UGUI ScrollRect滚动优化：无限循环利用Item

# Loop Scroll Rect ## v1.04 __This project works for Unity 5.2 or newer.__ I havn't tested older versions and any Pull Requests are welcomed. These scripts help make your ScrollRect `Reusable`, because it will only build cells when needed. If you have a large number of cells in a scroll rect, you absolutely need it! It will save a lot of time loading and draw call, along with memory in use, while still working smoothly. 中文说明请看[这里](http://qiankanglai.me/2015/08/15/LoopScrollRect/)。 ## Demo Demo for Loop Scroll Rect. Each cell knows its own index, and it is able to modify its content/size/color easily. Also ScrollBar is supported now! It supports both vertical & horizontal directions, back and forth.   Demo without mask. As you can see, the cells are only instantiated when needed and recycled.  **New**: Scroll to Index  ## Introduction The original idea comes from @ivomarel's [InfinityScroll](https://github.com/ivomarel/InfinityScroll). After serveral refactorisations, I almost rewrite all the codes: - Avoid using `sizeDelta` directly since it doesn't always mean size - Support GridLayout - Avoid blocking when dragging back - Take advantage of pool rather than instantiate/destroy every time - Improve some other details for performance - Supports reverse direction - **Supports ScrollBar** (this doesn't work in Infinite mode, and may behavior strange for cells with different size) Also, I modified [Easy Object Pool](https://www.assetstore.unity3d.com/cn/#!/content/31928) for recycling the cells. My scripts copies `ScrollRect` from [UGUI](https://bitbucket.org/Unity-Technologies/ui) rather than inherit `ScrollRect` like InfinityScroll. I need to modify some private variants to make dragging smooth. All my codes is wrapped with comments like `==========LoopScrollRect==========`, making maintaining a little easier. ### Infinite Version If you need scroll infinitely, you can simply set `totalCount` to a negative number. ### Quick Jump I've implemented a simple version with `Coroutine`. You can use the following API: public void SrollToCell(int index, float speed) Here is a corner case unsolved yet: You can't jump to the last cells which cannot be pulled to the start. ## Example: Loop Vertical Scroll Rect These steps may be confusing, so you can just open the demo scene and copy & paste :D You can also remove EasyObjPool and use your pool instead. - Prepare cell prefabs - The cell needs `Layout Element` attached and preferred width/height - You should add a script receiving message `void ScrollCellIndex (int idx) `  - Right click in Hierarchy and click **UI/Loop Horizontal Scroll Rect** or **UI/Loop Vertical Scroll Rect**. It is the same for these two in the Component Menu. - Init in Start: call Refill cells automatically when Start - Prefab Pool: the EasyObjPool gameObject - Prefab Pool Name: the corresponding pool in step 1 - Total Count: How many cells are available (index: 0 ~ TotalCount-1) - Threshold: How many additional pixels of content should be prepared before start or after end? - ReverseDirection: If you want scroll from bottom or right, you should toggle this - Clear Cells: remove existing cells and keep uninitialized - Refill Cells: initialize and fill up cells  If you need scroll from top or left, setting content's pivot to 1 and disable ReverseDirection. Otherwise, you should set 0 to pivot and enable ReverseDirection (I have made `VerticalScroll_Reverse` in the demo scene as reference). I highly suggests you trying these parameters by hand. More details can be found in the demo scene.