Modeling, Persist Data & Mantle
今天看到一个很好的开源库Mantle，顺便一路整理下Modeling，Persist Data的概念。
Modeling
首先来说说建模，一个好的模型应该是可重用的，针对特定领域表现其结构特征的。模型对象最好不要与用户交互和显示逻辑扯上关系。
Because model objects represent knowledge and expertise related to a specific problem domain, they can be reused when that problem domain is in effect. Ideally, a model object should have no explicit connection to the view objects that present its data and allow users to edit that data—in other words, it should not be concerned with user-interface and presentation issues.
Apple给出一个好的模型应该考虑这几个方面：
- Instance variables
- Accessor methods and declared properties
- Key-value coding
- Initialization and deallocation
- Object encoding
- Object copying
接着我们就来看看Object encoding和Object copying。
Persist Data
在说Object encoding之前，我们要说一下Persist Data，what和why。
Persistence is achieved in practice by storing the state as data in non-volatile storage such as a hard drive or flash memory, most basically via serialization of the data to a storable format, and then saving the data to a file.
Among the most important architectural decisions made when building an app is how to persist data between launches. The question of how, exactly, to re-create the state of the app from the time it was last opened; of how to describe the object graph in such a way that it can be flawlessly reconstructed next time.
Object graph
In an object-oriented program, groups of objects form a network through their relationships with each other—either through a direct reference to another object or through a chain of intermediate references. These groups of objects are referred to as object graphs.
Sometimes you may want to convert an object graph—usually just a section of the full object graph in the application—into a form that can be saved to a file or transmitted to another process or machine and then reconstructed. This process is known as archiving.
Object encoding
Object encoding converts an object’s class identity and state to a format that can be stored or transferred between processes. The class type and instance data are written to a byte stream that can persist after a program terminates. When the program is launched again, a newly allocated object can decode the stored representation of itself and restore itself to its previous runtime state. Encoding usually occurs in concert with archiving, which puts a graph of objects into a format (an archive) that can be written to the file system; unarchiving operates on an archive, asking each object in the stored graph to decode itself.
Classes that conform to NSCoding can be serialized and deserialized into data that can be either be archived to disk or distributed across a network.
概括
数据持久化的意义在于，每次打开app，都尽量给用户一种接续的体验：是从上次离开的地方继续开始的。
对离开时的state进行保存，实际就是对当时的object graph进行保存。将整个object graph进行序列化编码保存的过程就是archiving，archived的文件可以存本地或进行传输。
在archiving的过程中，每个object就会进行encoding，而满足encoding的前提是每个object都遵从<NSCoding>协议。
Object Copying
再来看看Object copying：
Copying an object creates a new object with the same class and properties as the original object. You copy an object when you want your own version of the data that the object contains. If you receive an object from elsewhere in an application but do not copy it, you share the object with its owner (and perhaps others), who might change the encapsulated contents.
NSCopying
An object can be copied if its class adopts the NSCopying protocol and implements its single method, copyWithZone:.
实现NSCopying协议就可以让该类及其子类被copy。
在copyWithZone:方法中可以实现复制初始化的方式。
Shallow or Deep
Copies of objects can be shallow or deep. Both shallow- and deep-copy approaches directly duplicate scalar properties but differ on how they handle pointer references, particularly references to objects. A deep copy duplicates the objects referenced while a shallow copy duplicates only the references to those objects.
copy是深拷贝，内容拷贝；retain是浅拷贝，指针拷贝。
Mantle
Mantle makes it easy to write a simple model layer for your Cocoa or Cocoa Touch application.
好吧，终于说到Mantle了。使用Mantle来构建model层，有两个很大的便利：
MTLModel
MTLModel已经默认实现了：
<NSCoding><NSCopying>-isEqual:-hash
在进行数据持久化和拷贝操作的时候，可以直接使用。
因为提供了-isEqual:和-hash的默认实现，可以把model作为NSDictionary的key来使用。
MTLJSONSerializing
Mantle提供了model对象直接序列化为JSON格式的双向方法，方便与服务器端交互。
使用时需要实现<MTLJSONSerializing>协议。