iOS底层原理 17 :KVC的原理

2020-10-27  本文已影响0人  smooth_lgh

KVC的全称是Key-Value Coding，翻译成中文是 键值编码，键值编码是由NSKeyValueCoding非正式协议启用的一种机制，对象采用该协议来间接访问其属性。既可以通过一个字符串key来访问某个属性。这种间接访问机制补充了实例变量及其相关的访问器方法所提供的直接访问。

我们可以通过官方文档Key-Value Coding Programming Guide 来研究一下,通过文档得到关于Setter的介绍：

Search Pattern for the Basic Setter

The default implementation of `setValue:forKey:`, given `key` and `value` parameters as input, attempts to set a property named `key` to `value` (or, for non-object properties, the unwrapped version of `value`, as described in [Representing Non-Object Values](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/DataTypes.html#//apple_ref/doc/uid/20002171-BAJEAIEE)) inside the object receiving the call, using the following procedure:

1.  Look for the first accessor named `set<Key>:` or `_set<Key>`, in that order. If found, invoke it with the input value (or unwrapped value, as needed) and finish.

2.  If no simple accessor is found, and if the class method `accessInstanceVariablesDirectly` returns `YES`, look for an instance variable with a name like `_<key>`, `_is<Key>`, `<key>`, or `is<Key>`, in that order. If found, set the variable directly with the input value (or unwrapped value) and finish.

3.  Upon finding no accessor or instance variable, invoke `setValue:forUndefinedKey:`. This raises an exception by default, but a subclass of `NSObject` may provide key-specific behavior.

当调用setValue:forKey:设置属性value时，其底层的执行流程为：

以下为以LGPerson的name属性为例，关于setter的流程图：


同样的KVC关于Getter的介绍为：

### Search Pattern for the Basic Getter

The default implementation of `valueForKey:`, given a `key` parameter as input, carries out the following procedure, operating from within the class instance receiving the `valueForKey:` call.

1.  Search the instance for the first accessor method found with a name like `get<Key>`, `<key>`, `is<Key>`, or `_<key>`, in that order. If found, invoke it and proceed to step 5 with the result. Otherwise proceed to the next step.

2.  If no simple accessor method is found, search the instance for methods whose names match the patterns `countOf<Key>` and `objectIn<Key>AtIndex:` (corresponding to the primitive methods defined by the `NSArray` class) and `<key>AtIndexes:` (corresponding to the `[NSArray](https://developer.apple.com/library/archive/documentation/LegacyTechnologies/WebObjects/WebObjects_3.5/Reference/Frameworks/ObjC/Foundation/Classes/NSArrayClassCluster/Description.html#//apple_ref/occ/cl/NSArray)` method `objectsAtIndexes:`).

    If the first of these and at least one of the other two is found, create a collection proxy object that responds to all `NSArray` methods and return that. Otherwise, proceed to step 3.

    The proxy object subsequently converts any `NSArray` messages it receives to some combination of `countOf<Key>`, `objectIn<Key>AtIndex:`, and `<key>AtIndexes:` messages to the key-value coding compliant object that created it. If the original object also implements an optional method with a name like `get<Key>:range:`, the proxy object uses that as well, when appropriate. In effect, the proxy object working together with the key-value coding compliant object allows the underlying property to behave as if it were an `NSArray`, even if it is not.

3.  If no simple accessor method or group of array access methods is found, look for a triple of methods named `countOf<Key>`, `enumeratorOf<Key>`, and `memberOf<Key>:` (corresponding to the primitive methods defined by the `[NSSet](https://developer.apple.com/library/archive/documentation/LegacyTechnologies/WebObjects/WebObjects_3.5/Reference/Frameworks/ObjC/Foundation/Classes/NSSetClassCluster/Description.html#//apple_ref/occ/cl/NSSet)` class).

    If all three methods are found, create a collection proxy object that responds to all `NSSet` methods and return that. Otherwise, proceed to step 4.

    This proxy object subsequently converts any `NSSet` message it receives into some combination of `countOf<Key>`, `enumeratorOf<Key>`, and `memberOf<Key>:` messages to the object that created it. In effect, the proxy object working together with the key-value coding compliant object allows the underlying property to behave as if it were an `NSSet`, even if it is not.

4.  If no simple accessor method or group of collection access methods is found, and if the receiver's class method `[accessInstanceVariablesDirectly](https://developer.apple.com/library/archive/documentation/LegacyTechnologies/WebObjects/WebObjects_3.5/Reference/Frameworks/ObjC/EOF/EOControl/Classes/NSObjectAdditions/Description.html#//apple_ref/occ/clm/NSObject/accessInstanceVariablesDirectly)` returns `YES`, search for an instance variable named `_<key>`, `_is<Key>`, `<key>`, or `is<Key>`, in that order. If found, directly obtain the value of the instance variable and proceed to step 5\. Otherwise, proceed to step 6.

5.  If the retrieved property value is an object pointer, simply return the result.

    If the value is a scalar type supported by `NSNumber`, store it in an `NSNumber` instance and return that.

    If the result is a scalar type not supported by NSNumber, convert to an `NSValue` object and return that.

6.  If all else fails, invoke `[valueForUndefinedKey:](https://developer.apple.com/documentation/objectivec/nsobject/1413457-value)`. This raises an exception by default, but a subclass of `NSObject` may provide key-specific behavior.

当调用valueForKey：时，其底层的执行流程如下:

以获取LGPerson的对象person的属性name为例，如下图的getter的流程图：


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