关于dependency inversion principle的理解

首先讲了这个原则的目的是为了 decouple objects in an application by inverting 反向 certain dependencies between them.

这个原则的应用可以使代码更 robust and maintainable.

The dependency inversion principle states that:

1. High-level objects should not depend on low-level objects. Both should depend on abstractions.

2. Abstractions should not depend on details. Details should depend on abstractions.

另外在 ios-programming-the-big-nerd-ranch-christian-keur 第六版 第十章 P185 中提到的 在Homepwner这个应用当中，针对上述提到的DIP中的高层模块和底层模块分别对应于

A store is a lower-level object that retrives 检索 and save Item instances through details that are only known to that class. 

ItemsViewControlleris a higher-level object that only knows that it will be provided with a utility object (the store) from which it can obtain a list

 of Iteminstances and to which it can pass new or updatedItem instances to be stored persistently 持久的.

This results in a decoupling becauseItemsViewControlleris not dependent onItemStore. In fact, as long as the store abstraction is

 respected,ItemStore could be replaced by another object that fetches  获取 Iteminstances differently (such as by using a web service)

 without any changes to ItemsViewController.

文末还提到了另外一种实现 dependency inversion principle的 a common pattern 叫做： dependency injection

... 之后再了解！