting the images from the Internet. Is there a way to lazy load images so while the text displays, the UI is not blocked and images are displayed as they are downloaded?
The total number of images is not fixed.

Here's what I created to hold the images that my app is currently displaying. Please note that the "Log" object in use here is my custom wrapper around the final Log class inside Android.
package com.wilson.android.library;
/*
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
*/
import java.io.IOException;
public class DrawableManager {
private final Map<String, Drawable> drawableMap;
public DrawableManager() {
drawableMap = new HashMap<String, Drawable>();
}
public Drawable fetchDrawable(String urlString) {
if (drawableMap.containsKey(urlString)) {
return drawableMap.get(urlString);
}
Log.d(this.getClass().getSimpleName(), "image url:" + urlString);
try {
InputStream is = fetch(urlString);
Drawable drawable = Drawable.createFromStream(is, "src");
if (drawable != null) {
drawableMap.put(urlString, drawable);
Log.d(this.getClass().getSimpleName(), "got a thumbnail drawable: " + drawable.getBounds() + ", "
+ drawable.getIntrinsicHeight() + "," + drawable.getIntrinsicWidth() + ", "
+ drawable.getMinimumHeight() + "," + drawable.getMinimumWidth());
} else {
Log.w(this.getClass().getSimpleName(), "could not get thumbnail");
}
return drawable;
} catch (MalformedURLException e) {
Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
} catch (IOException e) {
Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
}
}
public void fetchDrawableOnThread(final String urlString, final ImageView imageView) {
if (drawableMap.containsKey(urlString)) {
imageView.setImageDrawable(drawableMap.get(urlString));
}
final Handler handler = new Handler(Looper.getMainLooper()) {
#Override
public void handleMessage(Message message) {
imageView.setImageDrawable((Drawable) message.obj);
}
};
Thread thread = new Thread() {
#Override
public void run() {
//TODO : set imageView to a "pending" image
Drawable drawable = fetchDrawable(urlString);
Message message = handler.obtainMessage(1, drawable);
handler.sendMessage(message);
}
};
thread.start();
}
private InputStream fetch(String urlString) throws MalformedURLException, IOException {
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpGet request = new HttpGet(urlString);
HttpResponse response = httpClient.execute(request);
return response.getEntity().getContent();
}
}

I made a simple demo of a lazy list (located at GitHub) with images.
Basic Usage
ImageLoader imageLoader=new ImageLoader(context); ...
imageLoader.DisplayImage(url, imageView);
Don't forget to add the
following permissions to your AndroidManifest.xml:
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> Please
create only one instance of ImageLoader and reuse it all around your
application. This way image caching will be much more efficient.
It may be helpful to somebody. It downloads images in the background thread. Images are being cached on an SD card and in memory. The cache implementation is very simple and is just enough for the demo. I decode images with inSampleSize to reduce memory consumption. I also try to handle recycled views correctly.

I recommend open source instrument Universal Image Loader. It is originally based on Fedor Vlasov's project LazyList and has been vastly improved since then.
Multithread image loading
Possibility of wide tuning ImageLoader's configuration (thread executors, downloader, decoder, memory and disc cache, display image options, and others)
Possibility of image caching in memory and/or on the device's file system (or SD card)
Possibility to "listen" loading process
Possibility to customize every display image call with separated options
Widget support
Android 2.0+ support

Multithreading For Performance, a tutorial by Gilles Debunne.
This is from the Android Developers Blog. The suggested code uses:
AsyncTasks.
A hard, limited size, FIFO cache.
A soft, easily garbage collect-ed cache.
A placeholder Drawable while you download.

Update: Note that this answer is pretty ineffective now. The Garbage Collector acts aggressively on SoftReference and WeakReference, so this code is NOT suitable for new apps. (Instead, try libraries like Universal Image Loader suggested in other answers.)
Thanks to James for the code, and Bao-Long for the suggestion of using SoftReference. I implemented the SoftReference changes on James' code. Unfortunately, SoftReferences caused my images to be garbage collected too quickly. In my case, it was fine without the SoftReference stuff, because my list size is limited and my images are small.
There's a discussion from a year ago regarding the SoftReferences on google groups: link to thread. As a solution to the too-early garbage collection, they suggest the possibility of manually setting the VM heap size using dalvik.system.VMRuntime.setMinimumHeapSize(), which is not very attractive to me.
public DrawableManager() {
drawableMap = new HashMap<String, SoftReference<Drawable>>();
}
public Drawable fetchDrawable(String urlString) {
SoftReference<Drawable> drawableRef = drawableMap.get(urlString);
if (drawableRef != null) {
Drawable drawable = drawableRef.get();
if (drawable != null)
return drawable;
// Reference has expired so remove the key from drawableMap
drawableMap.remove(urlString);
}
if (Constants.LOGGING) Log.d(this.getClass().getSimpleName(), "image url:" + urlString);
try {
InputStream is = fetch(urlString);
Drawable drawable = Drawable.createFromStream(is, "src");
drawableRef = new SoftReference<Drawable>(drawable);
drawableMap.put(urlString, drawableRef);
if (Constants.LOGGING) Log.d(this.getClass().getSimpleName(), "got a thumbnail drawable: " + drawable.getBounds() + ", "
+ drawable.getIntrinsicHeight() + "," + drawable.getIntrinsicWidth() + ", "
+ drawable.getMinimumHeight() + "," + drawable.getMinimumWidth());
return drawableRef.get();
} catch (MalformedURLException e) {
if (Constants.LOGGING) Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
} catch (IOException e) {
if (Constants.LOGGING) Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
return null;
}
}
public void fetchDrawableOnThread(final String urlString, final ImageView imageView) {
SoftReference<Drawable> drawableRef = drawableMap.get(urlString);
if (drawableRef != null) {
Drawable drawable = drawableRef.get();
if (drawable != null) {
imageView.setImageDrawable(drawableRef.get());
return;
}
// Reference has expired so remove the key from drawableMap
drawableMap.remove(urlString);
}
final Handler handler = new Handler() {
#Override
public void handleMessage(Message message) {
imageView.setImageDrawable((Drawable) message.obj);
}
};
Thread thread = new Thread() {
#Override
public void run() {
//TODO : set imageView to a "pending" image
Drawable drawable = fetchDrawable(urlString);
Message message = handler.obtainMessage(1, drawable);
handler.sendMessage(message);
}
};
thread.start();
}

Picasso
Use Jake Wharton's Picasso Library.
(A Perfect ImageLoading Library from the developer of ActionBarSherlock)
A powerful image downloading and caching library for Android.
Images add much-needed context and visual flair to Android applications. Picasso allows for hassle-free image loading in your application—often in one line of code!
Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);
Many common pitfalls of image loading on Android are handled automatically by Picasso:
Handling ImageView recycling and download cancellation in an adapter.
Complex image transformations with minimal memory use.
Automatic memory and disk caching.
Picasso Jake Wharton's Library
Glide
Glide is a fast and efficient open-source media management framework for Android that wraps media decoding, memory and disk caching, and resource pooling into a simple and easy-to-use interface.
Glide supports fetching, decoding, and displaying video stills, images, and animated GIFs. Glide includes a flexible API that allows developers to plug into almost any network stack. By default, Glide uses a custom HttpUrlConnection based stack but also includes utility libraries plug-in to Google's Volley project or Square's OkHttp library instead.
Glide.with(this).load("your-url-here").into(imageView);
Glide's primary focus is on making scrolling any kind of a list of images as smooth and fast as possible, but Glide is also effective for almost any case where you need to fetch, resize, and display a remote image.
Glide Image Loading Library
Fresco by Facebook
Fresco is a powerful system for displaying images in Android applications.
Fresco takes care of image loading and display, so you don't have to. It will load images from the network, local storage, or local resources, and display a placeholder until the image has arrived. It has two levels of cache; one in memory and another in internal storage.
Fresco Github
In Android 4.x and lower, Fresco puts images in a special region of Android memory. This lets your application run faster - and suffer the dreaded OutOfMemoryError much less often.
Fresco Documentation

High-performance loader - after examining the methods suggested here,
I used Ben's solution with some changes -
I realized that working with drawable is faster than with bitmaps so I uses drawable instead
Using SoftReference is great, but it makes the cached image to be deleted too often, so I added a Linked list that holds images references, preventing the image to be deleted, until it reached a predefined size
To open the InputStream I used java.net.URLConnection which allows me to use web cache (you need to set a response cache first, but that's another story)
My code:
import java.util.Map;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Collections;
import java.util.WeakHashMap;
import java.lang.ref.SoftReference;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import android.graphics.drawable.Drawable;
import android.widget.ImageView;
import android.os.Handler;
import android.os.Message;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
public class DrawableBackgroundDownloader {
private final Map<String, SoftReference<Drawable>> mCache = new HashMap<String, SoftReference<Drawable>>();
private final LinkedList <Drawable> mChacheController = new LinkedList <Drawable> ();
private ExecutorService mThreadPool;
private final Map<ImageView, String> mImageViews = Collections.synchronizedMap(new WeakHashMap<ImageView, String>());
public static int MAX_CACHE_SIZE = 80;
public int THREAD_POOL_SIZE = 3;
/**
* Constructor
*/
public DrawableBackgroundDownloader() {
mThreadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
}
/**
* Clears all instance data and stops running threads
*/
public void Reset() {
ExecutorService oldThreadPool = mThreadPool;
mThreadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
oldThreadPool.shutdownNow();
mChacheController.clear();
mCache.clear();
mImageViews.clear();
}
public void loadDrawable(final String url, final ImageView imageView,Drawable placeholder) {
mImageViews.put(imageView, url);
Drawable drawable = getDrawableFromCache(url);
// check in UI thread, so no concurrency issues
if (drawable != null) {
//Log.d(null, "Item loaded from mCache: " + url);
imageView.setImageDrawable(drawable);
} else {
imageView.setImageDrawable(placeholder);
queueJob(url, imageView, placeholder);
}
}
private Drawable getDrawableFromCache(String url) {
if (mCache.containsKey(url)) {
return mCache.get(url).get();
}
return null;
}
private synchronized void putDrawableInCache(String url,Drawable drawable) {
int chacheControllerSize = mChacheController.size();
if (chacheControllerSize > MAX_CACHE_SIZE)
mChacheController.subList(0, MAX_CACHE_SIZE/2).clear();
mChacheController.addLast(drawable);
mCache.put(url, new SoftReference<Drawable>(drawable));
}
private void queueJob(final String url, final ImageView imageView,final Drawable placeholder) {
/* Create handler in UI thread. */
final Handler handler = new Handler() {
#Override
public void handleMessage(Message msg) {
String tag = mImageViews.get(imageView);
if (tag != null && tag.equals(url)) {
if (imageView.isShown())
if (msg.obj != null) {
imageView.setImageDrawable((Drawable) msg.obj);
} else {
imageView.setImageDrawable(placeholder);
//Log.d(null, "fail " + url);
}
}
}
};
mThreadPool.submit(new Runnable() {
#Override
public void run() {
final Drawable bmp = downloadDrawable(url);
// if the view is not visible anymore, the image will be ready for next time in cache
if (imageView.isShown())
{
Message message = Message.obtain();
message.obj = bmp;
//Log.d(null, "Item downloaded: " + url);
handler.sendMessage(message);
}
}
});
}
private Drawable downloadDrawable(String url) {
try {
InputStream is = getInputStream(url);
Drawable drawable = Drawable.createFromStream(is, url);
putDrawableInCache(url,drawable);
return drawable;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
private InputStream getInputStream(String urlString) throws MalformedURLException, IOException {
URL url = new URL(urlString);
URLConnection connection;
connection = url.openConnection();
connection.setUseCaches(true);
connection.connect();
InputStream response = connection.getInputStream();
return response;
}
}

I have followed this Android Training and I think it does an excellent job at downloading images without blocking the main UI. It also handles caching and dealing with scrolling through many images: Loading Large Bitmaps Efficiently

1. Picasso allows for hassle-free image loading in your application—often in one line of code!
Use Gradle:
implementation 'com.squareup.picasso:picasso:(insert latest version)'
Just one line of code!
Picasso.get().load("http://i.imgur.com/DvpvklR.png").into(imageView);
2. Glide An image loading and caching library for Android focused on smooth scrolling
Use Gradle:
repositories {
mavenCentral()
google()
}
dependencies {
implementation 'com.github.bumptech.glide:glide:4.11.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.11.0'
}
// For a simple view:
Glide.with(this).load("http://i.imgur.com/DvpvklR.png").into(imageView);
3. fresco is a powerful system for displaying images in Android
applications.Fresco takes care of image loading and display, so you don't have
to.
Getting Started with Fresco

I've written a tutorial that explains how to do lazy-loading of images in a listview. I go into some detail about the issues of recycling and concurrency. I also use a fixed thread pool to prevent spawning a lot of threads.
Lazy loading of images in Listview Tutorial

The way I do it is by launching a thread to download the images in the background and hand it a callback for each list item. When an image is finished downloading it calls the callback which updates the view for the list item.
This method doesn't work very well when you're recycling views however.

I just want to add one more good example, XML Adapters. As it's is used by Google and I am also using the same logic to avoid an OutOfMemory error.
Basically this ImageDownloader is your answer (as it covers most of your requirements). Some you can also implement in that.

This is a common problem on Android that has been solved in many ways by many people. In my opinion the best solution I've seen is the relatively new library called Picasso. Here are the highlights:
Open source, but headed up by Jake Wharton of ActionBarSherlock fame.
Asynchronously load images from network or app resources with one line of code
Automatic ListView detection
Automatic disk and memory caching
Can do custom transformations
Lots of configurable options
Super simple API
Frequently updated

I have been using NetworkImageView from the new Android Volley Library com.android.volley.toolbox.NetworkImageView, and it seems to be working pretty well. Apparently, this is the same view that is used in Google Play and other new Google applications. Definitely worth checking out.
Google I/O 2013 volley image cache tutorial
Developers Google events

Well, image loading time from the Internet has many solutions. You may also use the library Android-Query. It will give you all the required activity. Make sure what you want to do and read the library wiki page. And solve the image loading restriction.
This is my code:
#Override
public View getView(int position, View convertView, ViewGroup parent) {
View v = convertView;
if (v == null) {
LayoutInflater vi = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
v = vi.inflate(R.layout.row, null);
}
ImageView imageview = (ImageView) v.findViewById(R.id.icon);
AQuery aq = new AQuery(convertView);
String imageUrl = "http://www.vikispot.com/z/images/vikispot/android-w.png";
aq.id(imageview).progress(this).image(imageUrl, true, true, 0, 0, new BitmapAjaxCallback() {
#Override
public void callback(String url, ImageView iv, Bitmap bm, AjaxStatus status) {
iv.setImageBitmap(bm);
}
));
return v;
}
It should be solve your lazy loading problem.

I think this issue is very popular among Android developers, and there are plenty of such libraries that claims to resolve this issue, but only a few of them seems to be on the mark. AQuery is one such library, but it is better than most of them in all aspects and is worth trying for.

You must try this Universal Loader is best.
I am using this after done many RnD on lazy loading .
Universal Image Loader
Features
Multithread image loading (async or sync)
Wide customization of ImageLoader's configuration (thread executors, downloader, decoder, memory and disk cache, display image options, etc.)
Many customization options for every display image call (stub images, caching switch, decoding options, Bitmap processing and displaying, etc.)
Image caching in memory and/or on disk (device's file system or SD card)
Listening loading process (including downloading progress)
Android 2.0+ support

Have a look at Shutterbug, Applidium's lightweight SDWebImage (a nice library on iOS) port to Android.
It supports asynchronous caching, stores failed URLs, handles concurrency well, and helpful subclasses are included.
Pull requests (and bug reports) are welcome, too!

DroidParts has ImageFetcher that requires zero configuration to get started.
Uses a disk & in-memory Least Recently Used (LRU) cache.
Efficiently decodes images.
Supports modifying bitmaps in background thread.
Has simple cross-fade.
Has image loading progress callback.
Clone DroidPartsGram for an example:

Novoda also has a great lazy image loading library and many apps like Songkick, Podio, SecretDJ and ImageSearch use their library.
Their library is hosted here on Github and they have a pretty active issues tracker as well. Their project seems to be pretty active too, with over 300+ commits at the time of writing this reply.

Just a quick tip for someone who is in indecision regarding what library to use for lazy-loading images:
There are four basic ways.
DIY => Not the best solution but for a few images and if you want to go without the hassle of using others libraries
Volley's Lazy Loading library => From guys at android. It is nice and everything but is poorly documented and hence is a problem to use.
Picasso: A simple solution that just works, you can even specify the exact image size you want to bring in. It is very simple to use but might not be very "performant" for apps that has to deal with humongous amounts of images.
UIL: The best way to lazy load images. You can cache images(you need permission of course), initialize the loader once, then have your work done. The most mature asynchronous image loading library I have ever seen so far.

If you want to display Shimmer layout like Facebook there is a official facebook library for that. FaceBook Shimmer Android
It takes care of everything, You just need to put your desired design code in nested manner in shimmer frame.
Here is a sample code.
<com.facebook.shimmer.ShimmerFrameLayout
android:id=“#+id/shimmer_view_container”
android:layout_width=“wrap_content”
android:layout_height="wrap_content"
shimmer:duration="1000">
<here will be your content to display />
</com.facebook.shimmer.ShimmerFrameLayout>
And here is the java code for it.
ShimmerFrameLayout shimmerContainer = (ShimmerFrameLayout) findViewById(R.id.shimmer_view_container);
shimmerContainer.startShimmerAnimation();
Add this dependency in your gradle file.
implementation 'com.facebook.shimmer:shimmer:0.1.0#aar'
Here is how it looks like.

Check my fork of LazyList. Basically, I improve the LazyList by delaying the call of the ImageView and create two methods:
When you need to put something like "Loading image..."
When you need to show the downloaded image.
I also improved the ImageLoader by implementing a singleton in this object.

All above code have their own worth but with my personal experience just give a try with Picasso.
Picasso is a library specifically for this purpose, in-fact it will manage cache and all other network operations automatically.You will have to add library in your project and just write a single line of code to load image from remote URL.
Please visit here : http://code.tutsplus.com/tutorials/android-sdk-working-with-picasso--cms-22149

Use the glide library. It worked for me and will work for your code too.It works for both images as well as gifs too.
ImageView imageView = (ImageView) findViewById(R.id.test_image);
GlideDrawableImageViewTarget imagePreview = new GlideDrawableImageViewTarget(imageView);
Glide
.with(this)
.load(url)
.listener(new RequestListener<String, GlideDrawable>() {
#Override
public boolean onException(Exception e, String model, Target<GlideDrawable> target, boolean isFirstResource) {
return false;
}
#Override
public boolean onResourceReady(GlideDrawable resource, String model, Target<GlideDrawable> target, boolean isFromMemoryCache, boolean isFirstResource) {
return false;
}
})
.into(imagePreview);
}

I can recommend a different way that works like a charm: Android Query.
You can download that JAR file from here
AQuery androidAQuery = new AQuery(this);
As an example:
androidAQuery.id(YOUR IMAGEVIEW).image(YOUR IMAGE TO LOAD, true, true, getDeviceWidth(), ANY DEFAULT IMAGE YOU WANT TO SHOW);
It's very fast and accurate, and using this you can find many more features like animation when loading, getting a bitmap (if needed), etc.

Give Aquery a try. It has amazingly simple methods to load and cache images asynchronously.

URLImageViewHelper is an amazing library that helps you to do that.

public class ImageDownloader {
Map<String, Bitmap> imageCache;
public ImageDownloader() {
imageCache = new HashMap<String, Bitmap>();
}
// download function
public void download(String url, ImageView imageView) {
if (cancelPotentialDownload(url, imageView)) {
// Caching code right here
String filename = String.valueOf(url.hashCode());
File f = new File(getCacheDirectory(imageView.getContext()),
filename);
// Is the bitmap in our memory cache?
Bitmap bitmap = null;
bitmap = (Bitmap) imageCache.get(f.getPath());
if (bitmap == null) {
bitmap = BitmapFactory.decodeFile(f.getPath());
if (bitmap != null) {
imageCache.put(f.getPath(), bitmap);
}
}
// No? download it
if (bitmap == null) {
try {
BitmapDownloaderTask task = new BitmapDownloaderTask(
imageView);
DownloadedDrawable downloadedDrawable = new DownloadedDrawable(
task);
imageView.setImageDrawable(downloadedDrawable);
task.execute(url);
} catch (Exception e) {
Log.e("Error==>", e.toString());
}
} else {
// Yes? set the image
imageView.setImageBitmap(bitmap);
}
}
}
// cancel a download (internal only)
private static boolean cancelPotentialDownload(String url,
ImageView imageView) {
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
if (bitmapDownloaderTask != null) {
String bitmapUrl = bitmapDownloaderTask.url;
if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
bitmapDownloaderTask.cancel(true);
} else {
// The same URL is already being downloaded.
return false;
}
}
return true;
}
// gets an existing download if one exists for the imageview
private static BitmapDownloaderTask getBitmapDownloaderTask(
ImageView imageView) {
if (imageView != null) {
Drawable drawable = imageView.getDrawable();
if (drawable instanceof DownloadedDrawable) {
DownloadedDrawable downloadedDrawable = (DownloadedDrawable) drawable;
return downloadedDrawable.getBitmapDownloaderTask();
}
}
return null;
}
// our caching functions
// Find the dir to save cached images
private static File getCacheDirectory(Context context) {
String sdState = android.os.Environment.getExternalStorageState();
File cacheDir;
if (sdState.equals(android.os.Environment.MEDIA_MOUNTED)) {
File sdDir = android.os.Environment.getExternalStorageDirectory();
// TODO : Change your diretcory here
cacheDir = new File(sdDir, "data/ToDo/images");
} else
cacheDir = context.getCacheDir();
if (!cacheDir.exists())
cacheDir.mkdirs();
return cacheDir;
}
private void writeFile(Bitmap bmp, File f) {
FileOutputStream out = null;
try {
out = new FileOutputStream(f);
bmp.compress(Bitmap.CompressFormat.PNG, 80, out);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (out != null)
out.close();
} catch (Exception ex) {
}
}
}
// download asynctask
public class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
private String url;
private final WeakReference<ImageView> imageViewReference;
public BitmapDownloaderTask(ImageView imageView) {
imageViewReference = new WeakReference<ImageView>(imageView);
}
#Override
// Actual download method, run in the task thread
protected Bitmap doInBackground(String... params) {
// params comes from the execute() call: params[0] is the url.
url = (String) params[0];
return downloadBitmap(params[0]);
}
#Override
// Once the image is downloaded, associates it to the imageView
protected void onPostExecute(Bitmap bitmap) {
if (isCancelled()) {
bitmap = null;
}
if (imageViewReference != null) {
ImageView imageView = imageViewReference.get();
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
// Change bitmap only if this process is still associated with
// it
if (this == bitmapDownloaderTask) {
imageView.setImageBitmap(bitmap);
// cache the image
String filename = String.valueOf(url.hashCode());
File f = new File(
getCacheDirectory(imageView.getContext()), filename);
imageCache.put(f.getPath(), bitmap);
writeFile(bitmap, f);
}
}
}
}
static class DownloadedDrawable extends ColorDrawable {
private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;
public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
super(Color.WHITE);
bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(
bitmapDownloaderTask);
}
public BitmapDownloaderTask getBitmapDownloaderTask() {
return bitmapDownloaderTaskReference.get();
}
}
// the actual download code
static Bitmap downloadBitmap(String url) {
HttpParams params = new BasicHttpParams();
params.setParameter(CoreProtocolPNames.PROTOCOL_VERSION,
HttpVersion.HTTP_1_1);
HttpClient client = new DefaultHttpClient(params);
final HttpGet getRequest = new HttpGet(url);
try {
HttpResponse response = client.execute(getRequest);
final int statusCode = response.getStatusLine().getStatusCode();
if (statusCode != HttpStatus.SC_OK) {
Log.w("ImageDownloader", "Error " + statusCode
+ " while retrieving bitmap from " + url);
return null;
}
final HttpEntity entity = response.getEntity();
if (entity != null) {
InputStream inputStream = null;
try {
inputStream = entity.getContent();
final Bitmap bitmap = BitmapFactory
.decodeStream(inputStream);
return bitmap;
} finally {
if (inputStream != null) {
inputStream.close();
}
entity.consumeContent();
}
}
} catch (Exception e) {
// Could provide a more explicit error message for IOException or
// IllegalStateException
getRequest.abort();
Log.w("ImageDownloader", "Error while retrieving bitmap from "
+ url + e.toString());
} finally {
if (client != null) {
// client.close();
}
}
return null;
}
}

I had this issue and implemented lruCache. I believe you need API 12 and above or use the compatiblity v4 library. lurCache is fast memory, but it also has a budget, so if you're worried about that you can use a diskcache... It's all described in Caching Bitmaps.
I'll now provide my implementation which is a singleton I call from anywhere like this:
//Where the first is a string and the other is a imageview to load.
DownloadImageTask.getInstance().loadBitmap(avatarURL, iv_avatar);
Here's the ideal code to cache and then call the above in getView of an adapter when retrieving the web image:
public class DownloadImageTask {
private LruCache<String, Bitmap> mMemoryCache;
/* Create a singleton class to call this from multiple classes */
private static DownloadImageTask instance = null;
public static DownloadImageTask getInstance() {
if (instance == null) {
instance = new DownloadImageTask();
}
return instance;
}
//Lock the constructor from public instances
private DownloadImageTask() {
// Get max available VM memory, exceeding this amount will throw an
// OutOfMemory exception. Stored in kilobytes as LruCache takes an
// int in its constructor.
final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
// Use 1/8th of the available memory for this memory cache.
final int cacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
#Override
protected int sizeOf(String key, Bitmap bitmap) {
// The cache size will be measured in kilobytes rather than
// number of items.
return bitmap.getByteCount() / 1024;
}
};
}
public void loadBitmap(String avatarURL, ImageView imageView) {
final String imageKey = String.valueOf(avatarURL);
final Bitmap bitmap = getBitmapFromMemCache(imageKey);
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
} else {
imageView.setImageResource(R.drawable.ic_launcher);
new DownloadImageTaskViaWeb(imageView).execute(avatarURL);
}
}
private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemCache(key) == null) {
mMemoryCache.put(key, bitmap);
}
}
private Bitmap getBitmapFromMemCache(String key) {
return mMemoryCache.get(key);
}
/* A background process that opens a http stream and decodes a web image. */
class DownloadImageTaskViaWeb extends AsyncTask<String, Void, Bitmap> {
ImageView bmImage;
public DownloadImageTaskViaWeb(ImageView bmImage) {
this.bmImage = bmImage;
}
protected Bitmap doInBackground(String... urls) {
String urldisplay = urls[0];
Bitmap mIcon = null;
try {
InputStream in = new java.net.URL(urldisplay).openStream();
mIcon = BitmapFactory.decodeStream(in);
}
catch (Exception e) {
Log.e("Error", e.getMessage());
e.printStackTrace();
}
addBitmapToMemoryCache(String.valueOf(urldisplay), mIcon);
return mIcon;
}
/* After decoding we update the view on the main UI. */
protected void onPostExecute(Bitmap result) {
bmImage.setImageBitmap(result);
}
}
}

onPostExecute doesn't run, #override doesnt work android development

I've just gotten into android development, and while trying to create a login form i ran into some problems.
What I want to do is enter username and password and press login, when the login button is pushed I want it to do a JSON request with the account information to my server and get a response with whether or not the user is allowed to log in. If the user is allowed, I want to change to a new view.
My code receives the JSON information correctly, but from what I've understood the UI-code (pushing a new activity) should be done in onPostExecute(). My problem is that onPostExecute is never run, I've looked at other ppl with the same problem, but their solutions hasn't worked for me. What they have said is that i need to have an #Override before onPostExecute, but when I add that i get the compilation error that "the method does not override method from its superclass".
I've read solutions from people having that problem as well, and from what I have read the problem is that the method onPostExecute has to have the same parameters as the result parameter from doInBackground(). My problem is that I feel I already do, and when I try to look in what the superclass has (that is AsyncTask.java) it says that the method onPostExecute() looks like:
protected void onPostExecute(Result result) {
}
But I have no idea what class Result belongs to..
networkingTask is run using this line:
new networkingTask().execute(url);
If anyone could help me I'd be eternally grateful! :)
This is what my code looks like
private class networkingTask extends AsyncTask {
Context context;
private networkingTask(Context context) {
this.context = context.getApplicationContext();
}
public networkingTask() {
}
#Override
protected JSONObject doInBackground(Object... params) {
try {
String urlString = (String) params[0];
System.out.println(urlString);
// Creating JSON Parser instance
JSONParser jParser = new JSONParser();
// getting JSON string from URL
JSONObject json;
json = jParser.getJSONFromUrl(urlString);
String responseLogged = json.getString("logged");
System.out.println("can we log in?: "+ responseLogged);
return json;
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
#Override
protected void onPostExecute(JSONObject result) {
String responseLogged = "";
try {
responseLogged = result.getString("logged");
} catch (JSONException e) {
e.printStackTrace();
}
if(responseLogged.equals("true")){ //Login = true
Intent intent = new Intent(context, ShowListActivity.class);
intent.putExtra(EXTRA_JSON_OBJECT, result.toString());
startActivity(intent);
} else{ //Login = false
System.out.println("wrong password");
}
return;
}
}

In your line:
private class networkingTask extends AsyncTask
just change it to:
private class networkingTask extends AsyncTask<String, Void, JSONObject>
while String is the place for you to pass in the parameters, in your case it is url, the second parameter Void is for showing progress and the last one is the processed result to be passed from doInBackground to onPostExecute
For further explanation & info, please refer to Android Developers: AsyncTask

I think you may need to fill out the generic types for your AsyncTask. See the "Usage" section in the AsyncTask documentation.
Your class signature should look something like this:
private class networkingTask extends AsyncTask<Object, Void, JSONObject>
The types in the brackets here correspond to the argument types for doInBackground, onProgressUpdate, and onPostExecute, respectively, and are necessary if you're going to override these methods such that they are different from the generic method signatures.
Also, as a side note, it's a common convention in Java/Android to use upper CamelCase to start a class name. I'd also change the name of your class to "NetworkingTask" to help other developers looking at your code to better understand it.

The signatures don't match. You're attempting to override:
protected void onPostExecute(Result result)
with the signature
protected void onPostExecute(JSONObject result)
...which doesn't exist in the superclass.

I can't correctly access cached images while offline

I'm using AQuery to help cache some images. I know for fact the images are being cached correctly, but I'm having trouble accessing them unless I'm online.
I know they are being cached because I can see them on my SD card and if I replace mArtistImageURL in my return statement with the URL from my browser (copy/paste), the image will load while I'm offline, if it's already cached. I realize that I'm requesting to fetch the image URLs only if I'm online, but even so, I don't understand why the cached images won't load offline.
MusicUtils.isOnline() is basically a check to see if there is any connection. The main reason I have the bulk of my doInBackground() method wrapped in this is do to an Exception being thrown if I don't. FATAL EXCEPTION: AsyncTask #1 is all Logcat is putting out for me at the moment. I'm not sure why the stacktrace isn't complete.
I've tried using my AsyncTask in every way I know. I've returned a Bitmap, File, and String and I'm able to successfully load the image in each case, as long as I'm online. So, I have these cached images and I need some help putting them to use.
AsyncTask
public class loadArtistImage extends AsyncTask<String, Integer, Bitmap> {
#Override
protected Bitmap doInBackground(String... arg0) {
// Get artist image
if (MusicUtils.isOnline(mContext)) {
mArtistResults = Artist.getImages("Andrew Bird", 1, 1, key);
mArtistIterator = mArtistResults.getPageResults().iterator();
while (mArtistIterator.hasNext()) {
mArtistImageURL = mArtistIterator.next().getImageURL(
ImageSize.ORIGINAL);
}
}
aq.cache(mArtistImageURL, 60000 * 1440);
return aq.getCachedImage(mArtistImageURL);
}
#Override
protected void onPostExecute(Bitmap result) {
bm = result;
if (bm!= null) {
mArtistImage.postDelayed(new Runnable() {
#Override
public void run() {
mArtistImage.startAnimation(AnimationUtils
.loadAnimation(mContext, android.R.anim.fade_in));
MusicUtils.setArtistBackground(mArtistImage, bm);
}
}, 666);
} else {
// TODO something
}
super.onPostExecute(result);
}
}

Mm. While offline what the value of mArtistImageURL will be? Null? So at this point aq.cache(mArtistImageURL, 60000 * 1440); you are caching null, and at next line you are trying to get cache for null? Am I right?
Update: I suppose you cache images using url hash value for file name or something like this. So, modify your cache method to accept additional parameter for filename, for example aq.cache(mArtistImageURL,mArtistNameWithoutSpaces, 60000 * 1440); and when you want a file offline just call aq.getCachedImage(mArtistNameWithoutSpaces); so it will look at your cache folder and return a right file, or stub image if no file is found.
Update 2: Okay, if you dont want to modify AQuery cache methods you can try (againg) using shared preferences (so they would work) like this (not tested):
public String getUrlForArtist(String artistName) {
SharedPreferences settings = mContext.getSharedPreferences(
"artistImages", 0);
return settings.getString(artistName, "http://default.url");
}
public void setUrlForArtist(String artistName, String url) {
SharedPreferences settings = mContext.getSharedPreferences(
"artistImages", 0);
SharedPreferences.Editor editor = settings.edit();
editor.putString(artistName, url);
editor.commit();
}
So in the end your code will look something like this:
if (MusicUtils.isOnline(mContext)) {
mArtistResults = Artist.getImages("Andrew Bird", 1, 1, key);
mArtistIterator = mArtistResults.getPageResults().iterator();
while (mArtistIterator.hasNext()) {
mArtistImageURL = mArtistIterator.next().getImageURL(
ImageSize.ORIGINAL);
if(mArtistImageURL!=null)
setUrlForArtist("Andrew Bird", mArtistImageURL);
}
} else
mArtistImageURL = getUrlForArtist("Andrew Bird");
Oh, yeah, I dont know how AQuery behaves, but anyway you probably should only cache images while online.