Edit as of 30.12.2015 - The Ultimate Guide to image downloading
last major update: Mar 31 2016
TL;DR a.k.a. stop talking, just give me the code!!
Skip to the bottom of this post, copy the BasicImageDownloader (javadoc version here)
into your project, implement the OnImageLoaderListener interface
and you're done.
Note: though the BasicImageDownloader handles possible errors
and will prevent your app from crashing in case anything goes wrong, it will not perform
any post-processing (e.g. downsizing) on the downloaded Bitmaps.
Since this post has received quite a lot of attention, I have decided to completely rework it to prevent the folks from using deprecated technologies, bad programming practices or just doing silly things - like looking for "hacks" to run network on the main thread or accept all SSL certs.
I've created a demo project named "Image Downloader" that demonstrates how to download (and save) an image using my own downloader implementation, the Android's built-in DownloadManager as well as some popular open-source libraries. You can view the complete source code or download the project on GitHub.
Note: I have not adjusted the permission management for SDK 23+ (Marshmallow) yet, thus the project is targeting SDK 22 (Lollipop).
In my conclusion at the end of this post I will share my humble opinion about the proper use-case for each particular way of image downloading I've mentioned.
Let's start with an own implementation (you can find the code at the end of the post). First of all, this is a BasicImageDownloader and that's it. All it does is connecting to the given url, reading the data and trying to decode it as a Bitmap, triggering the OnImageLoaderListener interface callbacks when appropriate.
The advantage of this approach - it is simple and you have a clear overview of what's going on. A good way to go if all you need is downloading/displaying and saving some images, whilst you don't care about maintaining a memory/disk cache.
Note: in case of large images, you might need to scale them
down.
--
Android DownloadManager is a way to let the system handle the download for you. It's actually capable of downloading any kind of files, not just images. You may let your download happen silently and invisible to the user, or you can enable the user to see the download in the notification area. You can also register a BroadcastReceiver to get notified after you download is complete. The setup is pretty much straightforward, refer to the linked project for sample code.
Using the DownloadManager is generally not a good idea if you also want to display the image, since you'd need to read and decode the saved file instead of just setting the downloaded Bitmap into an ImageView. The DownloadManager also does not provide any API for you app to track the download progress.
--
Now the introduction of the great stuff - the libraries. They can do much more than just downloading and displaying images, including: creating and managing the memory/disk cache, resizing images, transforming them and more.
I will start with Volley, a powerful library created by Google and covered by the official documentation. While being a general-purpose networking library not specializing on images, Volley features quite a powerful API for managing images.
You will need to implement a Singleton class for managing Volley requests and you are good to go.
You might want to replace your ImageView with Volley's NetworkImageView, so the download basically becomes a one-liner:
((NetworkImageView) findViewById(R.id.myNIV)).setImageUrl(url, MySingleton.getInstance(this).getImageLoader());
If you need more control, this is what it looks like to create an ImageRequest with Volley:
ImageRequest imgRequest = new ImageRequest(url, new Response.Listener<Bitmap>() {
@Override
public void onResponse(Bitmap response) {
//do stuff
}
}, 0, 0, ImageView.ScaleType.CENTER_CROP, Bitmap.Config.ARGB_8888,
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
//do stuff
}
});
It is worth mentioning that Volley features an excellent error handling mechanism by providing the VolleyError class that helps you to determine the exact cause of an error. If your app does a lot of networking and managing images isn't its main purpose, then Volley it a perfect fit for you.
--
Square's Picasso is a well-known library which will do all of the image loading stuff for you. Just displaying an image using Picasso is as simple as:
Picasso.with(myContext)
.load(url)
.into(myImageView);
By default, Picasso manages the disk/memory cache so you don't need to worry about that. For more control you can implement the Target interface and use it to load your image into - this will provide callbacks similar to the Volley example. Check the demo project for examples.
Picasso also lets you apply transformations to the downloaded image and there are even other libraries around that extend those API. Also works very well in a RecyclerView/ListView/GridView.
--
Universal Image Loader is an another very popular library serving the purpose of image management. It uses its own ImageLoader that (once initialized) has a global instance which can be used to download images in a single line of code:
ImageLoader.getInstance().displayImage(url, myImageView);
If you want to track the download progress or access the downloaded Bitmap:
ImageLoader.getInstance().displayImage(url, myImageView, opts,
new ImageLoadingListener() {
@Override
public void onLoadingStarted(String imageUri, View view) {
//do stuff
}
@Override
public void onLoadingFailed(String imageUri, View view, FailReason failReason) {
//do stuff
}
@Override
public void onLoadingComplete(String imageUri, View view, Bitmap loadedImage) {
//do stuff
}
@Override
public void onLoadingCancelled(String imageUri, View view) {
//do stuff
}
}, new ImageLoadingProgressListener() {
@Override
public void onProgressUpdate(String imageUri, View view, int current, int total) {
//do stuff
}
});
The opts argument in this example is a DisplayImageOptions object. Refer to the demo project to learn more.
Similar to Volley, UIL provides the FailReason class that enables you to check what went wrong on download failure. By default, UIL maintains a memory/disk cache if you don't explicitly tell it not to do so.
Note: the author has mentioned that he is no longer maintaining the project as of Nov 27th, 2015. But since there are many contributors, we can hope that the Universal Image Loader will live on.
--
Facebook's Fresco is the newest and (IMO) the most advanced library that takes image management to a new level: from keeping Bitmaps off the java heap (prior to Lollipop) to supporting animated formats and progressive JPEG streaming.
To learn more about ideas and techniques behind Fresco, refer to this post.
The basic usage is quite simple. Note that you'll need to call Fresco.initialize(Context); only once, preferable in the Application class. Initializing Fresco more than once may lead to unpredictable behavior and OOM errors.
Fresco uses Drawees to display images, you can think of them as of ImageViews:
<com.facebook.drawee.view.SimpleDraweeView
android:id="@+id/drawee"
android:layout_width="match_parent"
android:layout_height="match_parent"
fresco:fadeDuration="500"
fresco:actualImageScaleType="centerCrop"
fresco:placeholderImage="@drawable/placeholder_grey"
fresco:failureImage="@drawable/error_orange"
fresco:placeholderImageScaleType="fitCenter"
fresco:failureImageScaleType="centerInside"
fresco:retryImageScaleType="centerCrop"
fresco:progressBarImageScaleType="centerInside"
fresco:progressBarAutoRotateInterval="1000"
fresco:roundAsCircle="false" />
As you can see, a lot of stuff (including transformation options) gets already defined in XML, so all you need to do to display an image is a one-liner:
mDrawee.setImageURI(Uri.parse(url));
Fresco provides an extended customization API, which, under circumstances, can be quite complex and requires the user to read the docs carefully (yes, sometimes you need to RTFM).
I have included examples for progressive JPEG's and animated images into the sample project.
Conclusion - "I have learned about the great stuff, what should I use now?"
Note that the following text reflects my personal opinion and should
not be taken as a postulate.
- If you only need to download/save/display some images, don't plan to use them in a
Recycler-/Grid-/ListView and don't need a whole bunch of images to be display-ready, the BasicImageDownloader should fit your needs.
- If your app saves images (or other files) as a result of a user or an automated action and you don't need the images to be displayed often, use the Android DownloadManager.
- In case your app does a lot of networking, transmits/receives
JSON data, works with images, but those are
