Fresco Image Management for Android

Fresco is an image management system for Android that handles loading and displaying images from various sources including the network, local storage, or local resources.

Key features include:

• Automatic Loading & Placeholders: Manages the lifecycle of image loading and displays placeholders until the image is ready.
• Two-Level Caching: Utilizes both an in-memory cache and an internal storage cache.
• Memory Management: On Android 4.x and lower, Fresco uses a special region of Android memory to reduce OutOfMemoryError occurrences.
• Advanced Formats: Supports streaming progressive JPEGs and displaying animated GIFs and WebPs.
• Customization: Provides extensive options for customizing how images are loaded and displayed.

Fresco's image pipeline manages the loading of images from various sources, including the network, local storage, or local resources. To optimize performance and reduce CPU usage, the pipeline utilizes a three-level caching system:

1. Two levels of in-memory cache
2. One level of internal storage cache

To create a custom progress indicator, extend the Drawable class. To ensure the progress bar accurately reflects the loading state, you must override the onLevelChange(int level) method.

Note that the level parameter is provided on a scale of 0 to 10,000, where 10,000 represents a fully downloaded image. Use this value within your implementation to update the drawable's appearance.

class CustomProgressBar extends Drawable {
   @Override
   protected boolean onLevelChange(int level) {
     // level is on a scale of 0-10,000
     // where 10,000 means fully downloaded

     // your app's logic to change the drawable's
     // appearance here based on progress
   }
}

The actual image is the primary target image. Unlike other branches, it is managed by the controller rather than the hierarchy.

To set the actual image, use:

• setImageURI(uri)
• Or build a custom controller using setController.

Specific properties available only for the actual image include:

• Scale Type: Defaults to centerCrop.
• Focus Point: Used specifically with the focusCrop scale type.
• Color Filter.

Fresco's caches implement DiskTrimmable and MemoryTrimmable interfaces. This allows your application to trigger emergency evictions when the device is low on storage or memory.

To implement this, you must configure the pipeline with objects implementing DiskTrimmableRegistry and MemoryTrimmableRegistry. These registries should maintain a list of trimmables and use your app-specific logic to decide when to notify them to perform a trim.

For advanced loading logic, you can provide a custom DataSource Supplier when building a DraweeController. This allows you to implement custom selection logic or compose existing suppliers.

Reference implementations include:

• FirstAvailableDataSourceSupplier
• IncreasingQualityDataSourceSupplier

See AbstractDraweeControllerBuilder for details on how to compose these suppliers.

If you do not need to keep the original version of an image (e.g., for circular profile pictures), use a BitmapTransformation instead of a PostProcessor.

Advantages:

• Applied immediately after decoding.
• The original image is not cached, saving memory.
• No additional Bitmap allocation is required because it modifies the original directly.

Implement the transform(Bitmap bitmap) method to perform the modification.

public class CircularBitmapTransformation implements BitmapTransformation {
  @Override
  public void transform(Bitmap bitmap) {
    NativeRoundingFilter.toCircle(bitmap);
  }

  @Override
  public boolean modifiesTransparency() {
    return true; // Indicates the transformation uses transparent pixels
  }
}