Image Sprite Switcher Track for Unity Timeline

ImageSpriteSwitcherBehaviour.cs

using System;
using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.UI;

[Serializable]
public class ImageSpriteSwitcherBehaviour : PlayableBehaviour {
	public Sprite sprite;

	public override void ProcessFrame(Playable playable, FrameData info, object playerData) {
		var image = playerData as Image;
		if (image != null) image.sprite = sprite;
	}
}

ImageSpriteSwitcherClip.cs

using System;
using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Timeline;

[Serializable]
public class ImageSpriteSwitcherClip : PlayableAsset, ITimelineClipAsset {
	public ImageSpriteSwitcherBehaviour template = new ImageSpriteSwitcherBehaviour();

	public ClipCaps clipCaps => ClipCaps.None;

	public override Playable CreatePlayable(PlayableGraph graph, GameObject owner) =>
		ScriptPlayable<ImageSpriteSwitcherBehaviour>.Create(graph, template);
}

ImageSpriteSwitcherDrawer.cs

using UnityEditor;
using UnityEngine;

/// <summary>
/// Put this inside Editor folder
/// </summary>
[CustomPropertyDrawer(typeof(ImageSpriteSwitcherBehaviour))]
public class ImageSpriteSwitcherDrawer : PropertyDrawer {
	public override float GetPropertyHeight(SerializedProperty property, GUIContent label) {
		const int fieldCount = 1;
		return fieldCount * EditorGUIUtility.singleLineHeight;
	}

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) {
		SerializedProperty ImageProp = property.FindPropertyRelative("sprite");

		Rect singleFieldRect = new Rect(position.x, position.y, position.width, EditorGUIUtility.singleLineHeight);
		EditorGUI.PropertyField(singleFieldRect, ImageProp);
	}
}

ImageSpriteSwitcherMixerBehaviour.cs

using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.UI;

public class ImageSpriteSwitcherMixerBehaviour : PlayableBehaviour {
	Sprite m_DefaultSprite;
	Image m_TrackBinding;
	bool m_FirstFrameHappened;

	// NOTE: This function is called at runtime and edit time.  Keep that in mind when setting the values of properties.
	public override void ProcessFrame(Playable playable, FrameData info, object playerData) {
		m_TrackBinding = playerData as Image;

		if (m_TrackBinding == null)
			return;

		if (!m_FirstFrameHappened) {
			m_DefaultSprite = m_TrackBinding.sprite;
			m_FirstFrameHappened = true;
		}

		var inputCount = playable.GetInputCount();

		Sprite spr = null;

		for (var i = 0; i < inputCount; i++) {
			var inputWeight = playable.GetInputWeight(i);
			var inputPlayable =
				(ScriptPlayable<ImageSpriteSwitcherBehaviour>) playable.GetInput(i);
			var input = inputPlayable.GetBehaviour();
			spr = inputWeight > 0.5f ? input.sprite : m_DefaultSprite;
		}

		m_TrackBinding.sprite = spr;
	}

	public override void OnPlayableDestroy(Playable playable) {
		m_FirstFrameHappened = false;

		if (m_TrackBinding == null)
			return;

		m_TrackBinding.sprite = m_DefaultSprite;
	}
}

ImageSpriteSwitcherTrack.cs

using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Timeline;
using UnityEngine.UI;

[TrackColor(0.3785221f, 0f, 0.4716981f)]
[TrackClipType(typeof(ImageSpriteSwitcherClip))]
[TrackBindingType(typeof(Image))]
public class ImageSpriteSwitcherTrack : TrackAsset {
	public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount) {
		return ScriptPlayable<ImageSpriteSwitcherMixerBehaviour>.Create(graph, inputCount);
	}

	public override void GatherProperties(PlayableDirector director, IPropertyCollector driver) {
#if UNITY_EDITOR
		var trackBinding = director.GetGenericBinding(this) as Image;
		if (trackBinding == null)
			return;

		var serializedObject = new UnityEditor.SerializedObject(trackBinding);
		var iterator = serializedObject.GetIterator();
		while (iterator.NextVisible(true)) {
			if (iterator.hasVisibleChildren)
				continue;

			driver.AddFromName<Image>(trackBinding.gameObject, iterator.propertyPath);
		}
#endif
		base.GatherProperties(director, driver);
	}
}