如何确保属性值支持KVC

Make a property KVC Compliant（如何确保属性值支持KVC）

自我感悟：KVC 无非就是通过 key 字符串的方式间接的访问类的属性。访问的途径有 setValue:forKeyPath: 或 valueForKeyPath:。那么如何让属性支持 KVC 呢，无非就是让属性的 getter 方法，setter 方法和实例变量都遵循 KVC 的 key 规则。如 setter 方法为 setKey:，getter 方法为 key or isKey，实例变量为 key or _key。

How you make a property KVC compliant depends on whether that property is an attribute, a to-one relationship, or a to-many relationship. For attributes and to-one relationships, a class must implement at least one of the following in the given order of preference (key refers to the property key)

如何使 property 支持 KVC，取决于 property 是 attribute，还是 to-one relationship，还是 to-many relationship。

Property is Attribute or To-One Relationship

若 property 是 attribute，或 to-one relationship，则类必须至少实现以下所列的一种。

• The class has a declared property with the name key.

  类必须以 key 为名声明属性

• Implement a method named key or isKey, or create an instance variable key or _key. The compiler typically does this for you when it automatically synthesizes properties.

  类必须要实现 key or isKey 命名的方法，或用 @systhesize 创建实例变量 key or _key。编译器自动合成属性的时候，通常会为你做这些事。

• If the property is mutable, implement the set: method. The compiler typically does this for you when you allow it to automatically synthesize your properties.

  若 property 是可变的，实现 setKey: 方法。编译器通常在合成属性时，为你自动做这些事情。

• If the property is a scalar, override the setNilValueForKey: method to gracefully handle the case where a nil value is assigned to the scalar property.

  如果属性是纯量（如 float, int, …），重写 setNilValueForKey: 方法来优雅地处理一个 nil 值付给了一个纯量属性。

Property is To-Many Relationship

Implementing KVC compliance for a to-many relationship is a more complicated procedure.

to-many relationship 支持 KVC 是更加复杂的过程。参考