ListViewExample.cs

using System;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;
    
public class ListViewExample : EditorWindow
{
    // Gradient used for the HP color indicator.
    private Gradient hpGradient;
    private GradientColorKey[] hpColorKey;
    private GradientAlphaKey[] hpAlphaKey;
    
    // ListView is kept for easy reference.
    private ListView listView;
    
    // List of CharacterInfo items, bound to the ListView.
    private List<CharacterInfo> items;
    [MenuItem("Window/ListView Custom Item")]
    
    public static void OpenWindow()
    {
        GetWindow<ListViewExample>().Show();
    }
    
    private void OnEnable()
    {
        SetGradient();
        
        // Create and populate the list of CharacterInfo objects.
        const int itemCount = 50;
        items = new List<CharacterInfo>(itemCount);
        for(int i = 1; i <= itemCount; i++)
        {
            CharacterInfo character = new CharacterInfo {name = $"Character {i}", maxHp = 100};
            character.currentHp = character.maxHp;
            items.Add(character);
        }
        
        // The ListView calls this to add visible items to the scroller.
        Func<VisualElement> makeItem = () =>
        {
            var characterInfoVisualElement = new CharacterInfoVisualElement();
            var slider = characterInfoVisualElement.Q<SliderInt>(name: "hp");
            slider.RegisterValueChangedCallback(evt =>
            {
                var hpColor = characterInfoVisualElement.Q<VisualElement>("hpColor");
                var i = (int)slider.userData;
                var characterInfo = items[i];
                characterInfo.currentHp = evt.newValue;
                SetHp(slider, hpColor, characterInfo);
            });
            return characterInfoVisualElement;
        };
        
        // The ListView calls this if a new item becomes visible when the item first appears on the screen, 
        // when a user scrolls, or when the dimensions of the scroller are changed.
        Action<VisualElement, int> bindItem = (e, i) => BindItem(e as CharacterInfoVisualElement, i);
        
        // Height used by the ListView to determine the total height of items in the list.
        int itemHeight = 55;
        
        // Use the constructor with initial values to create the ListView.
        listView = new ListView(items, itemHeight, makeItem, bindItem);
        listView.reorderable = false;
        listView.style.flexGrow = 1f; // Fills the window, at least until the toggle below.
        listView.showBorder = true;
        rootVisualElement.Add(listView);
        
        // Add a toggle to switch the reorderable property of the ListView.
        var reorderToggle = new Toggle("Reorderable");
        reorderToggle.style.marginTop = 10f;
        reorderToggle.value = false;
        reorderToggle.RegisterValueChangedCallback(evt => listView.reorderable = evt.newValue);
        rootVisualElement.Add(reorderToggle);
    }
    
    // Sets up the gradient.
    private void SetGradient()
    {
        hpGradient = new Gradient();
        
        // HP at 0%: Red. At 10%: Dark orange. At 40%: Yellow. At 100%: Green.
        hpColorKey = new GradientColorKey[4];
        hpColorKey[0] = new GradientColorKey(Color.red, 0f);
        hpColorKey[1] = new GradientColorKey(new Color(1f, 0.55f, 0f), 0.1f); // Dark orange
        hpColorKey[2] = new GradientColorKey(Color.yellow, 0.4f);
        hpColorKey[3] = new GradientColorKey(Color.green, 1f);
        
        // Alpha is always full.
        hpAlphaKey = new GradientAlphaKey[2];
        hpAlphaKey[0] = new GradientAlphaKey(1f, 0f);
        hpAlphaKey[1] = new GradientAlphaKey(1f, 1f);
        hpGradient.SetKeys(hpColorKey, hpAlphaKey);
    }
    
    // Bind the data (characterInfo) to the display (elem).
    private void BindItem(CharacterInfoVisualElement elem, int i)
    {
        var label = elem.Q<Label>(name: "nameLabel");
        var slider = elem.Q<SliderInt>(name: "hp");
        var hpColor = elem.Q<VisualElement>("hpColor");
        slider.userData = i;
        CharacterInfo characterInfo = items[i];
        label.text = characterInfo.name;
        SetHp(slider, hpColor, characterInfo);
    }
    
    private void SetHp(SliderInt slider, VisualElement colorIndicator, CharacterInfo characterInfo)
    {
        slider.highValue = characterInfo.maxHp;
        slider.SetValueWithoutNotify(characterInfo.currentHp);
        float ratio = (float)characterInfo.currentHp / characterInfo.maxHp;
        colorIndicator.style.backgroundColor = hpGradient.Evaluate(ratio);
    }
    
    // This class inherits from VisualElement to display and modify data to and from a CharacterInfo.
    public class CharacterInfoVisualElement : VisualElement
    {
        // Use Constructor when the ListView uses makeItem and returns a VisualElement to be 
        // bound to a CharacterInfo data class.
        public CharacterInfoVisualElement()
        {
            var root = new VisualElement();
            
            // The code below to style the ListView is for demo purpose. It's better to use a USS file
            // to style a visual element. 
            root.style.paddingTop = 3f;
            root.style.paddingRight = 0f;
            root.style.paddingBottom = 15f;
            root.style.paddingLeft = 3f;
            root.style.borderBottomColor = Color.gray;
            root.style.borderBottomWidth = 1f;
            var nameLabel = new Label() {name = "nameLabel"};
            nameLabel.style.fontSize = 14f;
            var hpContainer = new VisualElement();
            hpContainer.style.flexDirection = FlexDirection.Row;
            hpContainer.style.paddingLeft = 15f;
            hpContainer.style.paddingRight = 15f;
            hpContainer.Add(new Label("HP:"));
            var hpSlider = new SliderInt {name = "hp", lowValue = 0, highValue = 100};
            hpSlider.style.flexGrow = 1f;
            hpContainer.Add(hpSlider);
            var hpColor = new VisualElement();
            hpColor.name = "hpColor";
            hpColor.style.height = 15f;
            hpColor.style.width = 15f;
            hpColor.style.marginRight = 5f;
            hpColor.style.marginBottom = 5f;
            hpColor.style.marginLeft = 5f;
            hpColor.style.backgroundColor = Color.black;
            hpContainer.Add(hpColor);
            root.Add(nameLabel);
            root.Add(hpContainer);
            Add(root);
        }
    }

    // Basic data class used for a character, with a name and HP data. Use a list of CharacterInfo as
    // a data source for the ListView. The CharacterInfo can be bound to CharacterInfoVisualElement when needed.    
    [Serializable]
    public class CharacterInfo
    {
        public string name;
        public int maxHp;
        public int currentHp;
    }
}