/** 运行时的场景管理。 */
class SceneManager {
    private static _scene: Scene;
    private static _nextScene: Scene;
    public static sceneTransition: SceneTransition;
    public static stage: egret.Stage;

    constructor(stage: egret.Stage) {
        stage.addEventListener(egret.Event.ENTER_FRAME, SceneManager.update, this);

        SceneManager.stage = stage;
        SceneManager.initialize(stage);
    }

    public static get scene() {
        return this._scene;
    }
    public static set scene(value: Scene) {
        if (!value)
            throw new Error("场景不能为空");

        if (this._scene == null) {
            this._scene = value;
            this._scene.begin();
        } else {
            this._nextScene = value;
        }
    }

    public static initialize(stage: egret.Stage) {
        Input.initialize(stage);
    }

    public static update() {
        Time.update(egret.getTimer());

        if (SceneManager._scene) {
            for (let i = GlobalManager.globalManagers.length - 1; i >= 0; i--) {
                if (GlobalManager.globalManagers[i].enabled)
                    GlobalManager.globalManagers[i].update();
            }

            if (!SceneManager.sceneTransition ||
                (SceneManager.sceneTransition && (!SceneManager.sceneTransition.loadsNewScene || SceneManager.sceneTransition.isNewSceneLoaded))) {
                    SceneManager._scene.update();
            }

            if (SceneManager._nextScene) {
                SceneManager._scene.end();

                for (let i = 0; i < SceneManager._scene.entities.buffer.length; i++) {
                    let entity = SceneManager._scene.entities.buffer[i];
                    entity.destroy();
                }

                SceneManager._scene = SceneManager._nextScene;
                SceneManager._nextScene = null;

                SceneManager._scene.begin();
            }
        }

        SceneManager.render();
    }

    public static render() {
        if (this.sceneTransition){
            this.sceneTransition.preRender();

            if (this._scene && !this.sceneTransition.hasPreviousSceneRender){
                this._scene.render();
                this._scene.postRender();
                this.sceneTransition.onBeginTransition();
            } else if (this.sceneTransition) {
                if (this._scene && this.sceneTransition.isNewSceneLoaded) {
                    this._scene.render();
                    this._scene.postRender();
                }
    
                this.sceneTransition.render();
            }
        } else if (this._scene) {
            this._scene.render();

            Debug.render();

            this._scene.postRender();
        }
    }

    /**
     * 临时运行SceneTransition，允许一个场景过渡到另一个平滑的自定义效果。
     * @param sceneTransition 
     */
    public static startSceneTransition<T extends SceneTransition>(sceneTransition: T): T {
        if (this.sceneTransition) {
            console.warn("在前一个场景完成之前，不能开始一个新的场景转换。");
            return;
        }

        this.sceneTransition = sceneTransition;
        return sceneTransition;
    }
}