yichael_smartbow/Assets/DependAsset/JCUnityLib/Untiy/UI/BackgroundAdapter.cs

using UnityEngine;
using UnityEngine.UI;

namespace JC.Unity.UI { 
    [ExecuteAlways]
    public class BackgroundAdapter : MonoBehaviour
    {
        private CanvasScaler _scaler;

        void Start()
        {
            UpdateScale();
        }

        #if UNITY_EDITOR
        void Update()
        {
            UpdateScale();
        }
        #endif

        void UpdateScale()
        {
            if (_scaler == null) _scaler = GetComponentInParent<CanvasScaler>();
            if (_scaler == null) return;
            var resolution = _scaler.referenceResolution;
            var rt = _scaler.transform as RectTransform;
            if (rt == null) return;
            var screenSize = rt.sizeDelta;
            var factor = Mathf.Max(screenSize.x / resolution.x, screenSize.y / resolution.y);
            var scale = Vector3.one * factor;
            transform.localScale = scale;
        }
    }
}