如何在Android ListView中最好地从URL加载图像

Question

So I have been struggling with the best way to load images in a ListView in Android for a while.

The images come from a server, so can take some time to load. From what I understand there are only 2 ways to implement this:

1 - Load the images on the main thread - This leads to have the correct images display immediately when the view displays and scrolls, but gives poor scrolling performance and can lead to the app hanging and crashing.

2 - Load the images in an AsyncTask. This means the images will be blank when the list display or scroll, but eventually display. Because of caching done by the list, you can also get the wrong image for an item. But this gives good performance and scrolling, and does not hang/crash.

Seems like neither solution works correctly. There must be a solution that works?? I have seen other posts like this, but the answer seems to always be 1 or 2, but neither is a good solution...

My code for the list adapter is below. The HttpGetImageAction.fetchImage() method either executes an async task, or fetches the image on the main thread depending on a flag I set. I also cache the images locally and on disk once they have been loaded, so the issue mainly occur the first time (but the cache is only for 100 images, and the list has 1000s)

public class ImageListAdapter extends ArrayAdapter<WebMediumConfig> {

Activity activity;

public ImageListAdapter(Activity activity, int resourceId, List<WebMediumConfig> items) {
    super(activity, resourceId, items);
    this.activity = activity;
}

class ImageListViewHolder {
    ImageView imageView;
    TextView nameView;
    TextView descriptionView;
    TextView statView;
}

public View getView(int position, View convertView, ViewGroup parent) {
    ImageListViewHolder holder = null;
    WebMediumConfig config = getItem(position);

    LayoutInflater mInflater = (LayoutInflater)this.activity.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
    if (convertView == null) {
        convertView = mInflater.inflate(R.layout.image_list, null);
        holder = new ImageListViewHolder();
        holder.imageView = (ImageView)convertView.findViewById(R.id.imageView);
        holder.nameView = (TextView) convertView.findViewById(R.id.nameView);
        holder.descriptionView = (TextView)convertView.findViewById(R.id.descriptionView);
        holder.statView = (TextView)convertView.findViewById(R.id.statView);
        convertView.setTag(holder);
    } else {
        holder = (ImageListViewHolder) convertView.getTag();
    }

    holder.nameView.setText(Utils.stripTags(config.name));
    holder.descriptionView.setText(Utils.stripTags(config.description));
    holder.statView.setText(config.stats());
    if (MainActivity.showImages) {
        HttpGetImageAction.fetchImage(this.activity, config.avatar, holder.imageView);
    } else {
        holder.imageView.setVisibility(View.GONE);
    }

    return convertView;
  }
}

Answer 1

You should never load images on the main thread. Any network calls should be done asynchronously and should not be done on the main thread. Your images might load quickly (on the main thread) because you may have a good internet connection, have small images and/or have few images. But imagine what would happen if someone using your app has a slower internet and then one day your list grows to hundreds!

The approach below is a well accepted practice for loading scrollview content

• a) load your text content as you load the scrollview. Again the loading should be done async but you can show a loading view until the download completes
• b) Show image placeholders while the image loads on each cell
• c) Load the images asynchronously

Using a library such as Picasso would help immensely because there is a lot of boilerplate code which you'd need to handle otherwise such as

• cancelling image download when a cell is reused and
• starting a new download for a reused cell
• caching
• retrying a failed download

Hope this helps.

Answer 2

Use Glide [ https://github.com/bumptech/glide] or Picasso [ http://square.github.io/picasso/] - both libraries are very similar, and easy to use, have caching, work in background, allow placeholders ets. Usage is as simple as:

Picasso.get().load("http://i.imgur.com/DvpvklR.png").into(imageView);

Answer 3

Simple Solution: Glide.

Glide is a fast and efficient open source media management and image loading 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.

You can load the image like:

GlideApp.with(context)
    .load("http://via.placeholder.com/300.png")
    .into(imageView);

Refer for more info:

https://guides.codepath.com/android/Displaying-Images-with-the-Glide-Library https://github.com/bumptech/glide

Answer 4

Use Glide library.. Its recommended by Google .

Step 1:

Use latest Glide Version 4 dependency ( https://github.com/bumptech/glide )

implementation 'com.github.bumptech.glide:glide:4.8.0'

annotationProcessor 'com.github.bumptech.glide:compiler:4.8.0'

Step 2:

 Glide.with(MainActivity.this)
        .load(url)
        .apply(new RequestOptions().placeholder(R.drawable.placeholder).error(R.drawable.error_image))//this line optional - you can skip this line
        .into(imageview);

Answer 5

use picasso or Glide Library What these library do , is that they image url from u and download them and save it in cache so that whenever you open the same image another time it will load faster and it prevents the usage of the network

Picasso http://square.github.io/picasso/

Glide https://github.com/bumptech/glide

Answer 6

You can try UnivarsalImageLoader which is quite Fast and Optimised. It supports Multithread image loading (async or sync) and high level of customization.

https://github.com/nostra13/Android-Universal-Image-Loader

Answer 7

Here's how I do it

First I always create the RecyclerView.ViewHolder to which I pass a Func<int, Bitmap> image_provider (or you can pass some sort of lamda?) which when called will retrieve the image for a specific Id, if it can't it will show the default loading image

Everything is stored in a fragment (which I call a DataFragment) which has retainstate set to true.

First load a list of json objects that tell me which images I must show, this happens inside the data fragment so it will go on if there is a configuration change

I send that info to the adapter using a method like SetItems(MyImages)

Then I start a new thread that will load the images (while allowing the user to work)

I do this with the TPL library, the closest approximation for Android is Anko Async but you can also do it with AsyncTask, the problem is that I send a lot of messages to the main thread, so to do it with AsyncTask you have to give it a handler to the main thread which will send messages.

In the thread I loop through all the images I have to download, and download them and send a message to the DataFragment which sends it to the currently attached Activity which triggers the NotifyItemChanged method in the adapter

The adapter on creation receives a Func<int, Bitmap> image_provider which it invokes and retrieves the image from memory

I know it sounds a bit messy, so if you want code examples I can give them , but they are in C#

Answer 8

In your app level gradle implement below repository:

implementation 'com.squareup.picasso:picasso:2.5.2'

Put below line where you want to load image

 Picasso.with(getApplicationContext()).load(config.avatar).into(imageview);

Answer 9

just use Picasso.get().load(" http://i.imgur.com/DvpvklR.png ").into(imageView); or Glide.with(getApplicationContext()).load("image_url").asBitmap().centerCrop().into(new BitmapImageViewTarget(imageView)

in adapter like any view and done no extra efforts required