using System;
using UnityEngine;

namespace Chernobyl.Unity.Asset
{
    /// <summary>
    /// Utility and extension methods for <see cref="AssetBundle"/>.
    /// </summary>
    public static class AssetBundleExt
    {
        /// <summary>
        /// Returns a <see cref="Lazy{T}"/> that performs the same function as
        /// <see cref="AssetBundle.LoadFromFile(string)"/>.
        /// </summary>
        public static Lazy<AssetBundle> LazyLoadFromFile(string path) =>
            new Lazy<AssetBundle>(() => AssetBundle.LoadFromFile(path));

        /// <summary>
        /// Performs the same function as <see cref="AssetBundle.Unload(bool)"/> but only if the
        /// <see cref="AssetBundle"/> has been created.
        /// </summary>
        public static void Unload(this Lazy<AssetBundle> assetBundle, bool unloadAllLoadedObjects)
        {
            if (assetBundle.IsValueCreated) assetBundle.Value.Unload(unloadAllLoadedObjects);
        }

        /// <summary>
        /// Same as <see cref="AssetBundle.LoadAsset{T}(string)" /> but throws an exception if
        /// nothing is loaded.
        /// </summary>
        public static T LoadRequiredAsset<T>(this AssetBundle bundle, string name)
            where T : UnityEngine.Object
        {
            var asset = bundle.LoadAsset<T>(name);
            if (asset == null) throw new LoadAssetException(name, bundle);
            return asset;
        }

        /// <summary>
        /// Same as <see cref="AssetBundle.LoadAsset{T}(string)" /> but throws an exception if
        /// nothing is loaded.
        /// </summary>
        public static Lazy<T> LoadRequiredAsset<T>(this Lazy<AssetBundle> bundle, string name)
            where T : UnityEngine.Object => new Lazy<T>(() => bundle.Value.LoadRequiredAsset<T>(name));
    }

    /// <summary>
    /// Thrown when an asset could not be loaded from an <see cref="AssetBundle"/>.
    /// </summary>
    public class LoadAssetException : Exception
    {
        /// <param name="assetName">Name of the asset being loaded.</param>
        /// <param name="assetBundle">The instance that the asset was loaded from.</param>
        public LoadAssetException(string assetName, AssetBundle assetBundle)
            : base($"Failed to find the asset '{assetName}' in AssetBundle '{assetBundle.name}'.")
        { }
    }
}