KVC实现原理

KVC(Key-Value Coding)是Objective-C提供的一种利用字符串来间接访问对象属性的一种机制,它是通过访问器去访问对象属性的另一个可选方案。非正式协议NSKeyValueCoding对其接口进行了定义,NSObject中提供了接口的默认实现。

一、什么是KVC

官方文档:https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107i

 KVC(Key-Value Coding)是Objective-C提供的一种利用字符串来间接访问对象属性的一种机制,它是通过访问器去访问对象属性的另一个可选方案。非正式协议NSKeyValueCoding对其接口进行了定义,NSObject中提供了接口的默认实现。

二、常见方法


//获取方法
- (nullable id)valueForKey:(NSString *)key;

- (nullable id)valueForKeyPath:(NSString *)keyPath;

- (nullable id)valueForUndefinedKey:(NSString *)key;

//可变集合获取方法
- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;

- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key NS_AVAILABLE(10_7, 5_0);

- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;

- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath;

- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath NS_AVAILABLE(10_7, 5_0);

- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath;

//设置方法
- (void)setValue:(nullable id)value forKey:(NSString *)key;

- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;

- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;

- (void)setNilValueForKey:(NSString *)key;

三、KVC原理

通过查找头文件NSKeyValueCoding.h注释可以发现KVC的执行过程

  1. valueForKey:(valueForKeyPath:类似)
    • 首先,在对象类中按顺序查找存取器名称为-get<key>, -<key>, -is<key>的方法,如果找到则直接调用,如果方法返回结果类型为指针类型,则直接返回。如果方法返回结果类型是数量类型支持NSNumber转换,则返回NSNumber,否则转化成NSValue并返回(任意类型的结果转换成NSValue,不仅NSPoint,NSRect和NSSize);
    • 否则(找不到存取器方法),查找-countOf<Key>, -indexIn<Key>OfObject:, -objectIn<Key>AtIndex:-<key>AtIndexes:,如果count方法,index方法和另外两个方法中至少一个方法找到,返回一个能够响应NSOrderedSet所有方法的代理集合对象(NSKeyValueOrderedSet)。每个NSOrderedSet消息发送给代理集合对象时,当消息发送给原始接收器的valueForKey:都会被转换成-countOf<Key>, -indexIn<Key>OfObject:, -objectIn<Key>AtIndex:-<key>AtIndexes:的组合来返回。如果实现了可选方法-get<Key>:range:,方法也将被调用来进行性能优化。
    • 否则(找不到存取器方法和NSOrderedSet访问方法),查找-countOf<Key>, -objectIn<Key>AtIndex:-<key>AtIndexes:,如果count方法和另外两个方法中至少一个方法找到,返回一个能够响应NSArray所有方法的代理集合对象。每个NSArray消息发送给代理集合对象时(NSKeyValueArray),当消息发送给原始接收器的valueForKey:都会被转换成-countOf<Key>, -objectIn<Key>AtIndex:-<key>AtIndexes:的组合来返回。如果实现了可选方法-get<Key>:range:,方法也将被调用来进行性能优化。
    • 否则(找不到存取器方法、NSOrderedSet访问方法和array访问方法),查找-countOf<Key>, -enumeratorOf<Key>-memberOf<Key>:,如果三个方法都找到,返回一个能够响应NSSet所有方法的代理集合对象(NSKeyValueSet)。每个NSSet消息发送给代理集合对象时,当消息发送给原始接收器的valueForKey:都会被转换成-countOf<Key>, -enumeratorOf<Key>-memberOf<key>:的组合来返回。
    • 否则(找不到存取器方法和集合访问方法),如果+accessInstanceVariablesDirectly属性返回YES,则按顺序查找符合名称_< key >, _is< Key >, < key >, 或者 is< Key >的实例变量。如果找到实例变量,则返回实例变量的值,转换NSNumber和NSValue同步骤1.
    • 否则(找不到存取器方法,集合访问方法和实例变量),调用-valueForUndefinedKey:返回结果。-valueForUndefinedKey:的默认实现是抛出一个NSUndefinedKeyException异常,但是你覆盖此方法。

    注意:

    对于集合(NSArray, NSSet, NSOrderSet)像使用普通对象一样,则返回代理对象,需要实现以下方法

NSArray NSSet NSOrderSet
-countOf<Key> | ` -countOf` | ` -countOf`    
  -enumeratorOf<Key> | ` -indexInOfObject:`  
One of -memberOf<Key>: One of
-objectIn<Key>AtIndex: | | ` -objectInAtIndex:`    
-<key>AtIndexes: | | ` -AtIndexes:`    
Optional (performance)   Optional (performance)
-get<Key>:range: | | ` -get:range:`    
  1. setValue: forKey: (setValue: forKeyPath:类似)

    • 首先,查找类存取器方法-set<Key>:。如果找到此方法则检测参数类型。如果参数类型不是对象指针类型但值是nil,则调用-setNilValueForKey:方法,-setNilValueForKey:方法的默认实现是抛出一个NSInvalidArgumentException异常,但是你可以覆盖此方法。否则如果方法参数类型是对象指针类型,则直接调用此方法并传入value做为参数。如果方法参数类型是其他类型,NSNumber/NSValue的逆转换在方法-valueFor<Key>被调用的时候执行。
    • 否则(没有存取器方法),如果+accessInstanceVariablesDirectly属性返回YES,则按顺序查找符合名称_< key >, _is< Key >, < key >, 或者 is< Key >的实例变量。如果找到实例变量,并且它的类型是对象指针类型,则对旧值进行release操作,然后对value进行retain操作并赋值给实例变量。如果是其他类型,则同步骤1进行NSNumber/NSValue进行转换然后再赋值。
    • 否则(没有存取器方法和实例变量),调用setValue:forUndefinedkey:方法,setValue:forUndefinedkey:方法的默认实现是抛出一个NSUndefinedKeyException异常,但是你可以覆盖此方法。
  2. 可变集合-mutableArrayValueForKey:key-mutableOrderedSetValueForKey:key-mutableSetValueForKey:key

    默认实现存取器方法时和-valueForKey:key一样。返回代理对象时需要实现的方法有差别。如下:

NSMutableArray / NSMutableOrderedSet NSMutableSet
At least 1 insertion and 1 removal method * At least 1 addition and 1 removal method
-insertObject:in<Key>AtIndex: | ` -addObject:`  
-removeObjectFrom<Key>AtIndex: | ` -removeObject:`  
-insert<Key>:atIndexes: | ` -add:`  
-remove<Key>AtIndexes: | ` -remove:`  
Optional (performance) one of * Optional (performance)
-replaceObjectIn<Key>AtIndex:withObject: | ` -intersect:`  
-replace<Key>AtIndexes:with<Key>: | ` -set:`  

四、实例

1. 创建YKNoteKVCObject类

//
//  YKNoteKVCObject.h
//  YKNote
//
//  Created by wanyakun on 2016/11/10.
//  Copyright © 2016年 com.ucaiyuan. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface YKNoteKVCObject : NSObject {
    NSInteger _intVar;
    NSString *_strVar;
}

@property (nonatomic, assign) NSInteger intProperty;
@property (nonatomic, copy) NSString *strProperty;

@end

//
//  YKNoteKVCObject.m
//  YKNote
//
//  Created by wanyakun on 2016/11/10.
//  Copyright © 2016年 com.ucaiyuan. All rights reserved.
//

#import "YKNoteKVCObject.h"


@implementation YKNoteKVCObject

@synthesize intProperty = _intProperty;
@synthesize strProperty = _strProperty;

#pragma mark - method for orderSet
- (NSUInteger)countOfOrderSet {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return 5;
}

- (NSInteger)indexInOrderSetOfObject:(id)element {
    return 2;
}

- (id)objectInOrderSetAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return [NSString stringWithFormat:@"orderSet_%ld", index];
}

#pragma mark - method for array
- (NSUInteger)countOfArray {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return 10;
}

- (id)objectInArrayAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return [NSString stringWithFormat:@"array_%ld", index];
}

#pragma mark - mehtod for set
- (NSUInteger)countOfSet {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return 15;
}

- (NSEnumerator *)enumeratorOfSet {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    NSEnumerator *enumerator = [[NSEnumerator alloc] init];
    return enumerator;
}

- (NSString *)memberOfSet:(NSString *)object {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return [NSString stringWithFormat:@"member of set: %@", object];
}

#pragma mark - method for MutableOrderedSet
- (void)insertObject:(NSString *)object inMOrderSetAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)removeObjectFromMOrderSetAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

#pragma mark - method for MutableArray
- (void)insertObject:(NSString *)object inMArrayAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)removeObjectFromMArrayAtIndex:(NSUInteger)index {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

#pragma mark - method for mutableSet
- (void)addMSetObject:(NSString *)object {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)removeMSetObject:(NSString *)object {
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

#pragma mark - private method
- (id)valueForUndefinedKey:(NSString *)key {
    NSLog(@"%s\nValueForUndefinedKey:%@", __PRETTY_FUNCTION__, key);
    return @"undefinedKeyValue";
}

- (void)setNilValueForKey:(NSString *)key {
    NSLog(@"%s\nNilValueKey:%@", __PRETTY_FUNCTION__, key);
}

- (void)setValue:(id)value forUndefinedKey:(NSString *)key {
    NSLog(@"%s\nundefineKey:%@", __PRETTY_FUNCTION__, key);
}

+ (BOOL)accessInstanceVariablesDirectly {
    return YES;
}


#pragma mark getter setter
- (NSInteger)intProperty {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return _intProperty;
}

- (void)setIntProperty:(NSInteger)intProperty {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    _intProperty = intProperty;
}

- (NSString *)strProperty {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    return _strProperty;
}

- (void)setStrProperty:(NSString *)strProperty {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    _strProperty = [strProperty copy];
}

@end

2. 创建Controller,用来调用setValue:ForKey:和valueForKey:等

//
//  YKNoteKVCViewController.m
//  YKNote
//
//  Created by wanyakun on 2016/11/11.
//  Copyright © 2016年 com.ucaiyuan. All rights reserved.
//

#import "YKNoteKVCViewController.h"
#import "YKNoteKVCObject.h"
#import <objc/runtime.h>

@interface YKNoteKVCViewController ()

@property (nonatomic, strong) YKNoteKVCObject *yKNoteKVCObject;

@end

@implementation YKNoteKVCViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.title = @"KVC";
    self.view.backgroundColor = [UIColor whiteColor];
    //通过存取器访问
    [self.yKNoteKVCObject setValue:[NSNumber numberWithInteger:10] forKey:@"intProperty"];
    NSInteger intProperty = [[self.yKNoteKVCObject valueForKey:@"intProperty"] integerValue];
    NSLog(@"intProperty = %ld", intProperty);
    
    [self.yKNoteKVCObject setValue:@"I am strProperty" forKey:@"strProperty"];
    NSString *strProperty = [self.yKNoteKVCObject valueForKey:@"strProperty"];
    NSLog(@"strProperty = %@", strProperty);
    
    //通过实例变量访问
    [self.yKNoteKVCObject setValue:[NSNumber numberWithInteger:20] forKey:@"intVar"];
    NSInteger intVar = [[self.yKNoteKVCObject valueForKey:@"intVar"] integerValue];
    NSLog(@"intVar = %ld", intVar);

    [self.yKNoteKVCObject setValue:@"I am strVar" forKey:@"strVar"];
    NSString *strVar = [self.yKNoteKVCObject valueForKey:@"strVar"];
    NSLog(@"strVar = %@", strVar);

    //set undefineKey
    [self.yKNoteKVCObject setValue:@"undefine value" forKey:@"undefinedKey"];

    //为非Object pointer参数类型设置nil
    [self.yKNoteKVCObject setValue:nil forKey:@"intProperty"];

    //NSOrderSet, NSArray, NSSet代理对象
    id orderSet = [self.yKNoteKVCObject valueForKey:@"orderSet"];
    id array = [self.yKNoteKVCObject valueForKey:@"array"];
    id set = [self.yKNoteKVCObject valueForKey:@"set"];
    NSLog(@"\norderSet class:%@\narray class:%@\nset class:%@", object_getClass(orderSet), object_getClass(array), object_getClass(set));
    
    //NSMutableOrderSet, NSMutableArray, NSMutableSet代理对象
    id mutableOrderSet = [self.yKNoteKVCObject mutableOrderedSetValueForKey:@"mOrderSet"];
    id mutableArray = [self.yKNoteKVCObject mutableArrayValueForKey:@"mArray"];
    id mutableSet = [self.yKNoteKVCObject mutableSetValueForKey:@"mSet"];
    NSLog(@"\nmutableOrderSet class:%@\nmutableArray class:%@\nmutableSet class:%@", object_getClass(mutableOrderSet), object_getClass(mutableArray), object_getClass(mutableSet));
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

#pragma mark - getter
- (YKNoteKVCObject *)yKNoteKVCObject {
    if (_yKNoteKVCObject == nil) {
        _yKNoteKVCObject = [[YKNoteKVCObject alloc] init];
    }
    return _yKNoteKVCObject;
}

@end

3. 输出结果

-[YKNoteKVCObject setIntProperty:]
-[YKNoteKVCObject intProperty]
intProperty = 10
-[YKNoteKVCObject setStrProperty:]
-[YKNoteKVCObject strProperty]
strProperty = I am strProperty
intVar = 20
strVar = I am strVar
-[YKNoteKVCObject setValue:forUndefinedKey:]
undefineKey:undefinedKey
-[YKNoteKVCObject setNilValueForKey:]
NilValueKey:intProperty
orderSet class:NSKeyValueOrderedSet
array class:NSKeyValueArray
set class:NSKeyValueSet
mutableOrderSet class:NSKeyValueFastMutableOrderedSet2
mutableArray class:NSKeyValueFastMutableArray2
mutableSet class:NSKeyValueFastMutableSet2


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