Trigger Animated Vector Drawables with Kotlin in Android XML Development

Animated Vector Drawables (AVDs) in Android provide a powerful way to bring your UI to life with engaging animations. Instead of relying solely on traditional image resources, AVDs use XML to describe shapes and animations. This allows for smaller file sizes, smoother scaling, and intricate animations that respond to user interactions. Kotlin, with its modern syntax and powerful features, makes triggering and controlling AVDs straightforward. In this detailed guide, we’ll delve into how to use Animated Vector Drawables with Kotlin in your Android XML development to create captivating user experiences.

What are Animated Vector Drawables (AVDs)?

Animated Vector Drawables are XML-based drawable resources that define a vector graphic along with animations that modify the vector’s properties over time. These are efficient for creating scalable animations without increasing your app’s size dramatically. AVDs consist of two primary parts:

1. Vector Drawable: Describes the static vector graphic using paths, groups, and shapes.
2. Animation: Defines property animators that change attributes of the vector graphic, such as rotation, translation, scale, and pathData.

Why Use Animated Vector Drawables?

• Small Size: Vector graphics are lightweight, leading to smaller APK sizes compared to traditional raster images.
• Scalability: They scale smoothly to any screen density without pixelation.
• Performance: Animations are hardware-accelerated, providing smoother performance.
• Flexibility: You can animate almost any vector property, enabling complex and customized animations.

How to Trigger Animated Vector Drawables in Kotlin in Android XML

Step 1: Create the Vector Drawable

First, you need to create a vector drawable. For this example, let’s create a simple vector graphic representing a heart.

Create a file named vector_heart.xml in your res/drawable directory:

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="24dp"
    android:height="24dp"
    android:viewportWidth="24"
    android:viewportHeight="24">
    <path
        android:fillColor="#FF0000"
        android:pathData="M12,21.35l-1.45,-1.32C5.4,15.36 2,12.28 2,8.5 2,5.41 4.42,3 7.5,3c1.74,0 3.41,0.81 4.5,2.09C13.09,3.81 14.76,3 16.5,3 19.58,3 22,5.41 22,8.5 22,12.28 18.6,15.36 12,21.35z"/>
</vector>

Step 2: Create the Animated Vector Drawable

Next, create an animated vector drawable that animates the heart. This involves creating the animation definitions and linking them to the vector drawable.

Create a file named animated_heart.xml in the res/drawable directory:

<animated-vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:drawable="@drawable/vector_heart">
    <target
        android:name="heart"
        android:animation="@anim/heart_fill_animation"/>
</animated-vector>

Here, android:drawable points to our static vector drawable, and <target> links a specific part of the vector drawable (named “heart” – more on that below) to an animation defined in @anim/heart_fill_animation.

Step 3: Create the Animation Resource

Define the animation in the res/anim directory. Create a file named heart_fill_animation.xml:

<objectAnimator xmlns:android="http://schemas.android.com/apk/res/android"
    android:propertyName="fillColor"
    android:duration="500"
    android:valueFrom="#FF0000"
    android:valueTo="#00FF00"
    android:valueType="colorType"
    android:interpolator="@android:interpolator/fast_out_slow_in"/>

This animation changes the fill color of the heart from red (#FF0000) to green (#00FF00) over 500 milliseconds.

Step 4: Setting the Name Property in the Vector Drawable

Make sure that the path you want to animate in vector_heart.xml has a android:name attribute, which is then referenced by the target in animated_heart.xml:

<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="24dp"
    android:height="24dp"
    android:viewportWidth="24"
    android:viewportHeight="24">
    <path
        android:name="heart"
        android:fillColor="#FF0000"
        android:pathData="M12,21.35l-1.45,-1.32C5.4,15.36 2,12.28 2,8.5 2,5.41 4.42,3 7.5,3c1.74,0 3.41,0.81 4.5,2.09C13.09,3.81 14.76,3 16.5,3 19.58,3 22,5.41 22,8.5 22,12.28 18.6,15.36 12,21.35z"/>
</vector>

The android:name="heart" is what connects the animation to this specific path.

Step 5: Add the Animated Vector Drawable to an ImageView in XML

In your layout XML file (e.g., activity_main.xml), add an ImageView and set its source to the animated vector drawable:

<ImageView
    android:id="@+id/heartImageView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:src="@drawable/animated_heart"
    android:clickable="true"
    android:focusable="true"/>

Step 6: Trigger the Animation in Kotlin

In your Kotlin activity or fragment, retrieve the ImageView and start the animation:

import android.graphics.drawable.Animatable
import android.os.Bundle
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {

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

        val heartImageView: ImageView = findViewById(R.id.heartImageView)

        heartImageView.setOnClickListener {
            val drawable = heartImageView.drawable
            if (drawable is Animatable) {
                (drawable as Animatable).start()
            }
        }
    }
}

Here’s what’s happening:

1. We find the ImageView using its ID.
2. We attach an OnClickListener to it.
3. Inside the click listener, we get the drawable from the ImageView.
4. We check if the drawable is an instance of Animatable, which is the interface implemented by AnimatedVectorDrawable.
5. If it is, we cast the drawable to Animatable and call start() to begin the animation.

Best Practices and Advanced Tips

1. Reversible Animations

Sometimes you need an animation to play forwards and backward based on the state. Here’s how you can do that:

First, create two different ObjectAnimator animations for forward and reverse states:

<!-- Forward Animation (heart_fill_forward.xml) -->
<objectAnimator xmlns:android="http://schemas.android.com/apk/res/android"
    android:propertyName="fillColor"
    android:duration="500"
    android:valueFrom="#FF0000"
    android:valueTo="#00FF00"
    android:valueType="colorType"
    android:interpolator="@android:interpolator/fast_out_slow_in"/>

<!-- Reverse Animation (heart_fill_reverse.xml) -->
<objectAnimator xmlns:android="http://schemas.android.com/apk/res/android"
    android:propertyName="fillColor"
    android:duration="500"
    android:valueFrom="#00FF00"
    android:valueTo="#FF0000"
    android:valueType="colorType"
    android:interpolator="@android:interpolator/fast_out_slow_in"/>

Modify the animated vector drawable to use both animations:

<animated-vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:drawable="@drawable/vector_heart">
    <target
        android:name="heart"
        android:animation="@anim/heart_fill_forward"/>
    <target
        android:name="heart_reverse"
        android:animation="@anim/heart_fill_reverse"/>
</animated-vector>

Update your Kotlin code to start the appropriate animation based on state:

import android.graphics.drawable.AnimatedVectorDrawable
import android.os.Bundle
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {

    private var isFilled = false

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

        val heartImageView: ImageView = findViewById(R.id.heartImageView)
        heartImageView.setOnClickListener {
            val drawable = heartImageView.drawable as AnimatedVectorDrawable
            drawable.clearAnimationCallbacks()
            if (isFilled) {
                drawable.reset()  // Ensure it starts from the initial state
                drawable.setDrawableByName("heart_reverse") // Ensure the drawable has a target with this name. This is where AVD requires careful target selection in XML definition!
            } else {
                drawable.setDrawableByName("heart")
            }

            drawable.start()
            isFilled = !isFilled
        }
    }
}

2. Using Multiple Animations

For complex animations, you can define multiple animation targets:

<animated-vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:drawable="@drawable/vector_complex">
    <target
        android:name="part1"
        android:animation="@anim/part1_animation"/>
    <target
        android:name="part2"
        android:animation="@anim/part2_animation"/>
</animated-vector>

3. Graceful Fallback

For devices that don’t support AVDs (though rare these days), provide a fallback. One way to achieve this is by checking the API level and providing a static image resource for older devices.

4. Understanding VectorDrawableCompat and AnimatedVectorDrawableCompat

When targeting older Android versions (API level < 21), it’s beneficial to use VectorDrawableCompat and AnimatedVectorDrawableCompat, which are part of the support library. They allow you to use vector drawables on older devices. To use these, add the following dependency to your build.gradle file:

dependencies {
    implementation "androidx.vectordrawable:vectordrawable-animated:1.1.0"
    implementation "androidx.appcompat:appcompat:1.3.0"  // Ensure you have a compatible appcompat version
}

When using VectorDrawableCompat and AnimatedVectorDrawableCompat, the basic approach is similar. Here’s a modification snippet example in Kotlin for using them.

import android.graphics.drawable.Animatable2Compat
import android.graphics.drawable.Drawable
import android.os.Bundle
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity
import androidx.vectordrawable.graphics.drawable.Animatable2Compat
import androidx.vectordrawable.graphics.drawable.AnimatedVectorDrawableCompat

class MainActivity : AppCompatActivity() {

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

        val heartImageView: ImageView = findViewById(R.id.heartImageView)
        val avd = AnimatedVectorDrawableCompat.create(this, R.drawable.animated_heart)
        heartImageView.setImageDrawable(avd)


        (heartImageView.drawable as AnimatedVectorDrawableCompat).apply {
            registerAnimationCallback(object : Animatable2Compat.AnimationCallback() {
                override fun onAnimationEnd(drawable: Drawable?) {
                    heartImageView.post { start() }  // looping, careful to remove loop if needed
                }
            })
            start() // start on Activity creation itself, example of auto-start
        }

        heartImageView.setOnClickListener {
          (heartImageView.drawable as AnimatedVectorDrawableCompat).start()  // onClick as usual too
        }


    }
}

Troubleshooting

• Animation Not Starting: Ensure that the ImageView‘s src attribute is set correctly and that the drawable is indeed an instance of Animatable. Also, double-check that the names in the AnimatedVectorDrawable’s targets match the names defined in the VectorDrawable.
• Animation Not Looping: For looping animations, you may need to manually restart the animation using a callback or by resetting the drawable and starting it again.
• Performance Issues: Complex animations can sometimes impact performance. Profile your animations to ensure they run smoothly, and simplify your vector paths or reduce animation duration if needed.

Conclusion

Animated Vector Drawables provide an efficient, scalable, and flexible way to create engaging animations in Android. By combining the precision of XML vector graphics with the expressiveness of Kotlin, developers can craft intricate and captivating user experiences. From simple color changes to complex transformations, mastering AVDs unlocks a world of creative possibilities. Ensure that your application offers enhanced UX, maintains smooth performance, and minimizes its size by using Animated Vector Drawables effectively. Experiment with various animation properties, consider fallback mechanisms, and follow best practices to create visually appealing, responsive, and highly engaging user interfaces.