【Application.LoadLevel】
只有在File->Build Setting中设置了的按钮才能被加载。
当level加载完成后，MonoBehaviour.OnLevelWasLoaded is called on all active game objects。
When loading a new level all game objects that have been loaded before are destroyed. If you want to let an object survive when loading a new level, use Object.DontDestroyOnLoad.
Calling LoadLevel will update Application.loadedLevel and Application.loadedLevelName.
参考：file://localhost/Applications/Unity/Unity.app/Contents/Documentation/Documentation/ScriptReference/Application.LoadLevel.html
【Object.DontDestroyOnLoad】
When loading a new level all objects in the scene are destroyed, then the objects in the new level are loaded. In order to preserve an object during level loading call DontDestroyOnLoad on it. If the object is a component or game object then its entire transform hierarchy will not be destroyed either.
参考：file://localhost/Applications/Unity/Unity.app/Contents/Documentation/Documentation/ScriptReference/Object.DontDestroyOnLoad.html
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