What's LazyList?

Question

I can't find in any really credible source explaining what LazyList is. Anyone?

Answer 1

AFAIK, I'll explain you with the example If you list contain lot of images with Text, it will take some time for your list to load because you need to download images and you need to populate them in the list. Suppose if your list contains 100 images It will take lot of time to download each image and to show it the listitem. To make the user wait until the images loads is not user friendly. so What we need to do. At this point of time lazy list comes into picture. It is the idea that let the images be loaded in background and show text mean while.

Everybody know that listview recycle its views for every view. i.e if your listview contains 40 elemtns then listview won't allocate memory for 40 items instead it allocate memory for the visible items, i.e say you can see only 10 items at a time. so listview will allocate 10 items meemory.

So When ever you scroll the view, then the view will refresh. because of the you'll lose your reference to images and you need to download them agian. in order to avoid that, caching comes into picture.

This example is based on my knowledge in listview, I am not saying this is only correct. There might be wrong in the answer, if any body find feel free to inform me.

Answer 2

The best example of lazy list is facebook notifications,messages,requests. when you scroll then data will be load.

Answer 3

I think this is the other way around. AFAIK, Lazy Loading is the definition, where you actually load the data only when you need it, and it's a good design practice.

So I believe the same applies for this, only this time it's being referring to the List View.

If I'm wrong, please correct me.

Answer 4

Lazy List is lazy loading of images from sd-card or from server using urls. It is like on demand loading images.

Images can be cached to local sd-card or phone memory. Url is considered the key. If the key is present in sd-card, display images from sd-card else display image by downloading from server and cache the same to location of your choice. The cache limit can set. You can also choose your own location to cache images. Cache can also be cleared.

Instead of user waiting to download large images and then displaying lazy list, loads images on demand. Since images are cached you can display images offline.

https://github.com/thest1/LazyList. Lazy List

In your getview

imageLoader.DisplayImage(imageurl, imageview);

ImageLoader Display method

    public void DisplayImage(String url, ImageView imageView) //url and imageview as parameters
    {
    imageViews.put(imageView, url);
    Bitmap bitmap=memoryCache.get(url);   //get image from cache using url as key
    if(bitmap!=null)         //if image exists
        imageView.setImageBitmap(bitmap);  //dispaly iamge
     else   //downlaod image and dispaly. add to cache.
     {
        queuePhoto(url, imageView);
        imageView.setImageResource(stub_id);
     }
   }

An alternative to Lazy List is Universal Image Loader

https://github.com/nostra13/Android-Universal-Image-Loader. It is based on Lazy List(works on same principle). But it has lot of other configurations. I would prefer to use ****Universal Image Loader**** coz it gives you more configuration options. You can display a error image if downlaod failed. Can display images with rounded corners. Can cache on disc or memory. Can compress image.

In your custom adapter constructor

  File cacheDir = StorageUtils.getOwnCacheDirectory(a, "your folder");

 // Get singletone instance of ImageLoader
 imageLoader = ImageLoader.getInstance();
 // Create configuration for ImageLoader (all options are optional)
 ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(a)
          // You can pass your own memory cache implementation
         .discCache(new UnlimitedDiscCache(cacheDir)) // You can pass your own disc cache implementation
         .discCacheFileNameGenerator(new HashCodeFileNameGenerator())
         .enableLogging()
         .build();
 // Initialize ImageLoader with created configuration. Do it once.
 imageLoader.init(config);
 options = new DisplayImageOptions.Builder()
.showStubImage(R.drawable.stub_id)//display stub image
.cacheInMemory()
.cacheOnDisc()
.displayer(new RoundedBitmapDisplayer(20))
.build();

In your getView()

  ImageView image=(ImageView)vi.findViewById(R.id.imageview); 
  imageLoader.displayImage(imageurl, image,options);//provide imageurl, imageview and options

You can configure with other options to suit your needs.

Along with lazy loading/Universal Image Loader you can view holder for smooth scrolling and performance. http://developer.android.com/training/improving-layouts/smooth-scrolling.html.