学习unity3d，感觉事件顺序很重要。就翻译一下官方文档吧。
Execution Order of Event Functions
事件函数的执行顺序
In Unity scripting, there are a number of event functions that get executed in a predetermined order as a script executes. This execution order is described below:
Unity 脚本中，有大量的事件按照预定的顺序作为脚本来执行。
Editor
- Reset: Reset is called to initialize the script’s properties when it is first attached to the object and also when the Reset command is used.
- 当第一次附加到对象或执行Reset命令时，Reset事件会被调用，来初始化脚本的属性？（成员变量）
First Scene Load
These functions get called when a scene starts (once for each object in the scene).
- Awake: This function is always called before any Start functions and also just after a prefab is instantiated. (If a GameObject is inactive during start up Awake is not called until it is made active)
- Awake函数在所有的Start函数前,且在Prefab 初始化之后执行（如果GameObject 没有激活，那么对于此gameobject，awake事件阶段，就不会执行此gameobject的awake函数）
- OnEnable: (only called if the Object is active): This function is called just after the object is enabled. This happens when a MonoBehaviour instance is created, such as when a level is loaded or a GameObject with the script component is instantiated.
- 在gameobject是active状态下，当gameobject被实例化的时候，会先初始化gameobject 中的特殊compenent：monobehaviour， 而在这些之后，会执行脚本的onenable 事件。
- OnLevelWasLoaded: This function is executed to inform the game that a new level has been loaded.
Note that for objects added to the scene, the Awake and OnEnable functions for all scripts will be called before Start, Update, etc are called for any of them. Naturally, this cannot be enforced when an object is instantiated during gameplay.
对于scene中的所有object，脚本中的awake和onenable函数会在start，update等等函数之前调用，基本上，在对象初始化期间这是不可强制执行的。
Before the first frame update
- Start: Start is called before the first frame update only if the script instance is enabled.
For objects added to the scene, the Start function will be called on all scripts before Update, etc are called for any of them. Naturally, this cannot be enforced when an object is instantiated during gameplay.
start在第一帧更新之前调用。当然脚本必须是enable。因为是在第一帧之前调用，所以也就是说只会执行一次。
In between frames
每帧之间
- OnApplicationPause: This is called at the end of the frame where the pause is detected, effectively between the normal frame updates. One extra frame will be issued after OnApplicationPause is called to allow the game to show graphics that indicate the paused state.
Update Order
更新顺序
When you’re keeping track of game logic and interactions, animations, camera positions, etc., there are a few different events you can use. The common pattern is to perform most tasks inside the Update function, but there are also other functions you can use.
在处理游戏逻辑，交互，动画，摄像机位置等等的时候，有一些不同的事件是可以使用的。
大多数会采用在update方法中来处理，这是常用的方式。当然也有一些其他的函数可以使用。
- FixedUpdate: FixedUpdate is often called more frequently than Update. It can be called multiple times per frame, if the frame rate is low and it may not be called between frames at all if the frame rate is high. All physics calculations and updates occur immediately after FixedUpdate. When applying movement calculations inside FixedUpdate, you do not need to multiply your values by Time.deltaTime. This is because FixedUpdate is called on a reliable timer, independent of the frame rate.
fixedupdate：fixedupdate通常比update调用周期更短，如果帧率比较低，那么帧之间它可能被多次调用，，而如果帧率比较高那么可能有帧之间没有调用的情况。当fixedupdate调用之后，所有的物理计算和更新会立马更新。
当在fixedupdate中计算运动的时候，不需要使用time.deltatime来乘你的数值，因为fixedupdate会独立于帧率，并依靠一个可靠的定时器来调用。
（以下猜想：所以一般会把某些物理特性，交互等修改的时机放入到fixupdate中，因为之后引擎会固定调用物理和交互的更新，所以避免目标机显卡性能差，导致帧率过低，那么恰当的方式就是把物理特性和交互放入到fixupdate中，这样虽然显示慢，但整个游戏的内部逻辑运作是正常的）
- Update: Update is called once per frame. It is the main workhorse function for frame updates.
- LateUpdate: LateUpdate is called once per frame, after Update has finished. Any calculations that are performed in Update will have completed when LateUpdatebegins. A common use for LateUpdate would be a following third-person camera. If you make your character move and turn inside Update, you can perform all camera movement and rotation calculations in LateUpdate. This will ensure that the character has moved completely before the camera tracks its position.
Rendering
- OnPreCull: Called before the camera culls the scene. Culling determines which objects are visible to the camera. OnPreCull is called just before culling takes place.
- OnBecameVisible/OnBecameInvisible: Called when an object becomes visible/invisible to any camera.
- OnWillRenderObject: Called once for each camera if the object is visible.
- OnPreRender: Called before the camera starts rendering the scene.
- OnRenderObject: Called after all regular scene rendering is done. You can use GL class or Graphics.DrawMeshNow to draw custom geometry at this point.
- OnPostRender: Called after a camera finishes rendering the scene.
- OnRenderImage: Called after scene rendering is complete to allow post-processing of the image, see Post-processing Effects.
- OnGUI: Called multiple times per frame in response to GUI events. The Layout and Repaint events are processed first, followed by a Layout and keyboard/mouse event for each input event.
- OnDrawGizmos Used for drawing Gizmos in the scene view for visualisation purposes.
Coroutines
Normal coroutine updates are run after the Update function returns. A coroutine is a function that can suspend its execution (yield) until the given YieldInstruction finishes. Different uses of Coroutines:
- yield The coroutine will continue after all Update functions have been called on the next frame.
- yield WaitForSeconds Continue after a specified time delay, after all Update functions have been called for the frame
- yield WaitForFixedUpdate Continue after all FixedUpdate has been called on all scripts
- yield WWW Continue after a WWW download has completed.
- yield StartCoroutine Chains the coroutine, and will wait for the MyFunc coroutine to complete first.
When the Object is Destroyed
- OnDestroy: This function is called after all frame updates for the last frame of the object’s existence (the object might be destroyed in response to Object.Destroy or at the closure of a scene).
When Quitting
These functions get called on all the active objects in your scene:
- OnApplicationQuit: This function is called on all game objects before the application is quit. In the editor it is called when the user stops playmode.
- OnDisable: This function is called when the behaviour becomes disabled or inactive.
Script Lifecycle Flowchart
The following diagram summarises the ordering and repetition of event functions during a script’s lifetime.
