using UnityEngine;
using UnityEngine.UIElements;

namespace Uigenx.Effects
{
    /// <summary>
    /// Fills a <see cref="RenderTexture"/> from a camera or a blit and shows it as the background
    /// of a named element. The texture is created on enable and released on disable, so a disabled
    /// component costs nothing.
    /// </summary>
    [AddComponentMenu("UI Creator/Render Texture Source")]
    public sealed class UgxRenderTextureSource : MonoBehaviour
    {
        [SerializeField] UIDocument document;

        [Tooltip("Name of the element that shows the texture.")]
        [SerializeField] string elementName;

        [Header("Source")]
        [Tooltip("Rendered every update. Leave empty to blit `sourceTexture` through `blitMaterial` instead.")]
        [SerializeField] Camera sourceCamera;

        [SerializeField] Texture sourceTexture;
        [SerializeField] Material blitMaterial;

        [Header("Target")]
        [SerializeField] int width = 512;
        [SerializeField] int height = 512;
        [SerializeField] int depthBits = 16;
        [SerializeField] RenderTextureFormat format = RenderTextureFormat.ARGB32;

        [Tooltip("Updates per second. Zero renders once on enable.")]
        [Range(0f, 120f)]
        [SerializeField] float hz = 30f;

        /// <summary>Frames to keep looking for the target element before giving up.</summary>
        public const int ResolveRetryFrames = 8;

        RenderTexture _texture;
        VisualElement _element;
        double _nextRenderTime;
        RenderTexture _previousCameraTarget;
        int _resolveFramesLeft;

        public RenderTexture Texture => _texture;

        public VisualElement Element => _element;

        void OnEnable()
        {
            _resolveFramesLeft = ResolveRetryFrames;
            if (!TryStart()) return;

            StartTexture();
        }

        void StartTexture()
        {
            if (_element == null || _texture != null) return;

            _texture = new RenderTexture(Mathf.Max(1, width), Mathf.Max(1, height), depthBits, format)
            {
                name = "ugx-" + (string.IsNullOrEmpty(elementName) ? name : elementName),
                hideFlags = HideFlags.HideAndDontSave,
            };
            _texture.Create();

            if (sourceCamera != null)
            {
                _previousCameraTarget = sourceCamera.targetTexture;
                sourceCamera.targetTexture = _texture;
            }

            _element.style.backgroundImage = Background.FromRenderTexture(_texture);

            _nextRenderTime = 0d;
            RenderOnce();
        }

        void OnDisable()
        {
            // Clearing the inline value hands the element back to its stylesheet. Writing the
            // previously resolved Background instead would pin it forever.
            if (_element != null) _element.style.backgroundImage = StyleKeyword.Null;
            _element = null;

            if (sourceCamera != null)
            {
                sourceCamera.targetTexture = _previousCameraTarget;
                _previousCameraTarget = null;
            }

            if (_texture == null) return;
            _texture.Release();
            if (Application.isPlaying) Destroy(_texture);
            else DestroyImmediate(_texture);
            _texture = null;
        }

        void LateUpdate()
        {
            // UIDocument may not have built its root yet when this component is enabled, and
            // component order is not something a user should have to get right. Retry for a few
            // frames, then give up quietly; a wrong element name already reported itself.
            if (_texture == null && _resolveFramesLeft > 0)
            {
                _resolveFramesLeft--;
                if (TryStart()) StartTexture();
            }

            if (_texture == null || hz <= 0f) return;
            double now = Time.unscaledTimeAsDouble;
            if (now < _nextRenderTime) return;
            _nextRenderTime = now + 1d / hz;
            RenderOnce();
        }

        public void RenderOnce()
        {
            if (_texture == null) return;

            if (sourceCamera != null)
            {
                sourceCamera.Render();
                return;
            }
            if (sourceTexture == null) return;

            if (blitMaterial != null) Graphics.Blit(sourceTexture, _texture, blitMaterial);
            else Graphics.Blit(sourceTexture, _texture);
        }

        /// <summary>Resolves the target element. Reports a wrong name once, a missing root silently.</summary>
        bool TryStart()
        {
            if (_element != null) return true;

            if (document == null) document = GetComponent<UIDocument>();
            if (document == null)
            {
                Debug.LogError("[ugx] UgxRenderTextureSource needs a UIDocument", this);
                _resolveFramesLeft = 0;
                return false;
            }

            var root = document.rootVisualElement;
            if (root == null) return false;

            _element = string.IsNullOrEmpty(elementName) ? root : root.Q<VisualElement>(elementName);
            if (_element != null) return true;

            Debug.LogError("[ugx] UgxRenderTextureSource cannot find an element named '" + elementName + "'", this);
            _resolveFramesLeft = 0;
            return false;
        }
    }
}