com.unity.uiextensions/Runtime/Scripts/Controls/Accordion/AccordionElement.cs

///Credit ChoMPHi
///Sourced from - http://forum.unity3d.com/threads/accordion-type-layout.271818/

using System;
using System.Collections;
using UnityEngine.UI.Extensions.Tweens;

namespace UnityEngine.UI.Extensions
{
    [RequireComponent(typeof(RectTransform), typeof(LayoutElement))]
    [AddComponentMenu("UI/Extensions/Accordion/Accordion Element")]
    public class AccordionElement : Toggle
    {

        [SerializeField] private float m_MinHeight = 18f;

        public float MinHeight => m_MinHeight;

        [SerializeField] private float m_MinWidth = 40f;

        public float MinWidth => m_MinWidth;

        private Accordion m_Accordion;
        private RectTransform m_RectTransform;
        private LayoutElement m_LayoutElement;

        [NonSerialized]
        private readonly TweenRunner<FloatTween> m_FloatTweenRunner;

        protected AccordionElement()
        {
            if (this.m_FloatTweenRunner == null)
                this.m_FloatTweenRunner = new TweenRunner<FloatTween>();

            this.m_FloatTweenRunner.Init(this);
        }

        protected override void Awake()
        {
            base.Awake();
            base.transition = Transition.None;
            base.toggleTransition = ToggleTransition.None;
            this.m_Accordion = this.gameObject.GetComponentInParent<Accordion>();
            this.m_RectTransform = this.transform as RectTransform;
            this.m_LayoutElement = this.gameObject.GetComponent<LayoutElement>();
            this.onValueChanged.AddListener(OnValueChanged);
        }

        private new IEnumerator Start()
        {
            base.Start();
            yield return new WaitForEndOfFrame(); // Wait for the first frame
            OnValueChanged(this.isOn);
        }

#if UNITY_EDITOR
        protected override void OnValidate()
        {
            base.OnValidate();
            this.m_Accordion = this.gameObject.GetComponentInParent<Accordion>();

            if (this.group == null)
            {
                ToggleGroup tg = this.GetComponentInParent<ToggleGroup>();

                if (tg != null)
                {
                    this.group = tg;
                }
            }

            LayoutElement le = this.gameObject.GetComponent<LayoutElement>();

            if (le != null && m_Accordion != null)
            {
                if (this.isOn)
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        le.preferredHeight = -1f;
                    }
                    else
                    {
                        le.preferredWidth = -1f;
                    }
                }
                else
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        le.preferredHeight = this.m_MinHeight;
                    }
                    else
                    {
                        le.preferredWidth = this.m_MinWidth;

                    }
                }
            }
        }
#endif

        public void OnValueChanged(bool state)
        {
            if (this.m_LayoutElement == null)
                return;

            Accordion.Transition transition = (this.m_Accordion != null) ? this.m_Accordion.transition : Accordion.Transition.Instant;

            if (transition == Accordion.Transition.Instant && m_Accordion != null)
            {
                if (state)
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        this.m_LayoutElement.preferredHeight = -1f;
                    }
                    else
                    {
                        this.m_LayoutElement.preferredWidth = -1f;
                    }
                }
                else
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        this.m_LayoutElement.preferredHeight = this.m_MinHeight;
                    }
                    else
                    {
                        this.m_LayoutElement.preferredWidth = this.m_MinWidth;
                    }
                }
            }
            else if (transition == Accordion.Transition.Tween)
            {
                if (state)
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        this.StartTween(this.m_MinHeight, this.GetExpandedHeight());
                    }
                    else
                    {
                        this.StartTween(this.m_MinWidth, this.GetExpandedWidth());
                    }
                }
                else
                {
                    if (m_Accordion.ExpandVerticval)
                    {
                        this.StartTween(this.m_RectTransform.rect.height, this.m_MinHeight);
                    }
                    else
                    {
                        this.StartTween(this.m_RectTransform.rect.width, this.m_MinWidth);
                    }
                }
            }
        }

        protected float GetExpandedHeight()
        {
            if (this.m_LayoutElement == null)
                return this.m_MinHeight;

            float originalPrefH = this.m_LayoutElement.preferredHeight;
            this.m_LayoutElement.preferredHeight = -1f;
            float h = LayoutUtility.GetPreferredHeight(this.m_RectTransform);
            this.m_LayoutElement.preferredHeight = originalPrefH;

            return h;
        }

        protected float GetExpandedWidth()
        {
            if (this.m_LayoutElement == null)
                return this.m_MinWidth;

            float originalPrefW = this.m_LayoutElement.preferredWidth;
            this.m_LayoutElement.preferredWidth = -1f;
            float w = LayoutUtility.GetPreferredWidth(this.m_RectTransform);
            this.m_LayoutElement.preferredWidth = originalPrefW;

            return w;
        }

        protected void StartTween(float startFloat, float targetFloat)
        {
            float duration = (this.m_Accordion != null) ? this.m_Accordion.transitionDuration : 0.3f;

            FloatTween info = new FloatTween
            {
                duration = duration,
                startFloat = startFloat,
                targetFloat = targetFloat
            };
            if (m_Accordion.ExpandVerticval)
            {
                info.AddOnChangedCallback(SetHeight);
            }
            else
            {
                info.AddOnChangedCallback(SetWidth);
            }
            info.ignoreTimeScale = true;
            this.m_FloatTweenRunner.StartTween(info);
        }

        protected void SetHeight(float height)
        {
            if (this.m_LayoutElement == null)
                return;

            this.m_LayoutElement.preferredHeight = height;
        }

        protected void SetWidth(float width)
        {
            if (this.m_LayoutElement == null)
                return;

            this.m_LayoutElement.preferredWidth = width;
        }
    }
}