Introducing Coil, an open-source Android image loading library backed by Kotlin Coroutines

Yesterday, Colin White, a Senior Android Engineer at Instacart, introduced Coroutine Image Loader (Coil). It is a fast, lightweight, and modern image loading library for Android backed by Kotlin.

https://twitter.com/colinwhi/status/1160943333033648128

Currently, there are a number of image loading libraries for Android such as Glide, Fresco, Picasso, Mirage, among others. However, the Instacart team aims to introduce a new library that is “more modern and simpler” with Coil.

Key features in Coil

Backed by Kotlin

Coil offers a “simple, elegant API” by leveraging the Kotlin language features like extension functions, inlining, lambda params, and sealed classes. It provides strong support for non-blocking asynchronous computation and work cancellation while ensuring maximum thread reuse with the help of Kotlin Coroutines.

Leverages modern dependencies

Coil relies on dependencies that are standard and recommended such as OkHttp, Okio, and AndroidX Lifecycles. Square’s OkHttp and Okio are by default efficient and enables Coil to avoid reimplementing things like disk caching and stream buffering. Likewise, AndroidX Lifecycles is a recommended way for tracking the lifecycle state.

Lightweight

Coil’s codebase consists of 8x fewer lines of code as compared to Glide. It adds approximately 1500 methods to your APK, which is comparable to Picasso and significantly less than Glide and Fresco.

Supports extension

The image pipeline of Coil consists of three main classes: Mappers, Fetchers, and Decoders. You can use these interfaces to augment or override the base behavior and add support for new file types in Coil.

Supports dynamic image sampling

Coil comes with a new feature, dynamic image sampling. Consider you want to load a 500x500 image into a 100x100 ImageView. The library will load the image into memory at 100x100. But, what if you want the quality to be as the 500x500 image? In this case, the 100x100 image is used as a placeholder while the 500x500 image is read. Coil will take care of this automatically for all BitmapDrawables. The placeholder is set synchronously on the main thread preventing white flashes where the ImageView is empty for one frame. It also creates a visual effect where the image detail appears to fade in with the help of the crossfade animation.

To know more in detail about Coil, check out its official documentation and GitHub repository.

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