Using Coil for Image Loading in XML UI

While Jetpack Compose is the future of Android UI development, many Android applications still rely on XML layouts for their user interfaces. When working with XML, loading images efficiently is a common requirement. Coil, a modern image loading library for Android, simplifies the process and provides numerous benefits. This article will guide you through integrating and using Coil in your XML-based Android UI.

What is Coil?

Coil (Coroutine Image Loader) is a Kotlin-first image loading library for Android backed by Kotlin Coroutines. It is lightweight, easy to use, and modern. Coil handles memory management, caching, and image decoding efficiently, making it an excellent choice for Android developers.

Why Use Coil in XML UI?

• Simplicity: Easy to set up and use with minimal boilerplate.
• Performance: Built-in memory management and caching for optimized performance.
• Coroutines: Leverages Kotlin Coroutines for asynchronous image loading, preventing UI blocking.
• Modern: Designed with modern Android development practices in mind.

How to Integrate Coil in XML UI

Here’s a step-by-step guide on integrating and using Coil in your XML UI:

Step 1: Add Coil Dependency

Add the Coil dependency to your build.gradle file:

dependencies {
    implementation("io.coil-kt:coil:2.5.0")
}

Make sure to sync your project after adding the dependency.

Step 2: Update AndroidManifest.xml (Optional)

If your app targets Android 9 (API level 28) or lower, you might need to enable cleartext traffic for local URLs by adding android:usesCleartextTraffic="true" to your application tag in AndroidManifest.xml. This is only needed if you are loading images from non-HTTPS URLs:

<application
    android:usesCleartextTraffic="true"
    ...>
</application>

This step is generally not recommended for production apps unless necessary.

Step 3: Use Coil in Your XML Layout

Now, you can use Coil to load images in your XML layouts. Use the ImageView and set the image programmatically using Coil’s extension function.

First, define your ImageView in the XML layout:

<ImageView
    android:id="@+id/myImageView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:scaleType="centerCrop"
    xmlns:android="http://schemas.android.com/apk/res/android" />

Step 4: Load Image using Coil in Your Activity/Fragment

In your Activity or Fragment, load the image into the ImageView using Coil:

import android.os.Bundle
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity
import coil.load

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val imageView: ImageView = findViewById(R.id.myImageView)
        val imageUrl = "https://www.example.com/image.jpg"

        imageView.load(imageUrl) {
            placeholder(R.drawable.placeholder_image)
            error(R.drawable.error_image)
            crossfade(true)
        }
    }
}

In this example:

• We get a reference to the ImageView using findViewById().
• We use the load() extension function provided by Coil on the ImageView.
• We pass the imageUrl as the source of the image.
• We configure optional parameters like placeholder, error, and crossfade.
• placeholder(R.drawable.placeholder_image): Shows a placeholder image while the actual image is loading.
• error(R.drawable.error_image): Shows an error image if the image fails to load.
• crossfade(true): Animates the transition between the placeholder and the loaded image.

Step 5: Load Image from Resources

Coil also supports loading images from local resources:

imageView.load(R.drawable.my_image) {
    placeholder(R.drawable.placeholder_image)
    error(R.drawable.error_image)
}

Step 6: Load Image from Files

You can also load images from files using Coil:

import java.io.File

val imageFile = File(context.filesDir, "my_image.jpg")
imageView.load(imageFile) {
    placeholder(R.drawable.placeholder_image)
    error(R.drawable.error_image)
}

Advanced Coil Features

Coil offers several advanced features to fine-tune image loading:

Transformations

You can apply transformations to the loaded image using Coil. For example, applying a circular crop:

import coil.transform.CircleCropTransformation

imageView.load(imageUrl) {
    transformations(CircleCropTransformation())
    placeholder(R.drawable.placeholder_image)
    error(R.drawable.error_image)
}

Request Priority

Set the priority of an image loading request:

import coil.request.ImageRequest
import coil.request.Priority

imageView.load(imageUrl) {
    priority(Priority.HIGH)
    placeholder(R.drawable.placeholder_image)
    error(R.drawable.error_image)
}

Caching

Coil automatically handles caching. You can configure caching behavior using MemoryCachePolicy and DiskCachePolicy. Typically you will not need to configure this as the default policy is usually good enough for most cases.

import coil.request.CachePolicy

imageView.load(imageUrl) {
    memoryCachePolicy(CachePolicy.ENABLED)
    diskCachePolicy(CachePolicy.ENABLED)
    placeholder(R.drawable.placeholder_image)
    error(R.drawable.error_image)
}

Best Practices

• Use Placeholders: Always use placeholder images to improve the user experience while images are loading.
• Handle Errors: Provide error images to gracefully handle cases where images fail to load.
• Optimize Images: Ensure your images are optimized for size and resolution to reduce loading times.
• Leverage Caching: Take advantage of Coil’s built-in caching mechanisms to improve performance.
• Use Transformations Judiciously: Applying transformations can be resource-intensive, so use them wisely.

Conclusion

Integrating Coil into your XML-based Android UI simplifies image loading and provides numerous performance and usability benefits. With its easy-to-use API, Kotlin Coroutines support, and built-in caching, Coil is an excellent choice for modern Android development, whether you’re using XML or Jetpack Compose.