iOS 底层原理 - KVC

作者: yan0_0 | 来源:发表于2020-06-21 21:19 被阅读0次

KVC
KVC底层实现步骤
iOS-底层原理21：KVO底层原理
iOS - KVO
iOS 底层-KVC底层原理
iOS 底层原理 - KVC
iOS KVC底层原理
iOS KVC 底层原理
[iOS] KVC底层原理
iOS KVC底层原理

概述

KVC (Key-Value Coding), 也就是键值编码，是利用 NSKeyValueCoding 非正式协议实现的一种机制, 对象采用这种机制来提供对其属性的间接访问。如果想要看详细的解释，可以看下官方文档。KVC官方文档。

使用方式

1.基本类型

LGPerson *person = [[LGPerson alloc] init];
[person setValue:@"KC" forKey:@"name"];
[person setValue:@19 forKey:@"age"];
[person setValue:@"酷C" forKey:@"myName"];
NSLog(@"%@ - %@ - %@",[person valueForKey:@"name"],[person valueForKey:@"age"],[person valueForKey:@"myName"]);

2.集合类型

LGPerson *person = [[LGPerson alloc] init];
person.array = @[@"1",@"2",@"3"];
// 由于不是可变数组 - 无法做到
// person.array[0] = @"100";
NSArray *array = [person valueForKey:@"array"];
// 方法一 ：用 array 的值创建一个新的数组
array = @[@"100",@"2",@"3"];
[person setValue:array forKey:@"array"];
NSLog(@"%@",[person valueForKey:@"array"]);

// 方法二 ：
NSMutableArray *ma = [person mutableArrayValueForKey:@"array"];
ma[0] = @"10";
NSLog(@"%@",[person valueForKey:@"array"]);

3.访问非对象属性

ThreeFloats floats = {1., 2., 3.};
NSValue *value  = [NSValue valueWithBytes:&floats objCType:@encode(ThreeFloats)];
[person setValue:value forKey:@"threeFloats"];
NSValue *reslut = [person valueForKey:@"threeFloats"];
NSLog(@"%@",reslut);
    
ThreeFloats th;
[reslut getValue:&th] ;
NSLog(@"%f - %f - %f",th.x,th.y,th.z);

4.嵌套属性访问

    LGStudent *student = [[LGStudent alloc] init];
    student.subject    = @"iOS";
    person.student     = student;
    [person setValue:@"大师班" forKeyPath:@"student.subject"];
    NSLog(@"%@",[person valueForKeyPath:@"student.subject"]);

KVC原理分析

KVC 设值过程

KVC设值会调用setValue:forKey 方法：
1.先判断是否存在 setKey，或者_setKey，setIsKey这三种方法，如果找到立即调用相应方法并传递name参数设值，结束
2.如果没有找到这三种方法，判断accessInstanceVariablesDirectly方法返回值，如果返回YES,依次判断_key,_is<Key>,key,isKey等成员变量是否存在，根据顺序存在即赋值并结束。
3.如果accessInstanceVariablesDirectly方法，返回NO，或者上述所有成员变量均不存在，则调用setValue:forUndefinedKey:方法抛出异常并崩溃。

KVC取值过程

KVC取值会调用ValueForKey方法
1.按照顺序查找实例方法: get<Key>, <key>, is<Key>, 或者 <key>，如果找到就跳到第五步
2.判断是否是数组，如果是数组则查找countOf<Key>,objectIn<Key>AtIndex:或<key>AtIndexes，并返回一个新的数组。否则就执行步骤3
3.判断是否属于NSSet,基于是否有NSSet相关的方法:countOf<Key>, enumeratorOf<Key>或 and memberOf<Key>:
4.如果上述方法都不存在，判断对象的类方法accessInstanceVariablesDirectly 返回值，如果返回YES，按顺序查找成员变量<key>, _is<Key>, <key>, 或 is<Key>，如果找到直接获取实例变量的值并跳到5继续执行，否则执行6
5.检索属性值，如果是指针对象，直接返回结果；如果该值是可转化为NSNumber类型的值，那么将该值转化为NSNumber并返回；除此以外将该值转化为NSValue类型的值作为结果返回。
6.如果所有的方法均失败，则调用valueForUndefinedKey: 并抛出异常。

KVC异常处理

当我们在使用 setValue:forKey: 或者 valueForKey: 的时候，由于 key 需要自己手写且没有提示，所以很可能会不小心写错，然后就会报 setValue:forUndefinedKey: 或者 valueForUndefinedKey: 的崩溃。但是NSObject的子类可以通过重写这个方法来提供特定的操作。比如：

- (void)setValue:(id)value forUndefinedKey:(NSString *)key{
    NSLog(@" %@ 没有这个key",key);
}

- (id)valueForUndefinedKey:(NSString *)key{
    NSLog(@" %@ 没有这个key - 给你一个其他的吧!",key);
    return @"Master";
}

使用细节

1.针对Int类型的key的写法
[object setValue:@123 forKey:@"count"];
2.如果对Int类型的传一个NSString的值如：[object setValue:@"123" forKey:@"count"]，系统会自动帮我们转换成__NSCFNumber的类型，说明KVC具有自动转型的功能。

[person setValue:@"20" forKey:@"age"]; // int - string
NSLog(@"%@-%@",[person valueForKey:@"age"],[[person valueForKey:@"age"] class]);//__NSCFNumber
//打印结果
2020-06-21 20:31:03.379902+0800 004-KVC异常小技巧[2098:140868] 20-__NSCFNumber

3.结构体的取值以及设值要转换为NSValue作为中间媒介。

ThreeFloats floats = {1., 2., 3.};
NSValue *value  = [NSValue valueWithBytes:&floats objCType:@encode(ThreeFloats)];
[person setValue:value forKey:@"threeFloats"];
NSLog(@"%@-%@",[person valueForKey:@"threeFloats"],[[person valueForKey:@"threeFloats"] class]);

4.setNilValueForKey:的方法只针对NSNumber（int，bool, etc..）以及NSValue（结构体）相关的数据生效，针对指针对象赋值nil并不会走到这个方法。

[person setValue:nil forKey:@"age"]; // subject不会走 - 官方注释里面说只对 NSNumber - NSValue
[person setValue:nil forKey:@"subject"];

调用这个方法会走下面这个方法，发现subject不会走，subject是NSString类型，age是int类型

- (void)setNilValueForKey:(NSString *)key{
    NSLog(@" %@ 是空值",key);
}

自定义KVC

思路：
首先创建一个NSObject的扩展，根据上述分析的过程，自定义setValue：forKey：以及ValueForKey：的方法。

- (void)lg_setValue:(nullable id)value forKey:(NSString *)key{
    
    // 1:非空判断一下
    if (key == nil  || key.length == 0) return;
    
    // 2:找到相关方法 set<Key> _set<Key> setIs<Key>
    // key 要大写
    NSString *Key = key.capitalizedString;
    // 拼接方法
    NSString *setKey = [NSString stringWithFormat:@"set%@:",Key];
    NSString *_setKey = [NSString stringWithFormat:@"_set%@:",Key];
    NSString *setIsKey = [NSString stringWithFormat:@"setIs%@:",Key];
    
    if ([self lg_performSelectorWithMethodName:setKey value:value]) {
        NSLog(@"*********%@**********",setKey);
        return;
    }else if ([self lg_performSelectorWithMethodName:_setKey value:value]) {
        NSLog(@"*********%@**********",_setKey);
        return;
    }else if ([self lg_performSelectorWithMethodName:setIsKey value:value]) {
        NSLog(@"*********%@**********",setIsKey);
        return;
    }
    
    // 3:判断是否能够直接赋值实例变量
    if (![self.class accessInstanceVariablesDirectly] ) {
        @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ valueForUndefinedKey:]: this class is not key value coding-compliant for the key name.****",self] userInfo:nil];
    }
    
    // 4.找相关实例变量进行赋值
    // 4.1 定义一个收集实例变量的可变数组
    NSMutableArray *mArray = [self getIvarListName];
    // _<key> _is<Key> <key> is<Key>
    NSString *_key = [NSString stringWithFormat:@"_%@",key];
    NSString *_isKey = [NSString stringWithFormat:@"_is%@",Key];
    NSString *isKey = [NSString stringWithFormat:@"is%@",Key];
    if ([mArray containsObject:_key]) {
        // 4.2 获取相应的 ivar
       Ivar ivar = class_getInstanceVariable([self class], _key.UTF8String);
        // 4.3 对相应的 ivar 设置值
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:_isKey]) {
       Ivar ivar = class_getInstanceVariable([self class], _isKey.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:key]) {
       Ivar ivar = class_getInstanceVariable([self class], key.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }else if ([mArray containsObject:isKey]) {
       Ivar ivar = class_getInstanceVariable([self class], isKey.UTF8String);
       object_setIvar(self , ivar, value);
       return;
    }

    // 5:如果找不到相关实例
    @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ %@]: this class is not key value coding-compliant for the key name.****",self,NSStringFromSelector(_cmd)] userInfo:nil];
}

- (nullable id)lg_valueForKey:(NSString *)key{
    
    // 1:刷选key 判断非空
    if (key == nil  || key.length == 0) {
        return nil;
    }

    // 2:找到相关方法 get<Key> <key> countOf<Key>  objectIn<Key>AtIndex
    // key 要大写
    NSString *Key = key.capitalizedString;
    // 拼接方法
    NSString *getKey = [NSString stringWithFormat:@"get%@",Key];
    NSString *countOfKey = [NSString stringWithFormat:@"countOf%@",Key];
    NSString *objectInKeyAtIndex = [NSString stringWithFormat:@"objectIn%@AtIndex:",Key];
        
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    if ([self respondsToSelector:NSSelectorFromString(getKey)]) {
        return [self performSelector:NSSelectorFromString(getKey)];
    }else if ([self respondsToSelector:NSSelectorFromString(key)]){
        return [self performSelector:NSSelectorFromString(key)];
    }else if ([self respondsToSelector:NSSelectorFromString(countOfKey)]){
        if ([self respondsToSelector:NSSelectorFromString(objectInKeyAtIndex)]) {
            int num = (int)[self performSelector:NSSelectorFromString(countOfKey)];
            NSMutableArray *mArray = [NSMutableArray arrayWithCapacity:1];
            for (int i = 0; i<num-1; i++) {
                num = (int)[self performSelector:NSSelectorFromString(countOfKey)];
            }
            for (int j = 0; j<num; j++) {
                id objc = [self performSelector:NSSelectorFromString(objectInKeyAtIndex) withObject:@(num)];
                [mArray addObject:objc];
            }
            return mArray;
        }
    }
#pragma clang diagnostic pop
    
    // 3:判断是否能够直接赋值实例变量
    if (![self.class accessInstanceVariablesDirectly] ) {
        @throw [NSException exceptionWithName:@"LGUnknownKeyException" reason:[NSString stringWithFormat:@"****[%@ valueForUndefinedKey:]: this class is not key value coding-compliant for the key name.****",self] userInfo:nil];
    }
    
    // 4.找相关实例变量进行赋值
    // 4.1 定义一个收集实例变量的可变数组
    NSMutableArray *mArray = [self getIvarListName];
    // _<key> _is<Key> <key> is<Key>
    // _name -> _isName -> name -> isName
    NSString *_key = [NSString stringWithFormat:@"_%@",key];
    NSString *_isKey = [NSString stringWithFormat:@"_is%@",Key];
    NSString *isKey = [NSString stringWithFormat:@"is%@",Key];
    if ([mArray containsObject:_key]) {
        Ivar ivar = class_getInstanceVariable([self class], _key.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:_isKey]) {
        Ivar ivar = class_getInstanceVariable([self class], _isKey.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:key]) {
        Ivar ivar = class_getInstanceVariable([self class], key.UTF8String);
        return object_getIvar(self, ivar);;
    }else if ([mArray containsObject:isKey]) {
        Ivar ivar = class_getInstanceVariable([self class], isKey.UTF8String);
        return object_getIvar(self, ivar);;
    }

    return @"";
}

#pragma mark - 相关方法
- (BOOL)lg_performSelectorWithMethodName:(NSString *)methodName value:(id)value{
 
    if ([self respondsToSelector:NSSelectorFromString(methodName)]) {
        
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        [self performSelector:NSSelectorFromString(methodName) withObject:value];
#pragma clang diagnostic pop
        return YES;
    }
    return NO;
}

- (id)performSelectorWithMethodName:(NSString *)methodName{
    if ([self respondsToSelector:NSSelectorFromString(methodName)]) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        return [self performSelector:NSSelectorFromString(methodName) ];
#pragma clang diagnostic pop
    }
    return nil;
}

- (NSMutableArray *)getIvarListName{
    
    NSMutableArray *mArray = [NSMutableArray arrayWithCapacity:1];
    unsigned int count = 0;
    Ivar *ivars = class_copyIvarList([self class], &count);
    for (int i = 0; i<count; i++) {
        Ivar ivar = ivars[i];
        const char *ivarNameChar = ivar_getName(ivar);
        NSString *ivarName = [NSString stringWithUTF8String:ivarNameChar];
        NSLog(@"ivarName == %@",ivarName);
        [mArray addObject:ivarName];
    }
    free(ivars);
    return mArray;
}