Objective-C 键值编码

首先申明下，本文为笔者学习《Objective-C 基础教程》的笔记，并加入笔者自己的理解和归纳总结。

1. 键值编码

键值编码(Key-Value Coding)是一种间接更改对象状态的方式，简称KVC。

valueForKey和setValue:forKey分别用来查找属性值和设置属性值。
valueForKey会把一些基本类型（int、float等）自动装箱，返回NSNumber等类型。
调用setValue:forKey时要把那些基本类型主动装箱。

@interface ShapeBounds : NSObject {
    int width;
    int height;
}

@end

@implementation ShapeBounds 

- (NSString*) description {
    return [NSString stringWithFormat: @"(%d, %d)", width, height];
}

@end

int main(int argc, const char* argv[]) {
    @autoreleasepool {
        ShapeBounds* bounds = [[ShapeBounds alloc] init];
        NSLog(@"%@", bounds);

        [bounds setValue: [NSNumber numberWithInt: 20] forKey: @"width"];
        [bounds setValue: [NSNumber numberWithInt: 30] forKey: @"height"];
        NSLog(@"%@", bounds);

        NSLog(@"width = %@, height = %@", [bounds valueForKey: @"width"],
                [bounds valueForKey: @"height"]);
    }
    return 0;
}

2. 键路径

valueForKeyPath和setValueForKeyPath方法访问键路径。

@interface Shape : NSObject {
    ShapeBounds* bounds;
}

@end

@implementation Shape

- (id) init {
    if (self = [super init]) {
        bounds = [[ShapeBounds alloc] init];
    }
    return self;
}

- (NSString*) description {
    return [NSString stringWithFormat: @"Shape bounds = %@", bounds];
}

@end

int main(int argc, const char* argv[]) {
    @autoreleasepool {
        Shape* shape = [[Shape alloc] init];
        NSLog(@"%@", shape);

        [shape setValue: [NSNumber numberWithInt: 20] forKeyPath: @"bounds.width"];
        [shape setValue: [NSNumber numberWithInt: 30] forKeyPath: @"bounds.height"];
        NSLog(@"%@", shape);

        NSLog(@"width = %@, height = %@", [shape valueForKeyPath: @"bounds.width"],
            [shape valueForKeyPath: @"bounds.height"]);
    }
    return 0;
}

3. 整体操作

• 整体访问

  访问NSArray数组中的某个键值，存在一对多的情况，返回一个数组。

    Shape* createShape(int width, int height) {
        Shape* shape = [[Shape alloc] init];
  
        [shape setValue: [NSNumber numberWithInt: width] forKeyPath: @"bounds.width"];
        [shape setValue: [NSNumber numberWithInt: height] forKeyPath: @"bounds.height"];
  
        return shape;
    }
  
    int main(int argc, const char* argv[]) {
        @autoreleasepool {
            NSMutableArray* array = [NSMutableArray arrayWithCapacity: 17];
            [array addObject: createShape(20, 30)];
            [array addObject: createShape(40, 50)];
            [array addObject: createShape(60, 45)];
  
            NSLog(@"width = %@", [array valueForKeyPath: @"bounds.width"]);
            NSLog(@"height = %@", [array valueForKeyPath: @"bounds.height"]);
        }
        return 0;
    }
  

• 快速运算

  键路径不仅能引用对象值，还可以引用一些运算符来进行一些运行。@count，总数。@sum，总值。@avg，平均值。@min，最小值。@max，最大值。

    int main(int argc, const char* argv[]) {
        @autoreleasepool {
            NSMutableArray* array = [NSMutableArray arrayWithCapacity: 17];
            [array addObject: createShape(20, 30)];
            [array addObject: createShape(40, 50)];
            [array addObject: createShape(60, 45)];
  
            NSLog(@"count = %@", [array valueForKeyPath: @"bounds.@count"]);
            NSLog(@"sum.height = %@", [array valueForKeyPath: @"[email protected]"]);
            NSLog(@"avg.height = %@", [array valueForKeyPath: @"[email protected]"]);
            NSLog(@"min.height = %@", [array valueForKeyPath: @"[email protected]"]);
            NSLog(@"max.height = %@", [array valueForKeyPath: @"[email protected]"]);
        }
        return 0;
    }
  

• 批处理

  dictionaryWithValuesForKeys，接收一个字符串数组，对每个键使用valueForKey方法。
  setValuesForKeysWithDictionary，接收一个字典，调用setValue:forKey方法。

    int main(int argc, const char* argv[]) {
        @autoreleasepool {
            ShapeBounds* bounds = [[ShapeBounds alloc] init];
            [bounds setValue: [NSNumber numberWithInt: 20] forKey: @"width"];
            [bounds setValue: [NSNumber numberWithInt: 30] forKey: @"height"];
            NSLog(@"%@", bounds);
  
            NSArray* array = [NSArray arrayWithObjects: @"width", @"height", nil];
            NSDictionary* dict = [bounds dictionaryWithValuesForKeys:array];
            NSLog(@"dict = %@", dict);
  
            NSDictionary* newDict = [NSDictionary dictionaryWithObjectsAndKeys:
                    [NSNumber numberWithInt: 40], @"width",
                    [NSNumber numberWithInt: 50], @"height", nil];
            [bounds setValuesForKeysWithDictionary: newDict];
            NSLog(@"%@", bounds);
    }
    return 0;
  

  }

4. 处理nil和未定义的键

• (void) setNilValueForKey: (NSString*) key，处理nil值
• (void) setValue: (id) value forUndefinedKey: (NSString*) key，更改未知键的值
• (id) valueForUndefinedKey: (NSString*) key，处理未定义的值