Datasource 与 Delegate的总结
在Matt Gemmell的API Design文章中，Matt对Objective-c中的Data-Source Protocol和Delegate Protocol进行了总结：
所有的DataSource Protocol在设计上可以总结为：
- How many things do I have?
- What's the value for property Y of thing X?
同理，所有的Delegate Protocol也可总结为：
- Should this thing do that?
- This thingis about to do that.
- This thing just did that.
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