// Author: JohannesMP (2018-08-12)
//
// A wrapper that provides the means to safely serialize Scene Asset References.
// 
// Internally we serialize an Object to the SceneAsset which only exists at editor time.
// Any time the object is serialized, we store the path provided by this Asset (assuming it was valid).
// 
// This means that, come build time, the string path of the scene asset is always already stored, which if 
// the scene was added to the build settings means it can be loaded.
// 
// It is up to the user to ensure the scene exists in the build settings so it is loadable at runtime.
// To help with this, a custom PropertyDrawer displays the scene build settings state.
// 
// Known issues:
//     -   When reverting back to a prefab which has the asset stored as null, Unity will show the property 
//         as modified despite having just reverted. This only happens the fist time, and reverting again 
//    

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.Linq;
using System.IO;
#if UNITY_EDITOR
using UnityEditor;
#endif

[System.Serializable]
public class SceneReferenceLite : ISerializationCallbackReceiver
{
#if UNITY_EDITOR
    // What we use in editor to select the scene
    [SerializeField] private Object sceneAsset = null;
    bool IsValidSceneAsset
    {
        get
        {
            if (sceneAsset == null)
                return false;
            return sceneAsset.GetType().Equals(typeof(SceneAsset));
        }
    }
#endif

    // This should only ever be set during serialization/deserialization!
    [SerializeField]
    private string scenePath = string.Empty;

    // Use this when you want to actually have the scene path
    public string ScenePath
    {
        get
        {
#if UNITY_EDITOR
            // In editor we always use the asset's path
            return GetScenePathFromAsset();
#else
            // At runtime we rely on the stored path value which we assume was serialized correctly at build time.
            // See OnBeforeSerialize and OnAfterDeserialize
            return scenePath;
#endif
        }
        set
        {
            scenePath = value;
#if UNITY_EDITOR
            sceneAsset = GetSceneAssetFromPath();
#endif
        }
    }

    // Use this when you want to  have the scene name
    public string SceneName
    {
        get
        {
            return Path.GetFileNameWithoutExtension(ScenePath);
            // At runtime we rely on the stored path value which we assume was serialized correctly at build time.
            // See OnBeforeSerialize and OnAfterDeserialize
            //return scenePath;
        }
    }

    public static implicit operator string(SceneReferenceLite sceneReference)
    {
        return sceneReference.ScenePath;
    }

    // Called to prepare this data for serialization. Stubbed out when not in editor.
    public void OnBeforeSerialize()
    {
#if UNITY_EDITOR
        HandleBeforeSerialize();
#endif
    }

    // Called to set up data for deserialization. Stubbed out when not in editor.
    public void OnAfterDeserialize()
    {
#if UNITY_EDITOR
        // We sadly cannot touch assetdatabase during serialization, so defer by a bit.
        EditorApplication.update += HandleAfterDeserialize;
#endif
    }



#if UNITY_EDITOR
    private SceneAsset GetSceneAssetFromPath()
    {
        if (string.IsNullOrEmpty(scenePath))
            return null;
        return AssetDatabase.LoadAssetAtPath<SceneAsset>(scenePath);
    }

    private string GetScenePathFromAsset()
    {
        if (sceneAsset == null)
            return string.Empty;
        return AssetDatabase.GetAssetPath(sceneAsset);
    }

    private void HandleBeforeSerialize()
    {
        // Asset is invalid but have Path to try and recover from
        if (IsValidSceneAsset == false && string.IsNullOrEmpty(scenePath) == false)
        {
            sceneAsset = GetSceneAssetFromPath();
            if (sceneAsset == null)
                scenePath = string.Empty;

            UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
        }
        // Asset takes precendence and overwrites Path
        else
        {
            scenePath = GetScenePathFromAsset();
        }
    }

    private void HandleAfterDeserialize()
    {
        EditorApplication.update -= HandleAfterDeserialize;
        // Asset is valid, don't do anything - Path will always be set based on it when it matters
        if (IsValidSceneAsset)
            return;

        // Asset is invalid but have path to try and recover from
        if (string.IsNullOrEmpty(scenePath) == false)
        {
            sceneAsset = GetSceneAssetFromPath();
            // No asset found, path was invalid. Make sure we don't carry over the old invalid path
            if (sceneAsset == null)
                scenePath = string.Empty;

            if (Application.isPlaying == false)
                UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
        }
    }
#endif
}


#if UNITY_EDITOR
/// <summary>
/// Display a Scene Reference object in the editor.
/// If scene is valid, provides basic buttons to interact with the scene's role in Build Settings.
/// </summary>
[CustomPropertyDrawer(typeof(SceneReferenceLite))]
public class SceneReferenceLitePropertyDrawer : PropertyDrawer
{
    // The exact name of the asset Object variable in the SceneReference object
    const string sceneAssetPropertyString = "sceneAsset";
    // The exact name of  the scene Path variable in the SceneReference object
    const string scenePathPropertyString = "scenePath";

    static readonly RectOffset boxPadding = EditorStyles.helpBox.padding;
    static readonly float padSize = 2f;
    static readonly float lineHeight = EditorGUIUtility.singleLineHeight;
    static readonly float paddedLine = lineHeight + padSize;
    static readonly float footerHeight = 10f;

    /// <summary>
    /// Drawing the 'SceneReference' property
    /// </summary>
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var sceneAssetProperty = GetSceneAssetProperty(property);

        // Draw the Box Background
        position.height -= footerHeight;
        //GUI.Box(EditorGUI.IndentedRect(position), GUIContent.none, EditorStyles.helpBox);
        position = boxPadding.Remove(position);
        position.height = lineHeight;

        // Draw the main Object field
        //label.tooltip = "The actual Scene Asset reference.\nOn serialize this is also stored as the asset's path.";

        EditorGUI.BeginProperty(position, GUIContent.none, property);
        EditorGUI.BeginChangeCheck();
        int sceneControlID = GUIUtility.GetControlID(FocusType.Passive);
        var selectedObject = EditorGUI.ObjectField(position, label, sceneAssetProperty.objectReferenceValue, typeof(SceneAsset), false);

        if (EditorGUI.EndChangeCheck())
        {
            sceneAssetProperty.objectReferenceValue = selectedObject;
        }
        position.y += paddedLine;

        EditorGUI.EndProperty();
    }

    /// <summary>
    /// Ensure that what we draw in OnGUI always has the room it needs
    /// </summary>
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        int lines = 2;
        SerializedProperty sceneAssetProperty = GetSceneAssetProperty(property);
        if (sceneAssetProperty.objectReferenceValue == null)
            lines = 1;

        return boxPadding.vertical + lineHeight * lines + padSize * (lines - 1) + footerHeight;
    }


    static SerializedProperty GetSceneAssetProperty(SerializedProperty property)
    {
        return property.FindPropertyRelative(sceneAssetPropertyString);
    }

    static SerializedProperty GetScenePathProperty(SerializedProperty property)
    {
        return property.FindPropertyRelative(scenePathPropertyString);
    }
}
#endif