IOS键值编码KVC

KVC机制允许以字符串形式间接操作对象的属性

FKItem.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface FKItem : NSObject

@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) int price;

@end

NS_ASSUME_NONNULL_END

FKItem.m

#import "FKItem.h"

@implementation FKItem
{
    NSDate *date;
}

//处理非法设置不存在的key，在类的实现部分重写setValue: forUndefinedKey:方法，避免闪退
- (void)setValue:(id)value forUndefinedKey:(NSString *)key
{
    NSLog(@"你设置的key:[%@] 不存在!", key);
}

//处理非法访问不存在的key，在类的实现部分重写setValue: forUndefinedKey:方法，避免闪退
- (id)valueForUndefinedKey:(NSString *)key
{
    NSLog(@"你访问的key:[%@] 不存在!", key);
    return nil;
}

//给基本类型赋值为nil会报异常,在类的实现部分重写setNilValueForKey
- (void)setNilValueForKey:(NSString *)key
{
    if ([@"price" isEqualToString:key])
    {
        _price = 0;
    }
    else
    {
        //回调父类的setNilValueForKey,执行默认行为
        [super setNilValueForKey:key];
    }
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"name:%@ price:%d date:%@", _name, _price, date];
}
@end

FKOrder.h

#import <Foundation/Foundation.h>
#import "FKItem.h"
NS_ASSUME_NONNULL_BEGIN

@interface FKOrder : NSObject

@property (nonatomic, strong) FKItem *item;
@property (nonatomic, assign) int amount;

@end

NS_ASSUME_NONNULL_END

FKOrder.m

#import "FKOrder.h"

@implementation FKOrder

@end

main.m

#import <Foundation/Foundation.h>
#import "FKItem.h"
#import "FKOrder.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        FKItem *item = [[FKItem alloc] init];
        [item setValue:@"苹果" forKey:@"name"];
        [item setValue:[NSNumber numberWithInt:5] forKey:@"price"];
        [item setValue:[[NSDate alloc] init] forKey:@"date"];//可以给类实现部分定义的date成员赋值
        NSLog(@"name:%@ price:%@ date:%@", [item valueForKey:@"name"], [item valueForKey:@"price"], [item valueForKey:@"date"]);
        
        //访问不存在的key，会报异常
        [item setValue:@"233" forKey:@"namm"];
        [item valueForKey:@"namm"];
        //给基本类型赋值为nil，会报异常
        [item setValue:nil forKey:@"price"];
        NSLog(@"name:%@ price:%@ date:%@", [item valueForKey:@"name"], [item valueForKey:@"price"], [item valueForKey:@"date"]);
        
        FKOrder *order = [[FKOrder alloc] init];
        [order setValue:@"12" forKey:@"amount"];
        [order setValue:item forKey:@"item"];
        NSLog(@"%@ %@", [order valueForKey:@"amount"], [order valueForKey:@"item"]);
        
        //访问复合属性,用forKeyPath
        [order setValue:@"键盘" forKeyPath:@"item.name"];
        [order setValue:@"100" forKeyPath:@"item.price"];
        NSLog(@"%@ %@", [order valueForKeyPath:@"item.name"], [order valueForKeyPath:@"item.price"]);
    }
    return 0;
}

输出

2020-04-07 00:09:02.306795+0800 KVCTest[15122:475463] name:苹果 price:5 date:Tue Apr  7 00:09:02 2020
2020-04-07 00:09:02.306999+0800 KVCTest[15122:475463] 你设置的key:[namm] 不存在!
2020-04-07 00:09:02.307140+0800 KVCTest[15122:475463] 你访问的key:[namm] 不存在!
2020-04-07 00:09:02.307237+0800 KVCTest[15122:475463] name:苹果 price:0 date:Tue Apr  7 00:09:02 2020
2020-04-07 00:09:02.309560+0800 KVCTest[15122:475463] 12 name:苹果 price:0 date:2020-04-06 16:09:02 +0000
2020-04-07 00:09:02.309722+0800 KVCTest[15122:475463] 键盘 100